cppguide.xml

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<GUIDE title="Google C++ Style Guide">

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Revision 3.199
</p>


<address>
Benjy Weinberger<br/>
Craig Silverstein<br/>
Gregory Eitzmann<br/>
Mark Mentovai<br/>
Tashana Landray
</address>

<OVERVIEW>
<CATEGORY title="Important Note">
  <STYLEPOINT title="Displaying Hidden Details in this Guide">
     <SUMMARY>
       This style guide contains many details that are initially
       hidden from view.  They are marked by the triangle icon, which you
       see here on your left.  Click it now.
       You should see "Hooray" appear below.
     </SUMMARY>
     <BODY>
       <p>
        Hooray!  Now you know you can expand points to get more
        details.  Alternatively, there's an "expand all" at the
        top of this document.
       </p>
     </BODY>
  </STYLEPOINT>
</CATEGORY>
<CATEGORY title="Background">
  <p>
    C++ is the main development language
    
    used by many of Google's open-source
    projects.
    As every C++ programmer knows, the language has many powerful features,
    but this power brings with it complexity, which in turn can make code
    more bug-prone and harder to read and maintain.
  </p>
  <p>
    The goal of this guide is to manage this complexity by describing
    in detail the dos and don'ts of writing C++
    code. These rules exist to
    keep
    
    the
    code base manageable while still allowing coders to use C++ language
    features productively.
  </p>
  <p>
    <em>Style</em>, also known as readability, is what we call the
    conventions that govern our C++ code. The term Style is a bit of a
    misnomer, since these conventions cover far more than just source
    file formatting.
  </p>
  <p>
    One way in which we keep the code base manageable is by enforcing
    <em>consistency</em>.
    
    It is very important that any
    
    programmer
    be able to look at another's code and quickly understand it.
    Maintaining a uniform style and following conventions means that we can
    more easily use "pattern-matching" to infer what various symbols are
    and what invariants are true about them. Creating common, required
    idioms and patterns makes code much easier to understand.  In some
    cases there might be good arguments for changing certain style
    rules, but we nonetheless keep things as they are in order to
    preserve consistency.
  </p>
  <p>
    Another issue this guide addresses is that of C++ feature bloat.
    C++ is a huge language with many advanced features. In some cases
    we constrain, or even ban, use of certain features. We do this to
    keep code simple and to avoid the various common errors and
    problems that these features can cause.  This guide lists these
    features and explains why their use is restricted.
  </p>
  <p>
    
    Open-source projects developed by Google
    conform to the requirements in this guide.
  </p>
  
  <p>
    Note that this guide is not a C++ tutorial: we assume that the
    reader is familiar with the language.
    
  </p>
  
</CATEGORY>
</OVERVIEW>

<CATEGORY title="Header Files">
  <p>
    In general, every <code>.cc</code> file should have an associated
    <code>.h</code> file. There are some common exceptions, such as
    
    unittests
    and small <code>.cc</code> files containing just a <code>main()</code>
    function.
  </p>
  <p>
    Correct use of header files can make a huge difference to the
    readability, size and performance of your code.
  </p>
  <p>
    The following rules will guide you through the various pitfalls of
    using header files.
  </p>

  <STYLEPOINT title="The #define Guard">
    <SUMMARY>
      All header files should have <code>#define</code> guards to
      prevent multiple inclusion.  The format of the symbol name
      should be
      <code><i>&lt;PROJECT&gt;</i>_<i>&lt;PATH&gt;</i>_<i>&lt;FILE&gt;</i>_H_</code>.
    </SUMMARY>
    <BODY>
      
      <p>
        To guarantee uniqueness, they should be based on the full path
        in a project's source tree.  For example, the file
        <code>foo/src/bar/baz.h</code> in project <code>foo</code> should
        have the following guard:
      </p>
      <CODE_SNIPPET>
         #ifndef FOO_BAR_BAZ_H_
         #define FOO_BAR_BAZ_H_

         ...

         #endif  // FOO_BAR_BAZ_H_
      </CODE_SNIPPET>
      
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Header File Dependencies">
    <SUMMARY>
        Don't use an <code>#include</code> when a forward declaration
        would suffice.
    </SUMMARY>
    <BODY>
      <p>
        When you include a header file you introduce a dependency that
        will cause your code to be recompiled whenever the header file
        changes. If your header file includes other header files, any
        change to those files will cause any code that includes your
        header to be recompiled.  Therefore, we prefer to minimize
        includes, particularly includes of header files in other
        header files.
      </p>
      <p>
        You can significantly reduce the number of header files you
        need to include in your own header files by using forward
        declarations.  For example, if your header file uses the
        <code>File</code> class in ways that do not require access to
        the declaration of the <code>File</code> class, your header
        file can just forward declare <code>class File;</code> instead
        of having to <code>#include "file/base/file.h"</code>.
      </p>
      <p>
        How can we use a class <code>Foo</code> in a header file
        without access to its definition?
      </p>
      <ul>
        <li> We can declare data members of type <code>Foo*</code> or
             <code>Foo&amp;</code>.
             </li>
        <li> We can declare (but not define) functions with arguments,
             and/or return values, of type <code>Foo</code>.  (One
             exception is if an argument <code>Foo</code>
             or <code>const Foo&amp;</code> has a
             non-<code>explicit</code>, one-argument constructor,
             
             in which case we need the full definition to support
             automatic type conversion.)
             </li>
        <li> We can declare static data members of type
             <code>Foo</code>.  This is because static data members
             are defined outside the class definition.
             </li>
      </ul>
      <p>
        On the other hand, you must include the header file for
        <code>Foo</code> if your class subclasses <code>Foo</code> or
        has a data member of type <code>Foo</code>.
      </p>
      <p>
        Sometimes it makes sense to have pointer (or better,
        <code>scoped_ptr</code>)
        members instead of object members. However, this complicates code
        readability and imposes a performance penalty, so avoid doing
        this transformation if the only purpose is to minimize includes
        in header files.
      </p>
      <p>
        Of course, <code>.cc</code> files typically do require the
        definitions of the classes they use, and usually have to
        include several header files.
      </p>
      <SUBSECTION title="Note:">
        If you use a symbol <code>Foo</code> in your source file, you
        should bring in a definition for <code>Foo</code> yourself,
        either via an #include or via a forward declaration.  Do not
        depend on the symbol being brought in transitively via headers
        not directly included.  One exception is if <code>Foo</code>
        is used in <code>myfile.cc</code>, it's ok to #include (or
        forward-declare) <code>Foo</code> in <code>myfile.h</code>,
        instead of <code>myfile.cc</code>.
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Inline Functions">
    <SUMMARY>
      Define functions inline only when they are small, say, 10 lines
      or less.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        You can declare functions in a way that allows the compiler to
        expand them inline rather than calling them through the usual
        function call mechanism.
      </DEFINITION>
      <PROS>
        Inlining a function can generate more efficient object code,
        as long as the inlined function is small. Feel free to inline
        accessors and mutators, and other short, performance-critical
        functions.
      </PROS>
      <CONS>
        Overuse of inlining can actually make programs slower.
        Depending on a function's size, inlining it can cause the code
        size to increase or decrease.  Inlining a very small accessor
        function will usually decrease code size while inlining a very
        large function can dramatically increase code size.  On modern
        processors smaller code usually runs faster due to better use
        of the instruction cache.
      </CONS>
      <DECISION>
        <p>
          A decent rule of thumb is to not inline a function if it is
          more than 10 lines long. Beware of destructors, which are
          often longer than they appear because of implicit member-
          and base-destructor calls!
        </p>
        <p>
          Another useful rule of thumb: it's typically not cost
          effective to inline functions with loops or switch
          statements (unless, in the common case, the loop or switch
          statement is never executed).
        </p>
        <p>
          It is important to know that functions are not always
          inlined even if they are declared as such; for example,
          virtual and recursive functions are not normally inlined.
          Usually recursive functions should not be inline.  The main
          reason for making a virtual function inline is to place its
          definition in the class, either for convenience or to
          document its behavior, e.g., for accessors and mutators.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="The -inl.h Files">
    <SUMMARY>
      You may use file names with a <code>-inl.h</code> suffix to define
      complex inline functions when needed.
    </SUMMARY>
    <BODY>
      <p>
        The definition of an inline function needs to be in a header
        file, so that the compiler has the definition available for
        inlining at the call sites.  However, implementation code
        properly belongs in <code>.cc</code> files, and we do not like
        to have much actual code in <code>.h</code> files unless there
        is a readability or performance advantage.
      </p>
      <p>
        If an inline function definition is short, with very little,
        if any, logic in it, you should put the code in your
        <code>.h</code> file.  For example, accessors and mutators
        should certainly be inside a class definition.  More complex
        inline functions may also be put in a <code>.h</code> file for
        the convenience of the implementer and callers, though if this
        makes the <code>.h</code> file too unwieldy you can instead
        put that code in a separate <code>-inl.h</code> file.
        This separates the implementation from the class definition,
        while still allowing the implementation to be included where
        necessary.
      </p>
      <p>
        Another use of <code>-inl.h</code> files is for definitions of
        function templates.  This can be used to keep your template
        definitions easy to read.
      </p>
      <p>
        Do not forget that a <code>-inl.h</code> file requires a
        <a href="#The__define_Guard"><code>#define</code> guard</a> just
        like any other header file.
      </p>
      
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Parameter Ordering">
    <SUMMARY>
      When defining a function, parameter order is: inputs,
      then outputs.
    </SUMMARY>
    <BODY>
      <p>
        Parameters to C/C++ functions are either input to the
        function, output from the function, or both. Input parameters
        are usually values or <code>const</code> references, while output
        and input/output parameters will be non-<code>const</code>
        pointers. When ordering function parameters, put all input-only
        parameters before any output parameters.  In particular, do not add
        new parameters to the end of the function just because they are
        new; place new input-only parameters before the output
        parameters.
      </p>
      <p>
        This is not a hard-and-fast rule.  Parameters that are both
        input and output (often classes/structs) muddy the waters,
        and, as always, consistency with related functions may require
        you to bend the rule.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Names and Order of Includes">
    <SUMMARY>
      Use standard order for readability and to avoid hidden
      dependencies: C library, C++ library,
      
      other libraries' <code>.h</code>, your
      project's
      <code>.h</code>.
    </SUMMARY>
    <BODY>
      <p>
        
        All of a project's header files should be
        listed as descendants of the project's source directory
        without use of UNIX directory shortcuts <code>.</code> (the current
        directory) or <code>..</code> (the parent directory).  For
        example,
        
        <code>google-awesome-project/src/base/logging.h</code>
        should be included as
      </p>
      <CODE_SNIPPET>
        #include "base/logging.h"
      </CODE_SNIPPET>
      <p>
        In <code><var>dir/foo</var>.cc</code> or <code><var>dir/foo_test</var>.cc</code>,
	whose main purpose is to implement or test the stuff in
        <code><var>dir2/foo2</var>.h</code>, order your includes as
        follows:
      </p>
      <ol>
        <li> <code><var>dir2/foo2</var>.h</code> (preferred location
          &#8212; see details below).</li>
        <li> C system files.</li>
        <li> C++ system files.</li>
        <li> Other libraries' <code>.h</code> files.</li>
        <li> 
             Your project's
             <code>.h</code> files.</li>
      </ol>
      <p>
        With the preferred ordering, if <code><var>dir/foo2</var>.h</code>
        omits any necessary includes, the build of
        <code><var>dir/foo</var>.cc</code> or
        <code><var>dir/foo</var>_test.cc</code> will break.
        Thus, this rule ensures that build breaks show up first
        for the people working on these files, not for innocent people
        in other packages.
      </p>
      <p>
        <code><var>dir/foo</var>.cc</code> and
        <code><var>dir2/foo2</var>.h</code> are often in the same
        directory (e.g. <code>base/basictypes_test.cc</code> and
        <code>base/basictypes.h</code>), but can be in different
        directories too.
      </p>
      
      <p>
        Within each section it is nice to order the includes
        alphabetically.
      </p>
      <p>
        For example, the includes in
        
        <code>google-awesome-project/src/foo/internal/fooserver.cc</code>
        might look like this:
      </p>
      <CODE_SNIPPET>
        #include "foo/public/fooserver.h"  // Preferred location.

        #include &lt;sys/types.h&gt;
        #include &lt;unistd.h&gt;
        #include &lt;hash_map&gt;
        #include &lt;vector&gt;

        #include "base/basictypes.h"
        #include "base/commandlineflags.h"
        #include "foo/public/bar.h"
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Scoping">
  <STYLEPOINT title="Namespaces">
    <SUMMARY>
      Unnamed namespaces in <code>.cc</code> files are encouraged.  With
      named namespaces, choose the name based on the
      
      project, and possibly its path.
      Do not use a <SYNTAX>using-directive</SYNTAX>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        Namespaces subdivide the global scope into distinct, named
        scopes, and so are useful for preventing name collisions in
        the global scope.
      </DEFINITION>
      <PROS>
        <p>
          Namespaces provide a (hierarchical) axis of naming, in
          addition to the (also hierarchical) name axis provided by
          classes.
        </p>
        <p>
          For example, if two different projects have a class
          <code>Foo</code> in the global scope, these symbols may
          collide at compile time or at runtime.  If each project
          places their code in a namespace, <code>project1::Foo</code>
          and <code>project2::Foo</code> are now distinct symbols that
          do not collide.
        </p>
      </PROS>
      <CONS>
        <p>
          Namespaces can be confusing, because they provide an
          additional (hierarchical) axis of naming, in addition to the
          (also hierarchical) name axis provided by classes.
        </p>
        <p>
          Use of unnamed spaces in header files can easily cause
          violations of the C++ One Definition Rule (ODR).
        </p>
      </CONS>
      <DECISION>
        <p>
          Use namespaces according to the policy described below.
          Terminate namespaces with comments as shown in the given examples.
        </p>

        <SUBSECTION title="Unnamed Namespaces">
          <ul>
            <li> Unnamed namespaces are allowed and even encouraged in
                 <code>.cc</code> files, to avoid runtime naming
                 conflicts:
                 <CODE_SNIPPET>
                   namespace {                           // This is in a .cc file.

                   // The content of a namespace is not indented
                   enum { kUnused, kEOF, kError };       // Commonly used tokens.
                   bool AtEof() { return pos_ == kEOF; }  // Uses our namespace's EOF.

                   }  // namespace
                 </CODE_SNIPPET>

                 <p>
                   However, file-scope declarations that are
                   associated with a particular class may be declared
                   in that class as types, static data members or
                   static member functions rather than as members of
                   an unnamed namespace.
                 </p>
                 </li>
            <li> Do not use unnamed namespaces in <code>.h</code>
                 files.
                 </li>
          </ul>
        </SUBSECTION>

        <SUBSECTION title="Named Namespaces">
          <p>
            Named namespaces should be used as follows:
          </p>
          <ul>
            <li> Namespaces wrap the entire source file after includes,
                 
                 <a href="http://google-gflags.googlecode.com/">gflags</a>
                 definitions/declarations, and forward declarations of classes
                 from other namespaces:
                 <CODE_SNIPPET>
                   // In the .h file
                   namespace mynamespace {

                   // All declarations are within the namespace scope.
                   // Notice the lack of indentation.
                   class MyClass {
                    public:
                     ...
                     void Foo();
                   };

                   }  // namespace mynamespace
                 </CODE_SNIPPET>
                 <CODE_SNIPPET>
                   // In the .cc file
                   namespace mynamespace {

                   // Definition of functions is within scope of the namespace.
                   void MyClass::Foo() {
                     ...
                   }

                   }  // namespace mynamespace
                 </CODE_SNIPPET>
                 <p>
                   The typical <code>.cc</code> file might have more
                   complex detail, including the need to reference classes
                   in other namespaces.
                 </p>
                 <CODE_SNIPPET>
                   #include "a.h"

                   DEFINE_bool(someflag, false, "dummy flag");

                   class C;  // Forward declaration of class C in the global namespace.
                   namespace a { class A; }  // Forward declaration of a::A.

                   namespace b {

                   ...code for b...         // Code goes against the left margin.

                   }  // namespace b
                 </CODE_SNIPPET>
                 </li>

            
            <li> Do not declare anything in namespace
                 <code>std</code>, not even forward declarations of
                 standard library classes.  Declaring entities in
                 namespace <code>std</code> is undefined behavior,
                 i.e., not portable.  To declare entities from the
                 standard library, include the appropriate header
                 file.
                 </li>

            <li> You may not use a <SYNTAX>using-directive</SYNTAX> to
                 make all names from a namespace available.
                 <BAD_CODE_SNIPPET>
                   // Forbidden -- This pollutes the namespace.
                   using namespace foo;
                 </BAD_CODE_SNIPPET>
                 </li>

            <li> You may use a <SYNTAX>using-declaration</SYNTAX>
                 anywhere in a <code>.cc</code> file, and in functions,
                 methods or classes in <code>.h</code> files.
                 <CODE_SNIPPET>
                   // OK in .cc files.
                   // Must be in a function, method or class in .h files.
                   using ::foo::bar;
                 </CODE_SNIPPET>
                 </li>

            <li> Namespace aliases are allowed anywhere in a
                 <code>.cc</code> file, anywhere inside the named
                 namespace that wraps an entire <code>.h</code> file,
                 and in functions and methods.
                 <CODE_SNIPPET>
                   // Shorten access to some commonly used names in .cc files.
                   namespace fbz = ::foo::bar::baz;

                   // Shorten access to some commonly used names (in a .h file).
                   namespace librarian {
                   // The following alias is available to all files including
                   // this header (in namespace librarian):
                   // alias names should therefore be chosen consistently
                   // within a project.
                   namespace pd_s = ::pipeline_diagnostics::sidetable;

                   inline void my_inline_function() {
                     // namespace alias local to a function (or method).
                     namespace fbz = ::foo::bar::baz;
                     ...
                   }
                   }  // namespace librarian
                 </CODE_SNIPPET>
                 <p>
                 Note that an alias in a .h file is visible to everyone
                 #including that file, so public headers (those available
                 outside a project) and headers transitively #included by them,
                 should avoid defining aliases, as part of the general
                 goal of keeping public APIs as small as possible.
                 </p>
                 </li>
          </ul>
        </SUBSECTION>

        
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Nested Classes">
    <SUMMARY>
      Although you may use public nested classes when they are part of
      an interface, consider a <a HREF="#Namespaces">namespace</a> to
      keep declarations out of the global scope.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        A class can define another class within it; this is also
        called a <SYNTAX>member class</SYNTAX>.
        <CODE_SNIPPET>
          class Foo {

           private:
            // Bar is a member class, nested within Foo.
            class Bar {
              ...
            };

          };
        </CODE_SNIPPET>
      </DEFINITION>
      <PROS>
        This is useful when the nested (or member) class is only used
        by the enclosing class; making it a member puts it in the
        enclosing class scope rather than polluting the outer scope
        with the class name.  Nested classes can be forward declared
        within the enclosing class and then defined in the
        <code>.cc</code> file to avoid including the nested class
        definition in the enclosing class declaration, since the
        nested class definition is usually only relevant to the
        implementation.
      </PROS>
      <CONS>
        Nested classes can be forward-declared only within the
        definition of the enclosing class. Thus, any header file
        manipulating a <code>Foo::Bar*</code> pointer will have to
        include the full class declaration for <code>Foo</code>.
      </CONS>
      <DECISION>
        Do not make nested classes public unless they are actually
        part of the interface, e.g., a class that holds a set of
        options for some method.
        
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Nonmember, Static Member, and Global Functions">
    <SUMMARY>
      Prefer nonmember functions within a namespace or static member
      functions to global functions; use completely global functions
      rarely.
    </SUMMARY>
    <BODY>
      <PROS>
        Nonmember and static member functions can be useful in some
        situations.  Putting nonmember functions in a namespace avoids
        polluting the global namespace.
      </PROS>
      <CONS>
        Nonmember and static member functions may make more sense as
        members of a new class, especially if they access external
        resources or have significant dependencies.
      </CONS>
      <DECISION>
        <p>
          Sometimes it is useful, or even necessary, to define a
          function not bound to a class instance.  Such a function can
          be either a static member or a nonmember function.
          Nonmember functions should not depend on external variables,
          and should nearly always exist in a namespace.  Rather than
          creating classes only to group static member functions which
          do not share static data, use
          <a href="#Namespaces">namespaces</a> instead.
        </p>
        <p>
          Functions defined in the same compilation unit as production
          classes may introduce unnecessary coupling and link-time
          dependencies when directly called from other compilation
          units; static member functions are particularly susceptible
          to this.  Consider extracting a new class, or placing the
          functions in a namespace possibly in a separate library.
        </p>
        <p>
          If you must define a nonmember function and it is only
          needed in its <code>.cc</code> file, use an unnamed
          <a HREF="#Namespaces">namespace</a> or <code>static</code>
          linkage (eg <code>static int Foo() {...}</code>) to limit
          its scope.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Local Variables">
    <SUMMARY>
      Place a function's variables in the narrowest scope possible,
      and initialize variables in the declaration.
    </SUMMARY>
    <BODY>
      <p>
        C++ allows you to declare variables anywhere in a function.
        We encourage you to declare them in as local a scope as
        possible, and as close to the first use as possible. This
        makes it easier for the reader to find the declaration and see
        what type the variable is and what it was initialized to.  In
        particular, initialization should be used instead of
        declaration and assignment, e.g.
      </p>
      <BAD_CODE_SNIPPET>
        int i;
        i = f();      // Bad -- initialization separate from declaration.
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        int j = g();  // Good -- declaration has initialization.
      </CODE_SNIPPET>
      <p>
        Note that gcc implements <code>for (int i = 0; i
        &lt; 10; ++i)</code> correctly (the scope of <code>i</code> is
        only the scope of the <code>for</code> loop), so you can then
        reuse <code>i</code> in another <code>for</code> loop in the
        same scope.  It also correctly scopes declarations in
        <code>if</code> and <code>while</code> statements, e.g.
      </p>
      <CODE_SNIPPET>
        while (const char* p = strchr(str, '/')) str = p + 1;
      </CODE_SNIPPET>
      <p>
        There is one caveat:  if the variable is an object, its
        constructor is invoked every time it enters scope and is
        created, and its destructor is invoked every time it goes
        out of scope.
      </p>
      <BAD_CODE_SNIPPET>
        // Inefficient implementation:
        for (int i = 0; i &lt; 1000000; ++i) {
          Foo f;  // My ctor and dtor get called 1000000 times each.
          f.DoSomething(i);
        }
      </BAD_CODE_SNIPPET>
      <p>
        It may be more efficient to declare such a variable used in a
        loop outside that loop:
      </p>
      <CODE_SNIPPET>
        Foo f;  // My ctor and dtor get called once each.
        for (int i = 0; i &lt; 1000000; ++i) {
          f.DoSomething(i);
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Static and Global Variables">
    <SUMMARY>
      Static or global variables of class type are forbidden: they cause
      hard-to-find bugs due to indeterminate order of construction and
      destruction.
    </SUMMARY>
    <BODY>
      <p>
        Objects with static storage duration, including global variables,
        static variables, static class member variables, and function static
        variables, must be Plain Old Data (POD): only ints, chars, floats, or
        pointers, or arrays/structs of POD.
      </p>
      <p>
        The order in which class constructors and initializers for
        static variables are called is only partially specified in C++ and can
        even change from build to build, which can cause bugs that are difficult
        to find.  Therefore in addition to banning globals of class type, we do
        not allow static POD variables to be initialized with the result of a
        function, unless that function (such as getenv(), or getpid()) does not
        itself depend on any other globals.
      </p>
      <p>
        Likewise, the order in which destructors are called is defined to be the
        reverse of the order in which the constructors were called.  Since
        constructor order is indeterminate, so is destructor order.
        For example, at program-end time a static variable might have
        been destroyed, but code still running -- perhaps in another thread --
        tries to access it and fails.  Or the destructor for a static 'string'
        variable might be run prior to the destructor for another variable that
        contains a reference to that string.
      </p>
      <p>
        As a result we only allow static variables to contain POD data.  This
        rule completely disallows <code>vector</code> (use C arrays instead), or
        <code>string</code> (use <code>const char []</code>).
      </p>
      
      <p>
        If you need a static or global variable of a class type, consider
        initializing a pointer (which will never be freed), from either your
        main() function or from pthread_once().  Note that this must be a raw
        pointer, not a "smart" pointer, since the smart pointer's destructor
        will have the order-of-destructor issue that we are trying to avoid.
      </p>
      
      
    </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Classes">
  Classes are the fundamental unit of code in C++. Naturally, we use
  them extensively. This section lists the main dos and don'ts you
  should follow when writing a class.

  <STYLEPOINT title="Doing Work in Constructors">
    <SUMMARY>
      In general, constructors should merely set member variables to their
      initial values.  Any complex initialization should go in an explicit
      <code>Init()</code> method.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        It is possible to perform initialization in the body of the
        constructor.
      </DEFINITION>
      <PROS>
        Convenience in typing.  No need to worry about whether the
        class has been initialized or not.
      </PROS>
      <CONS>
        The problems with doing work in constructors are:
        <ul>
          <li> There is no easy way for constructors to signal errors,
               short of using exceptions (which are
               <a HREF="#Exceptions">forbidden</a>).
               </li>
          <li> If the work fails, we now have an object whose
               initialization code failed, so it may be an
               indeterminate state.
               </li>
          <li> If the work calls virtual functions, these calls will
               not get dispatched to the subclass implementations.
               Future modification to your class can quietly introduce
               this problem even if your class is not currently
               subclassed, causing much confusion.
               </li>
          <li> If someone creates a global variable of this type
               (which is against the rules, but still), the
               constructor code will be called before
               <code>main()</code>, possibly breaking some implicit
               assumptions in the constructor code.  For instance,
               
               <a href="http://google-gflags.googlecode.com/">gflags</a>
               will not yet have been initialized.
               </li>
        </ul>
      </CONS>
      <DECISION>
        If your object requires non-trivial initialization, consider
        having an explicit <code>Init()</code> method.  In particular,
        constructors should not call virtual functions, attempt to raise
        errors, access potentially uninitialized global variables, etc.
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Default Constructors">
    <SUMMARY>
      You must define a default constructor if your class defines
      member variables and has no other constructors.  Otherwise the
      compiler will do it for you, badly.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        The default constructor is called when we <code>new</code> a
        class object with no arguments.  It is always called when
        calling <code>new[]</code> (for arrays).
      </DEFINITION>
      <PROS>
        Initializing structures by default, to hold "impossible"
        values, makes debugging much easier.
      </PROS>
      <CONS>
        Extra work for you, the code writer.
      </CONS>
      <DECISION>
        <p>
          If your class defines member variables and has no other
          constructors you must define a default constructor (one that
          takes no arguments). It should preferably initialize the
          object in such a way that its internal state is consistent
          and valid.
        </p>
        <p>
          The reason for this is that if you have no other
          constructors and do not define a default constructor, the
          compiler will generate one for you. This compiler
          generated constructor may not initialize your object
          sensibly.
        </p>
        <p>
          If your class inherits from an existing class but you add no
          new member variables, you are not required to have a default
          constructor.
          
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Explicit Constructors">
    <SUMMARY>
      Use the C++ keyword <code>explicit</code> for constructors with
      one argument.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        Normally, if a constructor takes one argument, it can be used
        as a conversion.  For instance, if you define
        <code>Foo::Foo(string name)</code> and then pass a string to a
        function that expects a <code>Foo</code>, the constructor will
        be called to convert the string into a <code>Foo</code> and
        will pass the <code>Foo</code> to your function for you.  This
        can be convenient but is also a source of trouble when things
        get converted and new objects created without you meaning them
        to.  Declaring a constructor <code>explicit</code> prevents it
        from being invoked implicitly as a conversion.
      </DEFINITION>
      <PROS>
        Avoids undesirable conversions.
      </PROS>
      <CONS>
        None.
      </CONS>
      <DECISION>
        <p>
          We require all single argument constructors to be
          explicit. Always put <code>explicit</code> in front of
          one-argument constructors in the class definition:
          <code>explicit Foo(string name);</code>
        </p>
        <p>
          The exception is copy constructors, which, in the rare
          cases when we allow them, should probably not be
          <code>explicit</code>.
          
          Classes that are intended to be
          transparent wrappers around other classes are also
          exceptions.
          Such exceptions should be clearly marked with comments.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Copy Constructors">
    <SUMMARY>
      Provide a copy constructor and assignment operator only when necessary.
      Otherwise, disable them with <code>DISALLOW_COPY_AND_ASSIGN</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        The copy constructor and assignment operator are used to create copies
        of objects. The copy constructor is implicitly invoked by the
        compiler in some situations, e.g. passing objects by value.
      </DEFINITION>
      <PROS>
        Copy constructors make it easy to copy objects.  STL
        containers require that all contents be copyable and
        assignable. Copy constructors can be more efficient than
        <code>CopyFrom()</code>-style workarounds because they combine
        construction with copying, the compiler can elide them in some
        contexts, and they make it easier to avoid heap allocation.
      </PROS>
      <CONS>
        Implicit copying of objects in C++ is a rich source of bugs
        and of performance problems. It also reduces readability, as
        it becomes hard to track which objects are being passed around
        by value as opposed to by reference, and therefore where
        changes to an object are reflected.
      </CONS>
      <DECISION>
        <p>
          Few classes need to be copyable. Most should have neither a
          copy constructor nor an assignment operator. In many situations,
          a pointer or reference will work just as well as a copied value,
          with better performance. For example, you can pass function
          parameters by reference or pointer instead of by value, and you can
          store pointers rather than objects in an STL container.
        </p>
        <p>
          If your class needs to be copyable, prefer providing a copy method,
          such as <code>CopyFrom()</code> or <code>Clone()</code>, rather than
          a copy constructor, because such methods cannot be invoked
          implicitly. If a copy method is insufficient in your situation
          (e.g. for performance reasons, or because your class needs to be
          stored by value in an STL container), provide both a copy
          constructor and assignment operator.
        </p>
        <p>
          If your class does not need a copy constructor or assignment
          operator, you must explicitly disable them.
          
          
          To do so, add dummy declarations for the copy constructor and
          assignment operator in the <code>private:</code> section of your
          class, but do not provide any corresponding definition (so that
          any attempt to use them results in a link error). 
        </p>
        <p>
          For convenience, a <code>DISALLOW_COPY_AND_ASSIGN</code> macro
          can be used:
        </p>
        <CODE_SNIPPET>
          // A macro to disallow the copy constructor and operator= functions
          // This should be used in the private: declarations for a class
          #define DISALLOW_COPY_AND_ASSIGN(TypeName) \
            TypeName(const TypeName&amp;);               \
            void operator=(const TypeName&amp;)
        </CODE_SNIPPET>
        <p>
          Then, in <code>class Foo</code>:
        </p>
        <CODE_SNIPPET>
          class Foo {
           public:
            Foo(int f);
            ~Foo();

           private:
            DISALLOW_COPY_AND_ASSIGN(Foo);
          };
        </CODE_SNIPPET>
        <p>
        </p>
        
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Structs vs. Classes">
    <SUMMARY>
      Use a <code>struct</code> only for passive objects that carry data;
      everything else is a <code>class</code>.
    </SUMMARY>
    <BODY>
      <p>
        The <code>struct</code> and <code>class</code> keywords behave
        almost identically in C++.  We add our own semantic meanings
        to each keyword, so you should use the appropriate keyword for
        the data-type you're defining.
      </p>
      <p>
        <code>structs</code> should be used for passive objects that carry
        data, and may have associated constants, but lack any functionality
        other than access/setting the data members. The
        accessing/setting of fields is done by directly accessing the
        fields rather than through method invocations. Methods should
        not provide behavior but should only be used to set up the
        data members, e.g., constructor, destructor,
        <code>Initialize()</code>, <code>Reset()</code>,
        <code>Validate()</code>.
      </p>
      <p>
        If more functionality is required, a <code>class</code> is more
        appropriate.  If in doubt, make it a <code>class</code>.
      </p>
      <p>
        For consistency with STL, you can use <code>struct</code>
        instead of <code>class</code> for functors and traits.
      </p>
      <p>
        Note that member variables in structs and classes have
        <a HREF="#Variable_Names">different naming rules</a>.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Inheritance">
    <SUMMARY>
      Composition is often more appropriate than inheritance.  When
      using inheritance, make it <code>public</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        When a sub-class inherits from a base class, it includes the
        definitions of all the data and operations that the parent
        base class defines.  In practice, inheritance is used in two
        major ways in C++: implementation inheritance, in which
        actual code is inherited by the child, and <A HREF="#Interfaces">interface inheritance</A>, in which only
        method names are inherited.
      </DEFINITION>
      <PROS>
        Implementation inheritance reduces code size by re-using the
        base class code as it specializes an existing type.  Because
        inheritance is a compile-time declaration, you and the
        compiler can understand the operation and detect errors.
        Interface inheritance can be used to programmatically enforce
        that a class expose a particular API.  Again, the compiler
        can detect errors, in this case, when a class does not define
        a necessary method of the API.
      </PROS>
      <CONS>
        For implementation inheritance, because the code implementing
        a sub-class is spread between the base and the sub-class, it
        can be more difficult to understand an implementation.  The
        sub-class cannot override functions that are not virtual, so
        the sub-class cannot change implementation.  The base class
        may also define some data members, so that specifies physical
        layout of the base class.
      </CONS>
      <DECISION>
        <p>
          All inheritance should be <code>public</code>.  If you want to
          do private inheritance, you should be including an instance of
          the base class as a member instead.
        </p>
        <p>
          Do not overuse implementation inheritance.  Composition is
          often more appropriate. Try to restrict use of inheritance
          to the "is-a" case: <code>Bar</code> subclasses
          <code>Foo</code> if it can reasonably be said that
          <code>Bar</code> "is a kind of" <code>Foo</code>.
        </p>
        <p>
          Make your destructor <code>virtual</code> if necessary. If
          your class has virtual methods, its destructor
          
          should be virtual.
        </p>
        <p>
          Limit the use of <code>protected</code> to those member
          functions that might need to be accessed from subclasses.
          Note that <a href="#Access_Control">data members should
          be private</a>.
        </p>
        <p>
          When redefining an inherited virtual function, explicitly
          declare it <code>virtual</code> in the declaration of the
          derived class.  Rationale: If <code>virtual</code> is
          omitted, the reader has to check all ancestors of the
          class in question to determine if the function is virtual
          or not.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Multiple Inheritance">
    <SUMMARY>
      Only very rarely is multiple implementation inheritance actually
      useful.  We allow multiple inheritance only when at most one of
      the base classes has an implementation; all other base classes
      must be <A HREF="#Interfaces">pure interface</A> classes tagged
      with the <code>Interface</code> suffix.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        Multiple inheritance allows a sub-class to have more than one
        base class.  We distinguish between base classes that are
        <em>pure interfaces</em> and those that have an
        <em>implementation</em>.
      </DEFINITION>
      <PROS>
        Multiple implementation inheritance may let you re-use even more code
        than single inheritance (see <a HREF="#Inheritance">Inheritance</a>).
      </PROS>
      <CONS>
        Only very rarely is multiple <em>implementation</em>
        inheritance actually useful. When multiple implementation
        inheritance seems like the solution, you can usually find a
        different, more explicit, and cleaner solution.
      </CONS>
      <DECISION>
        Multiple inheritance is allowed only when all superclasses, with the
        possible exception of the first one, are <A HREF="#Interfaces">pure
        interfaces</A>.  In order to ensure that they remain pure interfaces,
        they must end with the <code>Interface</code> suffix.
        <SUBSECTION title="Note:">
          There is an <a HREF="#Windows_Code">exception</a> to this
          rule on Windows.
        </SUBSECTION>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Interfaces">
    <SUMMARY>
      Classes that satisfy certain conditions are allowed, but not required, to
      end with an <code>Interface</code> suffix.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        <p>
        A class is a pure interface if it meets the following requirements:
        </p>
        <ul>
          <li> It has only public pure virtual ("<code>= 0</code>") methods
               and static methods (but see below for destructor).
               </li>
          <li> It may not have non-static data members.
               </li>
          <li> It need not have any constructors defined.  If a constructor is
               provided, it must take no arguments and it must be protected.
               </li>
          <li> If it is a subclass, it may only be derived from classes
               that satisfy these conditions and are tagged with the
               <code>Interface</code> suffix.
               </li>
        </ul>
        <p>
          An interface class can never be directly instantiated
          because of the pure virtual method(s) it declares.  To make
          sure all implementations of the interface can be destroyed
          correctly, the interface must also declare a virtual destructor (in
          an exception to the first rule, this should not be pure).  See
          Stroustrup, <cite>The C++ Programming Language</cite>, 3rd
          edition, section 12.4 for details.
        </p>
      </DEFINITION>
      <PROS>
        Tagging a class with the <code>Interface</code> suffix lets
        others know that they must not add implemented methods or non
        static data members.  This is particularly important in the case of
        <A HREF="#Multiple_Inheritance">multiple inheritance</A>.
        Additionally, the interface concept is already well-understood by
        Java programmers.
      </PROS>
      <CONS>
        The <code>Interface</code> suffix lengthens the class name, which
        can make it harder to read and understand.  Also, the interface
        property may be considered an implementation detail that shouldn't
        be exposed to clients.
      </CONS>
      <DECISION>
        A class may end with <code>Interface</code> only if it meets the
        above requirements.  We do not require the converse, however:
        classes that meet the above requirements are not required to end
        with <code>Interface</code>.
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Operator Overloading">
    <SUMMARY>
      Do not overload operators except in rare, special circumstances.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        A class can define that operators such as <code>+</code> and
        <code>/</code> operate on the class as if it were a built-in
        type.
      </DEFINITION>
      <PROS>
        Can make code appear more intuitive because a class will
        behave in the same way as built-in types (such as
        <code>int</code>).  Overloaded operators are more playful
        names for functions that are less-colorfully named, such as
        <code>Equals()</code> or <code>Add()</code>.  For some
        template functions to work correctly, you may need to define
        operators.
      </PROS>
      <CONS>
        While operator overloading can make code more intuitive, it
        has several drawbacks:
        <ul>
          <li> It can fool our intuition into thinking that expensive
               operations are cheap, built-in operations.
               </li>
          <li> It is much harder to find the call sites for overloaded
               operators.  Searching for <code>Equals()</code> is much
               easier than searching for relevant invocations of
               <code>==</code>.
               </li>
          <li> Some operators work on pointers too, making it easy to
               introduce bugs.  <code>Foo + 4</code> may do one thing,
               while <code>&amp;Foo + 4</code> does something totally
               different. The compiler does not complain for either of
               these, making this very hard to debug.
               </li>
        </ul>
        Overloading also has surprising ramifications.  For instance,
        if a class overloads unary <code>operator&amp;</code>, it
        cannot safely be forward-declared.
      </CONS>
      <DECISION>
        <p>
          In general, do not overload operators. The assignment operator
          (<code>operator=</code>), in particular, is insidious and
          should be avoided.  You can define functions like
          <code>Equals()</code> and <code>CopyFrom()</code> if you
          need them.  Likewise, avoid the dangerous
          unary <code>operator&amp;</code> at all costs, if there's
          any possibility the class might be forward-declared.
        </p>
        <p>
          However, there may be rare cases where you need to overload
          an operator to interoperate with templates or "standard" C++
          classes (such as <code>operator&lt;&lt;(ostream&amp;, const
          T&amp;)</code> for logging).  These are acceptable if fully
          justified, but you should try to avoid these whenever
          possible.  In particular, do not overload <code>operator==</code>
          or <code>operator&lt;</code> just so that your class can be
          used as a key in an STL container; instead, you should
          create equality and comparison functor types when declaring
          the container.
        </p>
        <p>
          Some of the STL algorithms do require you to overload
          <code>operator==</code>, and you may do so in these cases,
          provided you document why.
        </p>
        <p>
          See also <a HREF="#Copy_Constructors">Copy Constructors</a>
          and <a HREF="#Function_Overloading">Function
          Overloading</a>.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Access Control">
    <SUMMARY>
      Make data members <code>private</code>, and provide
      access to them through accessor functions as needed (for
      technical reasons, we allow data members of a test fixture class
      to be <code>protected</code> when using
      
      <A HREF="http://code.google.com/p/googletest/">
      Google Test</A>). Typically a variable would be
      called <code>foo_</code> and the accessor function
      <code>foo()</code>.  You may also want a mutator function
      <code>set_foo()</code>.
      Exception: <code>static const</code> data members (typically
      called <code>kFoo</code>) need not be <code>private</code>.
    </SUMMARY>
    <BODY>
      <p>
        The definitions of accessors are usually inlined in the header
        file.
      </p>
      <p>
        See also <a HREF="#Inheritance">Inheritance</a> and <a HREF="#Function_Names">Function Names</a>.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Declaration Order">
    <SUMMARY>
      Use the specified order of declarations within a class:
      <code>public:</code> before <code>private:</code>, methods
      before data members (variables), etc.
    </SUMMARY>
    <BODY>
      <p>
        Your class definition should start with its <code>public:</code>
        section, followed by its <code>protected:</code> section and
        then its <code>private:</code> section. If any of these sections
        are empty, omit them.
      </p>
      <p>
        Within each section, the declarations generally should be in
        the following order:
      </p>
      <ul>
        <li> Typedefs and Enums</li>
        <li> Constants (<code>static const</code> data members)</li>
        <li> Constructors</li>
        <li> Destructor</li>
        <li> Methods, including static methods</li>
        <li> Data Members (except <code>static const</code> data members)</li>
      </ul>
      <p>
        Friend declarations should always be in the private section, and
        the <code>DISALLOW_COPY_AND_ASSIGN</code> macro invocation
        should be at the end of the <code>private:</code> section. It
        should be the last thing in the class. See <a HREF="#Copy_Constructors">Copy Constructors</a>.
      </p>
      <p>
        Method definitions in the corresponding <code>.cc</code> file
        should be the same as the declaration order, as much as possible.
      </p>
      <p>
        Do not put large method definitions inline in the class
        definition.  Usually, only trivial or performance-critical,
        and very short, methods may be defined inline.  See <a HREF="#Inline_Functions">Inline Functions</a> for more
        details.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Write Short Functions">
    <SUMMARY>
      Prefer small and focused functions.
    </SUMMARY>
    <BODY>
      <p>
        We recognize that long functions are sometimes appropriate, so
        no hard limit is placed on functions length. If a function
        exceeds about 40 lines, think about whether it can be broken
        up without harming the structure of the program.
      </p>
      <p>
        Even if your long function works perfectly now, someone
        modifying it in a few months may add new behavior. This could
        result in bugs that are hard to find.  Keeping your functions
        short and simple makes it easier for other people to read and
        modify your code.
      </p>
      <p>
        You could find long and complicated functions when working
        with
        
        some
        code.  Do not be intimidated by modifying existing
        code: if working with such a function proves to be difficult,
        you find that errors are hard to debug, or you want to use a
        piece of it in several different contexts, consider breaking
        up the function into smaller and more manageable pieces.
      </p>
    </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Google-Specific Magic">
  
  <p>
    There are various tricks and utilities that we use to make C++
    code more robust, and various ways we use C++ that may differ from
    what you see elsewhere.
  </p>

  
  <STYLEPOINT title="Smart Pointers">
    
    <SUMMARY>
      If you actually need pointer semantics, <code>scoped_ptr</code>
      is great.  You should only use <code>std::tr1::shared_ptr</code>
      with a non-const referent when it is truly necessary to share ownership
      of an object (e.g. inside an STL container). You should never use
      <code>auto_ptr</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        "Smart" pointers are objects that act like pointers, but automate
        management of the underlying memory. 
      </DEFINITION>
      <PROS>
        Smart pointers are extremely useful for preventing memory leaks, and
        are essential for writing exception-safe code. They also formalize
        and document the ownership of dynamically allocated memory.
      </PROS>
      <CONS>
        We prefer designs in which objects have single, fixed owners. Smart
        pointers which enable sharing or transfer of ownership can act as a
        tempting alternative to a careful design of ownership semantics,
        leading to confusing code and even bugs in which memory is never
        deleted. The semantics of smart pointers (especially
        <code>auto_ptr</code>) can be nonobvious and confusing. The
        exception-safety benefits of smart pointers are not decisive, since
        we do not allow exceptions.
      </CONS>
      <DECISION>
        <dl>
          <dt><code>scoped_ptr</code></dt>
          <dd>Straightforward and risk-free. Use wherever appropriate.</dd>
          <dt><code>auto_ptr</code></dt>
          <dd>Confusing and bug-prone ownership-transfer semantics. Do not use.
            </dd>
          <dt><code>shared_ptr</code></dt>
          <dd>
          Safe with const referents (i.e. <code>shared_ptr&lt;const
          T&gt;</code>). Reference-counted pointers with non-const referents
          can occasionally be the best design, but try to rewrite with single
          owners where possible. 
          </dd>
        </dl>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="cpplint">
    <SUMMARY>
      Use
      <code>cpplint.py</code>
      to detect style errors.
    </SUMMARY>
    <BODY>
      <p>
        <code>cpplint.py</code>
        is a tool that reads a source file and
        identifies many style errors.  It is not perfect, and has both false
        positives and false negatives, but it is still a valuable tool.  False
        positives can be ignored by putting <code>// NOLINT</code> at
        the end of the line.
      </p>
      
      <p>
        Some projects have instructions on how to run <code>cpplint.py</code>
        from their project tools. If the project you are contributing to does
        not, you can download <A HREF="http://google-styleguide.googlecode.com/svn/trunk/cpplint/cpplint.py"><code>cpplint.py</code></A> separately.
      </p>
    </BODY>
  </STYLEPOINT>

  
</CATEGORY>

<CATEGORY title="Other C++ Features">
  <STYLEPOINT title="Reference Arguments">
    <SUMMARY>
      All parameters passed by reference must be labeled
      <code>const</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        In C, if a function needs to modify a variable, the
        parameter must use a pointer, eg <code>int foo(int
        *pval)</code>.  In C++, the function can alternatively
        declare a reference parameter: <code>int foo(int
        &amp;val)</code>.
      </DEFINITION>
      <PROS>
        Defining a parameter as reference avoids ugly code like
        <code>(*pval)++</code>.  Necessary for some applications like
        copy constructors.  Makes it clear, unlike with pointers, that
        <code>NULL</code> is not a possible value.
      </PROS>
      <CONS>
        References can be confusing, as they have value syntax but
        pointer semantics.
      </CONS>
      <DECISION>
        <p>
          Within function parameter lists all references must be
          <code>const</code>:
        </p>
        <CODE_SNIPPET>
          void Foo(const string &amp;in, string *out);
        </CODE_SNIPPET>
        <p>
          In fact it is a very strong convention in Google code that input
          arguments are values or <code>const</code> references while
          output arguments are pointers.  Input parameters may be
          <code>const</code> pointers, but we never allow
          non-<code>const</code> reference parameters.
        </p>
        <p>

         However, there are some instances where using <code>const T*</code>
         is preferable to <code>const T&amp;</code> for input parameters. For
         example:

         You want to pass in NULL.

         The function saves a pointer or reference to the input.
         

         Remember that most of the time input parameters are going to be
         specified as <code>const T&amp;</code>.  Using <code>const T*</code>
         instead communicates to the reader that the input is somehow treated
         differently.  So if you choose <code>const T*</code> rather than
         <code>const T&amp;</code>, do so for a concrete reason; otherwise it
         will likely confuse readers by making them look for an explanation
         that doesn't exist.

        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Overloading">
    <SUMMARY>
      Use overloaded functions (including constructors) only if a
      reader looking at a call site can get a good idea of what is
      happening without having to first figure out exactly which
      overload is being called.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        <p>
          You may write a function that takes a
          <code>const string&amp;</code> and overload it with another that
          takes <code>const char*</code>.
        </p>
        <CODE_SNIPPET>
          class MyClass {
           public:
            void Analyze(const string &amp;text);
            void Analyze(const char *text, size_t textlen);
          };
        </CODE_SNIPPET>
      </DEFINITION>
      <PROS>
        Overloading can make code more intuitive by allowing an
        identically-named function to take different arguments.  It
        may be necessary for templatized code, and it can be
        convenient for Visitors.
      </PROS>
      <CONS>
        If a function is overloaded by the argument types alone, a
        reader may have to understand C++'s complex matching rules in
        order to tell what's going on.  Also many people are confused
        by the semantics of inheritance if a derived class overrides
        only some of the variants of a function.
      </CONS>
      <DECISION>
        If you want to overload a function, consider qualifying the
        name with some information about the arguments, e.g.,
        <code>AppendString()</code>, <code>AppendInt()</code> rather
        than just <code>Append()</code>.
        
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Default Arguments">
    <SUMMARY>
      We do not allow default function parameters, except in
      a few uncommon situations explained below.
    </SUMMARY>
    <BODY>
      <PROS>
        Often you have a function that uses lots of default values,
        but occasionally you want to override the defaults.  Default
        parameters allow an easy way to do this without having to
        define many functions for the rare exceptions.
      </PROS>
      <CONS>
        People often figure out how to use an
        API by looking at existing code that uses it.
        Default parameters are more difficult to maintain because
        copy-and-paste from previous code may not reveal all the
        parameters.  Copy-and-pasting of code segments can cause major
        problems when the default arguments are not appropriate for
        the new code.
      </CONS>
      <DECISION>
        <p>
          Except as described below, we require all arguments to be
          explicitly specified, to force programmers to consider the API
          and the values they are passing for each argument rather than
          silently accepting defaults they may not be aware of.
        </p>
        <p>
          One specific exception is when default arguments are used to
          simulate variable-length argument lists.
        </p>
        <CODE_SNIPPET>
          // Support up to 4 params by using a default empty AlphaNum.
          string StrCat(const AlphaNum &amp;a,
                        const AlphaNum &amp;b = gEmptyAlphaNum,
                        const AlphaNum &amp;c = gEmptyAlphaNum,
                        const AlphaNum &amp;d = gEmptyAlphaNum);
        </CODE_SNIPPET>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Variable-Length Arrays and alloca()">
    <SUMMARY>
      We do not allow variable-length arrays or <code>alloca()</code>.
    </SUMMARY>
    <BODY>
      <PROS>
        Variable-length arrays have natural-looking syntax.  Both
        variable-length arrays and <code>alloca()</code> are very
        efficient.
      </PROS>
      <CONS>
        Variable-length arrays and alloca are not part of Standard
        C++.  More importantly, they allocate a data-dependent amount
        of stack space that can trigger difficult-to-find memory
        overwriting bugs: "It ran fine on my machine, but dies
        mysteriously in production".
      </CONS>
      
      <DECISION>
        Use a safe allocator instead, such as
        <code>scoped_ptr</code>/<code>scoped_array</code>.
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Friends">
    <SUMMARY>
      We allow use of <code>friend</code> classes and functions,
      within reason.
    </SUMMARY>
    <BODY>
      <p>
        Friends should usually be defined in the same file so that the
        reader does not have to look in another file to find uses of
        the private members of a class.  A common use of
        <code>friend</code> is to have a <code>FooBuilder</code> class
        be a friend of <code>Foo</code> so that it can construct the
        inner state of <code>Foo</code> correctly, without exposing
        this state to the world.  In some cases it may be useful to
        make a unittest class a friend of the class it tests.
      </p>
      <p>
        Friends extend, but do not break, the encapsulation
        boundary of a class.  In some cases this is better than making
        a member public when you want to give only one other class
        access to it.  However, most classes should interact with
        other classes solely through their public members.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Exceptions">
    <SUMMARY>
      We do not use C++ exceptions.
    </SUMMARY>
    <BODY>
      <PROS>
        <ul>
          <li>Exceptions allow higher levels of an application to
          decide how to handle "can't happen" failures in deeply
          nested functions, without the obscuring and error-prone
          bookkeeping of error codes.</li>

          
          <li>Exceptions are used by most other modern
          languages.  Using them in C++ would make it more consistent with
          Python, Java, and the C++ that others are familiar with.</li>

          <li>Some third-party C++ libraries use exceptions, and turning
          them off internally makes it harder to integrate with those
          libraries.</li>

          <li>Exceptions are the only way for a constructor to fail.
          We can simulate this with a factory function or an
          <code>Init()</code> method, but these require heap
          allocation or a new "invalid" state, respectively.</li>

          <li>Exceptions are really handy in testing frameworks.</li>
        </ul>
      </PROS>
      <CONS>
        <ul>
          <li>When you add a <code>throw</code> statement to an existing
          function, you must examine all of its transitive callers. Either
          they must make at least the basic exception safety guarantee, or
          they must never catch the exception and be happy with the
          program terminating as a result. For instance, if
          <code>f()</code> calls <code>g()</code> calls
          <code>h()</code>, and <code>h</code> throws an exception
          that <code>f</code> catches, <code>g</code> has to be
          careful or it may not clean up properly.</li>

          <li>More generally, exceptions make the control flow of
          programs difficult to evaluate by looking at code: functions
          may return in places you don't expect. This causes
          maintainability and debugging difficulties. You can minimize
          this cost via some rules on how and where exceptions can be
          used, but at the cost of more that a developer needs to know
          and understand.</li>

          <li>Exception safety requires both RAII and different coding
          practices. Lots of supporting machinery is needed to make
          writing correct exception-safe code easy. Further, to avoid
          requiring readers to understand the entire call graph,
          exception-safe code must isolate logic that writes to
          persistent state into a "commit" phase. This will have both
          benefits and costs (perhaps where you're forced to obfuscate
          code to isolate the commit). Allowing exceptions would force
          us to always pay those costs even when they're not worth
          it.</li>

          <li>Turning on exceptions adds data to each binary produced,
          increasing compile time (probably slightly) and possibly
          increasing address space pressure.
          </li>

          <li>The availability of exceptions may encourage developers
          to throw them when they are not appropriate or recover from
          them when it's not safe to do so. For example, invalid user
          input should not cause exceptions to be thrown. We would
          need to make the style guide even longer to document these
          restrictions!</li>
        </ul>
      </CONS>
      <DECISION>
        <p>
          On their face, the benefits of using exceptions outweigh the
          costs, especially in new projects.  However, for existing code,
          the introduction of exceptions has implications on all dependent
          code.  If exceptions can be propagated beyond a new project, it
          also becomes problematic to integrate the new project into
          existing exception-free code.  Because most existing C++ code at
          Google is not prepared to deal with exceptions, it is
          comparatively difficult to adopt new code that generates
          exceptions.
        </p>
        <p>
          Given that Google's existing code is not exception-tolerant, the
          costs of using exceptions are somewhat greater than the costs in
          a new project.  The conversion process would be slow and
          error-prone.  We don't believe that the available alternatives to
          exceptions, such as error codes and assertions, introduce a
          significant burden.
          
        </p>
        <p>
          Our advice against using exceptions is not predicated on
          philosophical or moral grounds, but practical ones.
          
          Because we'd like to use our open-source
          projects at Google and it's difficult to do so if those projects
          use exceptions, we need to advise against exceptions in Google
          open-source projects as well.
          Things would probably be different if we had to do it all over
          again from scratch.
        </p>
        <p>
          There is an <a HREF="#Windows_Code">exception</a> to this
          rule (no pun intended) for Windows code.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Run-Time Type Information (RTTI)">
    <SUMMARY>
      We do not use Run Time Type Information (RTTI).
    </SUMMARY>
    <BODY>
      <DEFINITION>
        RTTI allows a programmer to query the C++ class of an
        object at run time.
      </DEFINITION>
      <PROS>
        <p>
          It is useful in some unittests. For example, it is useful in
          tests of factory classes where the test has to verify that a
          newly created object has the expected dynamic type.
        </p>
        <p>
          In rare circumstances, it is useful even outside of
          tests.
        </p>
      </PROS>
      <CONS>
        A query of type during run-time typically means a
        design problem.  If you need to know the type of an
        object at runtime, that is often an indication that
        you should reconsider the design of your class.
      </CONS>
      <DECISION>
        <p>
          Do not use RTTI, except in unittests.  If you find yourself
          in need of writing code that behaves differently based on
          the class of an object, consider one of the alternatives to
          querying the type.
        </p>
        <p>
          Virtual methods are the preferred way of executing different
          code paths depending on a specific subclass type.  This puts
          the work within the object itself.
        </p>
        <p>
          If the work belongs outside the object and instead in some
          processing code, consider a double-dispatch solution, such
          as the Visitor design pattern.  This allows a facility
          outside the object itself to determine the type of class
          using the built-in type system.
        </p>
        <p>
          If you think you truly cannot use those ideas,
          
          you may use RTTI.  But think twice
          about it.  :-)  Then think twice again.
          Do not hand-implement an RTTI-like workaround. The arguments
          against RTTI apply just as much to workarounds like class
          hierarchies with type tags.
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Casting">
    <SUMMARY>
      Use C++ casts like <code>static_cast&lt;&gt;()</code>.  Do not use
      other cast formats like <code>int y = (int)x;</code> or
      <code>int y = int(x);</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        C++ introduced a different cast system from C that
        distinguishes the types of cast operations.
      </DEFINITION>
      <PROS>
        The problem with C casts is the ambiguity of the operation;
        sometimes you are doing a <em>conversion</em> (e.g.,
        <code>(int)3.5</code>) and sometimes you are doing a
        <em>cast</em> (e.g., <code>(int)"hello"</code>); C++ casts
        avoid this.  Additionally C++ casts are more visible when
        searching for them.
      </PROS>
      <CONS>
        The syntax is nasty.
      </CONS>
      <DECISION>
        <p>
          Do not use C-style casts.  Instead, use these C++-style
          casts.
          
        </p>
        <ul>
          
          <li> Use <code>static_cast</code> as the equivalent of a
               C-style cast that does value conversion, or when you need to explicitly up-cast
               a pointer from a class to its superclass.
               </li>
          <li> Use <code>const_cast</code> to remove the <code>const</code>
               qualifier (see <a HREF="#Use_of_const">const</a>).
               </li>
          
          
          <li> Use <code>reinterpret_cast</code> to do unsafe
               conversions of pointer types to and from integer and
               other pointer types. Use this only if you know what you are
               doing and you understand the aliasing issues.
               
               </li>
          <li> Do not use <code>dynamic_cast</code> except in test code.
               If you need to know type information at runtime in this way
               outside of a unittest, you probably have a <A HREF="#Run-Time_Type_Information__RTTI_">design
               flaw</A>.
               </li>
        </ul>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Streams">
    <SUMMARY>
      Use streams only for logging.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        Streams are a replacement for <code>printf()</code> and
        <code>scanf()</code>.
      </DEFINITION>
      <PROS>
        With streams, you do not need to know the type of the object
        you are printing.  You do not have problems with format
        strings not matching the argument list.  (Though with gcc, you
        do not have that problem with <code>printf</code> either.)  Streams
        have automatic constructors and destructors that open and close the
        relevant files.
      </PROS>
      <CONS>
        Streams make it difficult to do functionality like
        <code>pread()</code>.  Some formatting (particularly the common
        format string idiom <code>%.*s</code>) is difficult if not
        impossible to do efficiently using streams without using
        <code>printf</code>-like hacks.  Streams do not support operator
        reordering (the <code>%1s</code> directive), which is helpful for
        internationalization.
      </CONS>
      <DECISION>
        
        <p>
          Do not use streams, except where required by a logging interface.
          Use <code>printf</code>-like routines instead.
        </p>
        <p>
          There are various pros and cons to using streams, but in
          this case, as in many other cases, consistency trumps the
          debate.  Do not use streams in your code.
        </p>

        <SUBSECTION title="Extended Discussion">
          <p>
            There has been debate on this issue, so this explains the
            reasoning in greater depth.  Recall the Only One Way
            guiding principle: we want to make sure that whenever we
            do a certain type of I/O, the code looks the same in all
            those places.  Because of this, we do not want to allow
            users to decide between using streams or using
            <code>printf</code> plus Read/Write/etc.  Instead, we should
            settle on one or the other.  We made an exception for logging
            because it is a pretty specialized application, and for
            historical reasons.
          </p>
          <p>
            Proponents of streams have argued that streams are the obvious
            choice of the two, but the issue is not actually so clear.  For
            every advantage of streams they point out, there is an
            equivalent disadvantage.  The biggest advantage is that
            you do not need to know the type of the object to be
            printing.  This is a fair point.  But, there is a
            downside: you can easily use the wrong type, and the
            compiler will not warn you.  It is easy to make this
            kind of mistake without knowing when using streams.
          </p>
          <CODE_SNIPPET>
            cout &lt;&lt; this;  // Prints the address
            cout &lt;&lt; *this;  // Prints the contents
          </CODE_SNIPPET>
          <p>
            The compiler does not generate an error because
            <code>&lt;&lt;</code> has been overloaded.  We discourage
            overloading for just this reason.
          </p>
          <p>
            Some say <code>printf</code> formatting is ugly and hard to
            read, but streams are often no better.  Consider the following
            two fragments, both with the same typo.  Which is easier to
            discover?
          </p>
          <CODE_SNIPPET>
             cerr &lt;&lt; "Error connecting to '" &lt;&lt; foo-&gt;bar()-&gt;hostname.first
                  &lt;&lt; ":" &lt;&lt; foo-&gt;bar()-&gt;hostname.second &lt;&lt; ": " &lt;&lt; strerror(errno);

             fprintf(stderr, "Error connecting to '%s:%u: %s",
                     foo-&gt;bar()-&gt;hostname.first, foo-&gt;bar()-&gt;hostname.second,
                     strerror(errno));
          </CODE_SNIPPET>
          <p>
            And so on and so forth for any issue you might bring up.
            (You could argue, "Things would be better with the right
            wrappers," but if it is true for one scheme, is it not
            also true for the other?  Also, remember the goal is to
            make the language smaller, not add yet more machinery that
            someone has to learn.)
          </p>
          <p>
            Either path would yield different advantages and
            disadvantages, and there is not a clearly superior
            solution.  The simplicity doctrine mandates we settle on
            one of them though, and the majority decision was on
            <code>printf</code> + <code>read</code>/<code>write</code>.
          </p>
        </SUBSECTION>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Preincrement and Predecrement">
    <SUMMARY>
      Use prefix form (<code>++i</code>) of the increment and
      decrement operators with iterators and other template objects.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        When a variable is incremented (<code>++i</code> or
        <code>i++</code>) or decremented (<code>--i</code> or
        <code>i--</code>) and the value of the expression is not used,
        one must decide whether to preincrement (decrement) or
        postincrement (decrement).
      </DEFINITION>
      <PROS>
        When the return value is ignored, the "pre" form
        (<code>++i</code>) is never less efficient than the "post"
        form (<code>i++</code>), and is often more efficient.  This is
        because post-increment (or decrement) requires a copy of
        <code>i</code> to be made, which is the value of the
        expression.  If <code>i</code> is an iterator or other
        non-scalar type, copying <code>i</code> could be expensive.
        Since the two types of increment behave the same when the
        value is ignored, why not just always pre-increment?
      </PROS>
      <CONS>
        The tradition developed, in C, of using post-increment when
        the expression value is not used, especially in <code>for</code>
        loops.  Some find post-increment easier to read, since the
        "subject" (<code>i</code>) precedes the "verb" (<code>++</code>),
        just like in English.
      </CONS>
      <DECISION>
        For simple scalar (non-object) values there is no reason to
        prefer one form and we allow either.  For iterators and other
        template types, use pre-increment.
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Use of const">
    <SUMMARY>
      We strongly recommend that you use <code>const</code> whenever
      it makes sense to do so.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        Declared variables and parameters can be preceded by the
        keyword <code>const</code> to indicate the variables are not
        changed (e.g., <code>const int foo</code>).  Class functions
        can have the <code>const</code> qualifier to indicate the
        function does not change the state of the class member
        variables (e.g., <code>class Foo { int Bar(char c) const;
        };</code>).
      </DEFINITION>
      <PROS>
        Easier for people to understand how variables are being used.
        Allows the compiler to do better type checking, and,
        conceivably, generate better code.  Helps people convince
        themselves of program correctness because they know the
        functions they call are limited in how they can modify your
        variables.  Helps people know what functions are safe to use
        without locks in multi-threaded programs.
      </PROS>
      <CONS>
        <code>const</code> is viral: if you pass a <code>const</code>
        variable to a function, that function must have <code>const</code>
        in its prototype (or the variable will need a
        <code>const_cast</code>).  This can be a particular problem
        when calling library functions.
      </CONS>
      <DECISION>
        <p>
          <code>const</code> variables, data members, methods and
          arguments add a level of compile-time type checking; it
          is better to detect errors as soon as possible.
          Therefore we strongly recommend that you use
          <code>const</code> whenever it makes sense to do so:
        </p>
        <ul>
          <li> If a function does not modify an argument passed by
               reference or by pointer, that argument should be
               <code>const</code>.
               </li>
          <li> Declare methods to be <code>const</code> whenever
               possible. Accessors should almost always be
               <code>const</code>. Other methods should be const if they do
               not modify any data members, do not call any
               non-<code>const</code> methods, and do not return a
               non-<code>const</code> pointer or non-<code>const</code>
               reference to a data member.
               </li>
          <li> Consider making data members <code>const</code>
               whenever they do not need to be modified after
               construction.
               </li>
        </ul>
        <p>
          However, do not go crazy with <code>const</code>.  Something like
          <code>const int * const * const x;</code> is likely
          overkill, even if it accurately describes how const x is.
          Focus on what's really useful to know: in this case,
          <code>const int** x</code> is probably sufficient.
        </p>
        <p>
          The <code>mutable</code> keyword is allowed but is unsafe
          when used with threads, so thread safety should be carefully
          considered first.
        </p>
      </DECISION>
      <SUBSECTION title="Where to put the const">
        <p>
          Some people favor the form <code>int const *foo</code> to
          <code>const int* foo</code>.  They argue that this is more
          readable because it's more consistent: it keeps the rule
          that <code>const</code> always follows the object it's
          describing.  However, this consistency argument doesn't
          apply in this case, because the "don't go crazy" dictum
          eliminates most of the uses you'd have to be consistent with.
          
          Putting the <code>const</code> first is arguably more readable,
          since it follows English in putting the "adjective"
          (<code>const</code>) before the "noun" (<code>int</code>).
        </p>
        <p>
          That said, while we encourage putting <code>const</code> first,
          we do not require it.  But be consistent with the code around
          you!
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Integer Types">
    <SUMMARY>
      Of the built-in C++ integer types, the only one used
      
      is <code>int</code>.  If a program needs a variable of a different
      size, use
      
      a precise-width integer type from
      <code>&lt;stdint.h&gt;</code>, such as <code>int16_t</code>.
    </SUMMARY>
    <BODY>
      <DEFINITION>
        C++ does not specify the sizes of its integer types. Typically
        people assume that <code>short</code> is 16 bits,
        <code>int</code> is 32 bits, <code>long</code> is 32 bits and
        <code>long long</code> is 64 bits.
      </DEFINITION>
      <PROS>
        Uniformity of declaration.
      </PROS>
      <CONS>
        The sizes of integral types in C++ can vary based on compiler
        and architecture.
      </CONS>
      <DECISION>
        <p>
          
          <code>&lt;stdint.h&gt;</code> defines
          types like <code>int16_t</code>, <code>uint32_t</code>,
          <code>int64_t</code>, etc.
          You should always use those in preference to
          <code>short</code>, <code>unsigned long long</code> and the
          like, when you need a guarantee on the size of an integer.
          Of the C integer types, only <code>int</code> should be
          used.  When appropriate, you are welcome to use standard
          types like <code>size_t</code> and <code>ptrdiff_t</code>.
        </p>
        <p>
          We use <code>int</code> very often, for integers we know are not
          going to be too big, e.g., loop counters. Use plain old
          <code>int</code> for such things. You should assume that an
          <code>int</code> is
          
          at least 32 bits,
          but don't assume that it has more than 32 bits.
          If you need a 64-bit integer type, use
          <code>int64_t</code> or
          <code>uint64_t</code>.
        </p>
        <p>
          For integers we know can be "big",
           use
          <code>int64_t</code>.
          
        </p>
        <p>
          You should not use the unsigned integer types such as
          <code>uint32_t</code>,
          unless the quantity you are representing is really a bit pattern
          rather than a number, or unless you need defined
          twos-complement overflow. In particular, do not use unsigned
          types to say a number will never be negative.  Instead, use
          
          assertions for this.
        </p>
        
      </DECISION>

      <SUBSECTION title="On Unsigned Integers">
        <p>
          Some people, including some textbook authors, recommend
          using unsigned types to represent numbers that are never
          negative.  This is intended as a form of self-documentation.
          However, in C, the advantages of such documentation are
          outweighed by the real bugs it can introduce.  Consider:
        </p>
        <CODE_SNIPPET>
          for (unsigned int i = foo.Length()-1; i &gt;= 0; --i) ...
        </CODE_SNIPPET>
        <p>
          This code will never terminate!  Sometimes gcc will notice
          this bug and warn you, but often it will not.  Equally bad
          bugs can occur when comparing signed and unsigned
          variables.  Basically, C's type-promotion scheme causes
          unsigned types to behave differently than one might expect.
        </p>
        <p>
          So, document that a variable is non-negative using
          assertions.
          Don't use an unsigned type.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="64-bit Portability">
    <SUMMARY>
      Code should be 64-bit and 32-bit friendly.  Bear in mind problems of
      printing, comparisons, and structure alignment.
    </SUMMARY>
    <BODY>
      <ul>
        <li>
          <p>
            <code>printf()</code> specifiers for some types are
            not cleanly portable between 32-bit and 64-bit
            systems. C99 defines some portable format
            specifiers. Unfortunately, MSVC 7.1 does not
            understand some of these specifiers and the
            standard is missing a few, so we have to define our
            own ugly versions in some cases (in the style of the
            standard include file <code>inttypes.h</code>):
          </p>
          <CODE_SNIPPET>
            // printf macros for size_t, in the style of inttypes.h
            #ifdef _LP64
            #define __PRIS_PREFIX "z"
            #else
            #define __PRIS_PREFIX
            #endif

            // Use these macros after a % in a printf format string
            // to get correct 32/64 bit behavior, like this:
            // size_t size = records.size();
            // printf("%"PRIuS"\n", size);

            #define PRIdS __PRIS_PREFIX "d"
            #define PRIxS __PRIS_PREFIX "x"
            #define PRIuS __PRIS_PREFIX "u"
            #define PRIXS __PRIS_PREFIX "X"
            #define PRIoS __PRIS_PREFIX "o"
          </CODE_SNIPPET>
          <table border="1" summary="portable printf specifiers">
            <TBODY>
              <tr align="center">
                <th>Type</th>
                <th>DO NOT use</th>
                <th>DO use</th>
                <th>Notes</th>
              </tr>
              <tr align="center">
                <td><code>void *</code> (or any pointer)</td>
                <td><code>%lx</code></td>
                <td><code>%p</code></td>
                <td> </td>
              </tr>
              
              <tr align="center">
                <td><code>int64_t</code></td>
                <td><code>%qd</code>,
                     <code>%lld</code></td>
                <td><code>%"PRId64"</code></td>
                <td/>
              </tr>
              
              <tr align="center">
                <td><code>uint64_t</code></td>
                <td><code>%qu</code>,
                    <code>%llu</code>,
                    <code>%llx</code></td>
                <td><code>%"PRIu64"</code>,
                    <code>%"PRIx64"</code></td>
                <td/>
              </tr>
              
              <tr align="center">
                <td><code>size_t</code></td>
                <td><code>%u</code></td>
                <td><code>%"PRIuS"</code>,
                    <code>%"PRIxS"</code></td>
                <td>C99 specifies <code>%zu</code></td>
              </tr>
              <tr align="center">
                <td><code>ptrdiff_t</code></td>
                <td><code>%d</code></td>
                <td><code>%"PRIdS"</code></td>
                <td>C99 specifies <code>%zd</code></td>
              </tr>
              
            </TBODY>
          </table>
          <p>
            Note that the <code>PRI*</code> macros expand to independent
            strings which are concatenated by the compiler. Hence
            if you are using a non-constant format string, you
            need to insert the value of the macro into the format,
            rather than the name. It is still possible, as usual,
            to include length specifiers, etc., after the
            <code>%</code> when using the <code>PRI*</code>
            macros. So, e.g.  <code>printf("x = %30"PRIuS"\n",
            x)</code> would expand on 32-bit Linux to
            <code>printf("x = %30" "u" "\n", x)</code>, which the
            compiler will treat as <code>printf("x = %30u\n",
            x)</code>.
          </p>
          
          </li>

        <li> Remember that <code>sizeof(void *)</code> !=
             <code>sizeof(int)</code>.  Use <code>intptr_t</code> if
             you want a pointer-sized integer.
             </li>

        <li> You may need to be careful with structure alignments,
             particularly for structures being stored on disk. Any
             class/structure with a
             
             <code>int64_t</code>/<code>uint64_t</code>
             member will by default end up being 8-byte aligned on a 64-bit
             system. If you have such structures being shared on disk
             between 32-bit and 64-bit code, you will need to ensure
             that they are packed the same on both architectures.
             
             Most compilers offer a way to alter
             structure alignment.  For gcc, you can use
             <code>__attribute__((packed))</code>.  MSVC offers
             <code>#pragma pack()</code> and
             <code>__declspec(align())</code>.
             </li>

        <li>
             
             Use the <code>LL</code> or <code>ULL</code> suffixes as
             needed to create 64-bit constants.  For example:
             
             <CODE_SNIPPET>
             int64_t my_value = 0x123456789LL;
             uint64_t my_mask = 3ULL &lt;&lt; 48;
             </CODE_SNIPPET>
             </li>

        <li> If you really need different code on 32-bit and 64-bit
             systems, use <code>#ifdef _LP64</code> to choose between
             the code variants. (But please avoid this if
             possible, and keep any such changes localized.)
             </li>
      </ul>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Preprocessor Macros">
    <SUMMARY>
      Be very cautious with macros.  Prefer inline functions, enums,
      and <code>const</code> variables to macros.
    </SUMMARY>
    <BODY>
      <p>
        Macros mean that the code you see is not the same as the code
        the compiler sees.  This can introduce unexpected behavior,
        especially since macros have global scope.
      </p>
      <p>
        Luckily, macros are not nearly as necessary in C++ as they are
        in C.  Instead of using a macro to inline performance-critical
        code, use an inline function.  Instead of using a macro to
        store a constant, use a <code>const</code> variable.  Instead of
        using a macro to "abbreviate" a long variable name, use a
        reference.  Instead of using a macro to conditionally compile code
        ... well, don't do that at all (except, of course, for the
        <code>#define</code> guards to prevent double inclusion of
        header files).  It makes testing much more difficult.
      </p>
      <p>
        Macros can do things these other techniques cannot, and you do
        see them in the codebase, especially in the lower-level
        libraries.  And some of their special features (like
        stringifying, concatenation, and so forth) are not available
        through the language proper.  But before using a macro,
        consider carefully whether there's a non-macro way to achieve
        the same result.
      </p>
      <p>
        The following usage pattern will avoid many problems with
        macros; if you use macros, follow it whenever possible:
      </p>
      <ul>
        <li> Don't define macros in a <code>.h</code> file.
             </li>
        <li> <code>#define</code> macros right before you use them,
             and <code>#undef</code> them right after.
             </li>
        <li> Do not just <code>#undef</code> an existing macro before
             replacing it with your own; instead, pick a name that's
             likely to be unique.
             </li>
        <li> Try not to use macros that expand to unbalanced C++
             constructs, or at least document that behavior well.
             </li>
        <li> Prefer not using <code>##</code> to generate function/class/variable
             names.
             </li>
      </ul>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="0 and NULL">
  <SUMMARY>
    Use <code>0</code> for integers, <code>0.0</code> for reals,
    <code>NULL</code> for pointers, and <code>'\0'</code> for chars.
  </SUMMARY>
  <BODY>
    <p>
      Use <code>0</code> for integers and <code>0.0</code> for reals.
      This is not controversial.
    </p>
    <p>
      For pointers (address values), there is a choice between <code>0</code>
      and <code>NULL</code>.  Bjarne Stroustrup prefers an unadorned
      <code>0</code>.  We prefer <code>NULL</code> because it looks like a
      pointer.  In fact, some C++ compilers, such as gcc 4.1.0, provide special
      definitions of <code>NULL</code> which enable them to give useful
      warnings, particularly in situations where <code>sizeof(NULL)</code>
      is not equal to <code>sizeof(0)</code>.
    </p>
    <p>
      Use <code>'\0'</code> for chars.
      This is the correct type and also makes code more readable.
    </p>
  </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="sizeof">
  <SUMMARY>
    Use <code>sizeof(<var>varname</var>)</code> instead of
    <code>sizeof(<var>type</var>)</code> whenever possible.
  </SUMMARY>
  <BODY>
    <p>
      Use <code>sizeof(<var>varname</var>)</code> because it will update
      appropriately if the type of the variable changes.
      <code>sizeof(<var>type</var>)</code> may make sense in some cases,
      but should generally be avoided because it can fall out of sync if
      the variable's type changes.
    </p>
    <p>
      <CODE_SNIPPET>
        Struct data;
        memset(&amp;data, 0, sizeof(data));
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        memset(&amp;data, 0, sizeof(Struct));
      </BAD_CODE_SNIPPET>
    </p>
  </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Boost">
  <SUMMARY>
    Use only approved libraries from the Boost library collection.
  </SUMMARY>
  <BODY>
    <DEFINITION>
      The <a href="http://www.boost.org/">Boost library collection</a> is
      a popular collection of peer-reviewed, free, open-source C++ libraries.
    </DEFINITION>
    <PROS>
      Boost code is generally very high-quality, is widely portable, and fills
      many important gaps in the C++ standard library, such as type traits,
      better binders, and better smart pointers. It also provides an
      implementation of the TR1 extension to the standard library.
    </PROS>
    <CONS>
      Some Boost libraries encourage coding practices which can hamper
      readability, such as metaprogramming and other advanced template
      techniques, and an excessively "functional" style of programming.
      
    </CONS>
    <DECISION>
      
      <div>
        
        In order to maintain a high level of readability for all contributors
        who might read and maintain code, we only allow an approved subset of
        Boost features.  Currently, the following libraries are permitted:
        <ul>
          <li> <a href="http://www.boost.org/libs/utility/call_traits.htm">
               Call Traits</a> from <code>boost/call_traits.hpp</code>
               </li>
          <li> <a href="http://www.boost.org/libs/utility/compressed_pair.htm">
               Compressed Pair</a> from <code>boost/compressed_pair.hpp</code>
               </li>
          <li> <a href="http://www.boost.org/libs/ptr_container/">
               Pointer Container</a> from <code>boost/ptr_container</code> except
               serialization and wrappers for containers not in the C++03
               standard (<code>ptr_circular_buffer.hpp</code> and
               <code>ptr_unordered*</code>)
               </li>
          <li> <a href="http://www.boost.org/libs/array/">
               Array</a> from <code>boost/array.hpp</code>
               </li>
          <li> <a href="http://www.boost.org/libs/graph/">
               The Boost Graph Library (BGL)</a> from <code>boost/graph</code>,
               except serialization (<code>adj_list_serialize.hpp</code>) and
               parallel/distributed algorithms and data structures
               (<code>boost/graph/parallel/*</code> and
               <code>boost/graph/distributed/*</code>).
               </li>
          <li> <a href="http://www.boost.org/libs/property_map/">
               Property Map</a> from <code>boost/property_map</code>, except
               parallel/distributed property maps
               (<code>boost/property_map/parallel/*</code>).
               </li>
          <li> The part of
               <a href="http://www.boost.org/libs/iterator/">
               Iterator</a> that deals with defining iterators:
               <code>boost/iterator/iterator_adaptor.hpp</code>,
               <code>boost/iterator/iterator_facade.hpp</code>, and
               <code>boost/function_output_iterator.hpp</code></li>
        </ul>
        We are actively considering adding other Boost features to the list, so
        this rule may be relaxed in the future.
      </div>
    </DECISION>
  </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="C++11">
  <SUMMARY>
    Use only approved libraries and language extensions from C++11 (formerly
    known as C++0x).
    Currently, none are approved.
  </SUMMARY>
  <BODY>
    <DEFINITION>
      C++11 is the latest ISO C++ standard.
      It contains
      <a href="http://en.wikipedia.org/wiki/C%2B%2B11">significant
      changes</a> both to the language and libraries.
      
    </DEFINITION>
    <PROS>
      C++11 has become the official standard, and eventually will
      be supported by most C++ compilers.  It standardizes some common C++
      extensions that we use already, allows shorthands for some operations,
      and has some performance and safety improvements.
    </PROS>
    <CONS>
      <p>
        The C++11 standard is substantially more complex than its predecessor
        (1,300 pages versus 800 pages), and is
        unfamilar to many developers.  The long-term effects of some
        features on code readability and maintenance are unknown.  We cannot
        predict when its various features will be implemented uniformly by
        tools that may be of interest (gcc, icc, clang, Eclipse, etc.).
      </p>
      <p>
        As with <a href="#Boost">Boost</a>, some C++11 extensions encourage
        coding practices that hamper readability&#8212;for example by removing
        checked redundancy (such as type names) that may be helpful to readers,
        or by encouraging template metaprogramming.  Other extensions
        duplicate functionality available through existing
        mechanisms, which may lead to
        confusion and conversion costs.
      </p>
    </CONS>
    <DECISION>
      Use only C++11 libraries and language features that have been approved
      for use.  Currently, no such features are approved.
      Features will be approved individually as appropriate.
      Avoid writing code that is incompatible with C++11 (even though it
      works in C++03).
      
    </DECISION>
  </BODY>
  </STYLEPOINT>

</CATEGORY>

<CATEGORY title="Naming">
  <p>
    The most important consistency rules are those that govern
    naming. The style of a name immediately informs us what sort of
    thing the named entity is: a type, a variable, a function, a
    constant, a macro, etc., without requiring us to search for the
    declaration of that entity. The pattern-matching engine in our
    brains relies a great deal on these naming rules.
    
  </p>
  <p>
    Naming rules are pretty arbitrary, but
    
    we feel that consistency is more important than individual preferences
    in this area, so regardless of whether you find them sensible or not,
    the rules are the rules.
  </p>

  <STYLEPOINT title="General Naming Rules">
    <SUMMARY>
      Function names, variable names, and filenames should be
      descriptive; eschew abbreviation.  Types and variables should be
      nouns, while functions should be "command" verbs.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="How to Name">
        <p>
          Give as descriptive a name as possible, within reason. Do
          not worry about saving horizontal space as it is far more
          important to make your code immediately understandable by a
          new reader. Examples of well-chosen names:
        </p>
        <CODE_SNIPPET>
          int num_errors;                  // Good.
          int num_completed_connections;   // Good.
        </CODE_SNIPPET>
        <p>
          Poorly-chosen names use ambiguous abbreviations or arbitrary
          characters that do not convey meaning:
        </p>
        <BAD_CODE_SNIPPET>
          int n;                           // Bad - meaningless.
          int nerr;                        // Bad - ambiguous abbreviation.
          int n_comp_conns;                // Bad - ambiguous abbreviation.
        </BAD_CODE_SNIPPET>
        <p>
          Type and variable names should typically be nouns: e.g.,
          <code>FileOpener</code>,
          
          <code>num_errors</code>.
        </p>
        <p>
          Function names should typically be imperative (that is they
          should be commands): e.g., <code>OpenFile()</code>,
          <code>set_num_errors()</code>.  There is an exception for
          accessors, which, described more completely in <a HREF="#Function_Names">Function Names</a>, should be named
          the same as the variable they access.
        </p>
      </SUBSECTION>

      <SUBSECTION title="Abbreviations">
        <p>
          Do not use abbreviations unless they are extremely well
          known outside your project. For example:
        </p>
        <CODE_SNIPPET>
          // Good
          // These show proper names with no abbreviations.
          int num_dns_connections;  // Most people know what "DNS" stands for.
          int price_count_reader;   // OK, price count. Makes sense.
        </CODE_SNIPPET>
        <BAD_CODE_SNIPPET>
          // Bad!
          // Abbreviations can be confusing or ambiguous outside a small group.
          int wgc_connections;  // Only your group knows what this stands for.
          int pc_reader;        // Lots of things can be abbreviated "pc".
        </BAD_CODE_SNIPPET>
        <p>
          Never abbreviate by leaving out letters:
        </p>
        <CODE_SNIPPET>
          int error_count;  // Good.
        </CODE_SNIPPET>
        <BAD_CODE_SNIPPET>
          int error_cnt;    // Bad.
        </BAD_CODE_SNIPPET>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="File Names">
    <SUMMARY>
      Filenames should be all lowercase and can include underscores
      (<code>_</code>) or dashes (<code>-</code>).  Follow the
      convention that your
      
      project
      uses. If there is no consistent local pattern to follow, prefer "_".
    </SUMMARY>
    <BODY>
      <p>
        Examples of acceptable file names:
      </p>
      <p>
        <code>
          my_useful_class.cc<br/>
          my-useful-class.cc<br/>
          myusefulclass.cc<br/>
          myusefulclass_test.cc  // _unittest and _regtest are deprecated.<br/>
        </code>
      </p>
      <p>
        C++ files should end in <code>.cc</code> and header files
        should end in <code>.h</code>.
      </p>
      <p>
        Do not use filenames that already exist
        in <code>/usr/include</code>, such as <code>db.h</code>.
      </p>
      <p>
        In general, make your filenames very specific.  For example,
        use <code>http_server_logs.h</code> rather
        than <code>logs.h</code>.  A very common case is to have a
        pair of files called, e.g., <code>foo_bar.h</code>
        and <code>foo_bar.cc</code>, defining a class
        called <code>FooBar</code>.
      </p>
      <p>
        Inline functions must be in a <code>.h</code> file. If your
        inline functions are very short, they should go directly into your
        <code>.h</code> file. However, if your inline functions
        include a lot of code, they may go into a third file that
        ends in <code>-inl.h</code>.  In a class with a lot of inline
        code, your class could have three files:
      </p>
      <CODE_SNIPPET>
        url_table.h      // The class declaration.
        url_table.cc     // The class definition.
        url_table-inl.h  // Inline functions that include lots of code.
      </CODE_SNIPPET>
      <p>
        See also the section <a href="#The_-inl.h_Files">-inl.h Files</a>
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Type Names">
    <SUMMARY>
      Type names start with a capital letter and have a capital
      letter for each new word, with no underscores:
      <code>MyExcitingClass</code>, <code>MyExcitingEnum</code>.
    </SUMMARY>
    <BODY>
      <p>
        The names of all types &#8212; classes, structs, typedefs, and enums
        &#8212; have the same naming convention.  Type names should start
        with a capital letter and have a capital letter for each new
        word.  No underscores.  For example:
      </p>
      <CODE_SNIPPET>
        // classes and structs
        class UrlTable { ...
        class UrlTableTester { ...
        struct UrlTableProperties { ...

        // typedefs
        typedef hash_map&lt;UrlTableProperties *, string&gt; PropertiesMap;

        // enums
        enum UrlTableErrors { ...
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Variable Names">
    <SUMMARY>
      Variable names are all lowercase, with underscores between
      words.  Class member variables have trailing underscores.  For
      instance: <code>my_exciting_local_variable</code>,
      <code>my_exciting_member_variable_</code>.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Common Variable names">
        <p>
          For example:
        </p>
        <CODE_SNIPPET>
          string table_name;  // OK - uses underscore.
          string tablename;   // OK - all lowercase.
        </CODE_SNIPPET>
        <BAD_CODE_SNIPPET>
          string tableName;   // Bad - mixed case.
        </BAD_CODE_SNIPPET>
      </SUBSECTION>

      <SUBSECTION title="Class Data Members">
        <p>
          Data members (also called instance variables or member
          variables) are lowercase with optional underscores like
          regular variable names, but always end with a trailing
          underscore.
        </p>
        <CODE_SNIPPET>
          string table_name_;  // OK - underscore at end.
          string tablename_;   // OK.
        </CODE_SNIPPET>
      </SUBSECTION>

      <SUBSECTION title="Struct Variables">
        <p>
          Data members in structs should be named like regular
          variables without the trailing underscores that data members
          in classes have.
        </p>
        <CODE_SNIPPET>
          struct UrlTableProperties {
            string name;
            int num_entries;
          }
        </CODE_SNIPPET>
        <p>
          See <a HREF="#Structs_vs._Classes">Structs vs. Classes</a> for a
          discussion of when to use a struct versus a class.
        </p>
      </SUBSECTION>

      <SUBSECTION title="Global Variables">
        <p>
          There are no special requirements for global variables,
          which should be rare in any case, but if you use one,
          consider prefixing it with <code>g_</code> or some other
          marker to easily distinguish it from local variables.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Constant Names">
    <SUMMARY>
      Use a <code>k</code> followed by mixed case:
      <code>kDaysInAWeek</code>.
    </SUMMARY>
    <BODY>
      <p>
        All compile-time constants, whether they are declared locally,
        globally, or as part of a class, follow a slightly different
        naming convention from other variables. Use a <code>k</code>
        followed by words with uppercase first letters:
      </p>
      <CODE_SNIPPET>
        const int kDaysInAWeek = 7;
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Names">
    <SUMMARY>
      Regular functions have mixed case; accessors and mutators match
      the name of the variable: <code>MyExcitingFunction()</code>,
      <code>MyExcitingMethod()</code>,
      <code>my_exciting_member_variable()</code>,
      <code>set_my_exciting_member_variable()</code>.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Regular Functions">
        <p>
          Functions should start with a capital letter and have a
          capital letter for each new word. No underscores.
        </p>
        <p>
          If your function crashes upon an error, you should append OrDie to
          the function name. This only applies to functions which could be
          used by production code and to errors that are reasonably
          likely to occur during normal operation.
        </p>
        <CODE_SNIPPET>
          AddTableEntry()
          DeleteUrl()
          OpenFileOrDie()
        </CODE_SNIPPET>
      </SUBSECTION>

      <SUBSECTION title="Accessors and Mutators">
        <p>
          Accessors and mutators (get and set functions) should match
          the name of the variable they are getting and setting.  This
          shows an excerpt of a class whose instance variable is
          <code>num_entries_</code>.
        </p>
        <CODE_SNIPPET>
          class MyClass {
           public:
            ...
            int num_entries() const { return num_entries_; }
            void set_num_entries(int num_entries) { num_entries_ = num_entries; }

           private:
            int num_entries_;
          };
        </CODE_SNIPPET>
        <p>
          You may also use lowercase letters for other very short
          inlined functions. For example if a function were so cheap
          you would not cache the value if you were calling it in a
          loop, then lowercase naming would be acceptable.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Namespace Names">
    
    <SUMMARY>
      Namespace names are all lower-case, and based on project names and
      possibly their directory structure:
      <code>google_awesome_project</code>.
    </SUMMARY>
    <BODY>
      <p>
        See <a HREF="#Namespaces">Namespaces</a> for a discussion of
        namespaces and how to name them.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Enumerator Names">
    <SUMMARY>
      Enumerators should be named <i>either</i> like
      <A HREF="#Constant_Names">constants</A> or like
      <A HREF="#Macro_Names">macros</A>: either <code>kEnumName</code>
      or <code>ENUM_NAME</code>.
    </SUMMARY>
    <BODY>
      <p>
        Preferably, the individual enumerators should be named like
        <A HREF="#Constant_Names">constants</A>.  However, it is also
        acceptable to name them like <A HREF="#Macro_Names">macros</A>.    The enumeration name,
        <code>UrlTableErrors</code> (and
        <code>AlternateUrlTableErrors</code>), is a type, and
        therefore mixed case.
      </p>
      <CODE_SNIPPET>
        enum UrlTableErrors {
          kOK = 0,
          kErrorOutOfMemory,
          kErrorMalformedInput,
        };
        enum AlternateUrlTableErrors {
          OK = 0,
          OUT_OF_MEMORY = 1,
          MALFORMED_INPUT = 2,
        };
      </CODE_SNIPPET>
      <p>
        Until January 2009, the style was to name enum values like
        <A HREF="#Macro_Names">macros</A>.  This caused problems with
        name collisions between enum values and macros.  Hence, the
        change to prefer constant-style naming was put in place.  New
        code should prefer constant-style naming if possible.
        However, there is no reason to change old code to use
        constant-style names, unless the old names are actually
        causing a compile-time problem.
      </p>
      
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Macro Names">
    <SUMMARY>
      You're not really going to <A HREF="#Preprocessor_Macros">define
      a macro</A>, are you?  If you do, they're like this:
      <code>MY_MACRO_THAT_SCARES_SMALL_CHILDREN</code>.
    </SUMMARY>
    <BODY>
      <p>
        Please see the <a href="#Preprocessor_Macros">description of
        macros</a>; in general macros should <em>not</em> be used.
        However, if they are absolutely needed, then they should be
        named with all capitals and underscores.
      </p>
      <CODE_SNIPPET>
        #define ROUND(x) ...
        #define PI_ROUNDED 3.0
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Exceptions to Naming Rules">
    <SUMMARY>
      If you are naming something that is analogous to an existing C
      or C++ entity then you can follow the existing naming convention
      scheme.
    </SUMMARY>
    <BODY>
      <p>
        <dl>
          <dt> <code>bigopen()</code> </dt>
          <dd>   function name, follows form of <code>open()</code> </dd>
          <dt> <code>uint</code> </dt>
          <dd>   <code>typedef</code> </dd>
          <dt> <code>bigpos</code> </dt>
          <dd>   <code>struct</code> or <code>class</code>, follows form of
                 <code>pos</code> </dd>
          <dt> <code>sparse_hash_map</code> </dt>
          <dd>   STL-like entity; follows STL naming conventions </dd>
          <dt> <code>LONGLONG_MAX</code> </dt>
          <dd>   a constant, as in <code>INT_MAX</code> </dd>
        </dl>
      </p>
    </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Comments">
  <p>
    Though a pain to write, comments are absolutely vital to keeping our
    code readable.  The following rules describe what you should
    comment and where.  But remember: while comments are very
    important, the best code is self-documenting.  Giving sensible
    names to types and variables is much better than using obscure
    names that you must then explain through comments.
  </p>
  <p>
    When writing your comments, write for your audience: the next
    
    contributor
    who will need to understand your code.  Be generous &#8212; the next
    one may be you!
  </p>

  
  <STYLEPOINT title="Comment Style">
    <SUMMARY>
      Use either the <code>//</code> or <code>/* */</code> syntax, as long
      as you are consistent.
    </SUMMARY>
    <BODY>
      <p>
        You can use either the <code>//</code> or the <code>/* */</code>
        syntax; however, <code>//</code> is <em>much</em> more common.
        Be consistent with how you comment and what style you use where.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="File Comments">
    <SUMMARY>
      Start each file with a copyright notice, followed by a
      description of the contents of the file.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Legal Notice and Author Line">
        
        
        <p>
          Every file should contain the following items, in order:
          <ul>
            <li>a copyright statement (for example,
                <code>Copyright 2008 Google Inc.</code>)</li>
            <li>a license boilerplate.  Choose the appropriate boilerplate
                for the license used by the project (for example,
                Apache 2.0, BSD, LGPL, GPL)</li>
            <li>an author line to identify the original author of the
                file</li>
          </ul>
        </p>
        <p>
          If you make significant changes to a file that someone else
          originally wrote, add yourself to the author line. This can
          be very helpful when another
          
          contributor
          has questions about the file and needs to know whom to contact
          about it.
        </p>
      </SUBSECTION>

      <SUBSECTION title="File Contents">
        <p>
          Every file should have a comment at the top, below the copyright
          notice and author line, that describes the contents of the file.
        </p>
        <p>
          Generally a <code>.h</code> file will describe the classes
          that are declared in the file with an overview of what they
          are for and how they are used. A <code>.cc</code> file
          should contain more information about implementation details
          or discussions of tricky algorithms. If you feel the
          implementation details or a discussion of the algorithms
          would be useful for someone reading the <code>.h</code>,
          feel free to put it there instead, but mention in the
          <code>.cc</code> that the documentation is in the
          <code>.h</code> file.
        </p>
        <p>
          Do not duplicate comments in both the <code>.h</code> and
          the <code>.cc</code>. Duplicated comments diverge.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Class Comments">
    <SUMMARY>
      Every class definition should have an accompanying comment that
      describes what it is for and how it should be used.
    </SUMMARY>
    <BODY>
      <CODE_SNIPPET>
        // Iterates over the contents of a GargantuanTable.  Sample usage:
        //    GargantuanTableIterator* iter = table-&gt;NewIterator();
        //    for (iter-&gt;Seek("foo"); !iter-&gt;done(); iter-&gt;Next()) {
        //      process(iter-&gt;key(), iter-&gt;value());
        //    }
        //    delete iter;
        class GargantuanTableIterator {
          ...
        };
      </CODE_SNIPPET>
      <p>
        If you have already described a class in detail in the
        comments at the top of your file feel free to simply state
        "See comment at top of file for a complete description", but
        be sure to have some sort of comment.
      </p>
      <p>
        Document the synchronization assumptions the class makes, if
        any.  If an instance of the class can be accessed by multiple
        threads, take extra care to document the rules and invariants
        surrounding multithreaded use.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Comments">
    <SUMMARY>
      Declaration comments describe use of the function; comments at
      the definition of a function describe operation.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Function Declarations">
        <p>
          Every function declaration should have comments immediately
          preceding it that describe what the function does and how to
          use it.  These comments should be descriptive ("Opens the
          file") rather than imperative ("Open the file"); the comment
          describes the function, it does not tell the function what
          to do.  In general, these comments do not describe how the
          function performs its task.  Instead, that should be left to
          comments in the function definition.
        </p>
        <p>
          Types of things to mention in comments at the function
          declaration:
        </p>
        <ul>
          <li> What the inputs and outputs are.
               </li>
          <li> For class member functions:  whether the object
               remembers reference arguments beyond the
               duration of the method call, and whether it will
               free them or not.
               </li>
          <li> If the function allocates memory that the caller
               must free.
               </li>
          <li> Whether any of the arguments can be <code>NULL</code>.
               </li>
          <li> If there are any performance implications of how a
               function is used.
               </li>
          <li> If the function is re-entrant.  What are its
               synchronization assumptions?
               </li>
        </ul>
        <p>
          Here is an example:
        </p>
        <CODE_SNIPPET>
          // Returns an iterator for this table.  It is the client's
          // responsibility to delete the iterator when it is done with it,
          // and it must not use the iterator once the GargantuanTable object
          // on which the iterator was created has been deleted.
          //
          // The iterator is initially positioned at the beginning of the table.
          //
          // This method is equivalent to:
          //    Iterator* iter = table-&gt;NewIterator();
          //    iter-&gt;Seek("");
          //    return iter;
          // If you are going to immediately seek to another place in the
          // returned iterator, it will be faster to use NewIterator()
          // and avoid the extra seek.
          Iterator* GetIterator() const;
        </CODE_SNIPPET>
        <p>
          However, do not be unnecessarily verbose or state the
          completely obvious.  Notice below that it is not necessary
          to say "returns false otherwise" because this is implied.
        </p>
        <CODE_SNIPPET>
          // Returns true if the table cannot hold any more entries.
          bool IsTableFull();
        </CODE_SNIPPET>
        <p>
          When commenting constructors and destructors, remember that
          the person reading your code knows what constructors and
          destructors are for, so comments that just say something like
          "destroys this object" are not useful.  Document what
          constructors do with their arguments (for example, if they
          take ownership of pointers), and what cleanup the destructor
          does.  If this is trivial, just skip the comment.  It is
          quite common for destructors not to have a header comment.
        </p>
      </SUBSECTION>

      <SUBSECTION title="Function Definitions">
        <p>
          Each function definition should have a comment describing
          what the function does if there's anything tricky about how it does
          its job.  For example, in the definition comment you might
          describe any coding tricks you use, give an overview of the
          steps you go through, or explain why you chose to implement
          the function in the way you did rather than using a viable
          alternative.  For instance, you might mention why it must
          acquire a lock for the first half of the function but why it
          is not needed for the second half.
        </p>
        <p>
          Note you should <em>not</em> just repeat the comments given
          with the function declaration, in the <code>.h</code> file or
          wherever.  It's okay to recapitulate briefly what the function
          does, but the focus of the comments should be on how it does it.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Variable Comments">
    <SUMMARY>
      In general the actual name of the variable should be descriptive
      enough to give a good idea of what the variable is used for.  In
      certain cases, more comments are required.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Class Data Members">
        <p>
          Each class data member (also called an instance variable or
          member variable) should have a comment describing what it is
          used for.  If the variable can take sentinel values with
          special meanings, such as <code>NULL</code> or -1, document this.
          For example:
        </p>
        <CODE_SNIPPET>
          private:
           // Keeps track of the total number of entries in the table.
           // Used to ensure we do not go over the limit. -1 means
           // that we don't yet know how many entries the table has.
           int num_total_entries_;
        </CODE_SNIPPET>
      </SUBSECTION>

      <SUBSECTION title="Global Variables">
        <p>
          As with data members, all global variables should have a
          comment describing what they are and what they are used for.
          For example:
        </p>
        <CODE_SNIPPET>
          // The total number of tests cases that we run through in this regression test.
          const int kNumTestCases = 6;
        </CODE_SNIPPET>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Implementation Comments">
    <SUMMARY>
      In your implementation you should have comments in tricky,
      non-obvious, interesting, or important parts of your code.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Class Data Members">
        <p>
          Tricky or complicated code blocks should have comments
          before them. Example:
        </p>
        <CODE_SNIPPET>
          // Divide result by two, taking into account that x
          // contains the carry from the add.
          for (int i = 0; i &lt; result-&gt;size(); i++) {
            x = (x &lt;&lt; 8) + (*result)[i];
            (*result)[i] = x &gt;&gt; 1;
            x &amp;= 1;
          }
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="Line Comments">
        <p>
          Also, lines that are non-obvious should get a comment at the
          end of the line. These end-of-line comments should be
          separated from the code by 2 spaces.  Example:
        </p>
        <CODE_SNIPPET>
          // If we have enough memory, mmap the data portion too.
          mmap_budget = max&lt;int64&gt;(0, mmap_budget - index_-&gt;length());
          if (mmap_budget &gt;= data_size_ &amp;&amp; !MmapData(mmap_chunk_bytes, mlock))
            return;  // Error already logged.
        </CODE_SNIPPET>
        <p>
          Note that there are both comments that describe what the
          code is doing, and comments that mention that an error has
          already been logged when the function returns.
        </p>
        <p>
          If you have several comments on subsequent lines, it can
          often be more readable to line them up:
        </p>
        <CODE_SNIPPET>
          DoSomething();                  // Comment here so the comments line up.
          DoSomethingElseThatIsLonger();  // Comment here so there are two spaces between
                                          // the code and the comment.
          { // One space before comment when opening a new scope is allowed,
            // thus the comment lines up with the following comments and code.
            DoSomethingElse();  // Two spaces before line comments normally.
          }
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="NULL, true/false, 1, 2, 3...">
        <p>
          When you pass in <code>NULL</code>, boolean, or literal integer
          values to functions, you should consider adding a comment about
          what they are, or make your code self-documenting by using
          constants. For example, compare:
        </p>
        <BAD_CODE_SNIPPET>
          bool success = CalculateSomething(interesting_value,
                                            10,
                                            false,
                                            NULL);  // What are these arguments??
        </BAD_CODE_SNIPPET>
        <p>
          versus:
        </p>
        <CODE_SNIPPET>
          bool success = CalculateSomething(interesting_value,
                                            10,     // Default base value.
                                            false,  // Not the first time we're calling this.
                                            NULL);  // No callback.
        </CODE_SNIPPET>
        <p>
          Or alternatively, constants or self-describing variables:
        </p>
        <CODE_SNIPPET>
          const int kDefaultBaseValue = 10;
          const bool kFirstTimeCalling = false;
          Callback *null_callback = NULL;
          bool success = CalculateSomething(interesting_value,
                                            kDefaultBaseValue,
                                            kFirstTimeCalling,
                                            null_callback);
        </CODE_SNIPPET>
      </SUBSECTION>

      <SUBSECTION title="Don'ts">
        <p>
          Note that you should <em>never</em> describe the code
          itself. Assume that the person reading the code knows C++
          better than you do, even though he or she does not know what
          you are trying to do:
        </p>
        <BAD_CODE_SNIPPET>
           // Now go through the b array and make sure that if i occurs,
           // the next element is i+1.
           ...        // Geez.  What a useless comment.
        </BAD_CODE_SNIPPET>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Punctuation, Spelling and Grammar">
    <SUMMARY>
      Pay attention to punctuation, spelling, and grammar; it is
      easier to read well-written comments than badly written ones.
    </SUMMARY>
    <BODY>
      <p>
        Comments should usually be written as complete
        sentences with proper capitalization and periods at the end.
        Shorter comments, such as comments at the end of a line of
        code, can sometimes be less formal, but you should be
        consistent with your style.  Complete sentences are more
        readable, and they provide some assurance that the comment is
        complete and not an unfinished thought.
      </p>
      <p>
        Although it can be frustrating to have a code reviewer point
        out that you are using a comma when you should be using a
        semicolon, it is very important that source code maintain a
        high level of clarity and readability.  Proper punctuation,
        spelling, and grammar help with that goal.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="TODO Comments">
    <SUMMARY>
      Use <code>TODO</code> comments for code that is temporary, a
      short-term solution, or good-enough but not perfect.
    </SUMMARY>
    <BODY>
      <p>
        <code>TODO</code>s should include the string <code>TODO</code> in
        all caps, followed by the
        
        name, e-mail address, or other
        identifier
        of the person who can best provide context about the problem
        referenced by the <code>TODO</code>.  A colon is optional.  The main
        purpose is to have a consistent <code>TODO</code> format that can be
        searched to find the person who can provide more details upon request.
        A <code>TODO</code> is not a commitment that the person referenced
        will fix the problem.  Thus when you create a <code>TODO</code>, it is
        almost always your
        
        name
        that is given.
      </p>
      
      <CODE_SNIPPET>
        // TODO(kl@gmail.com): Use a "*" here for concatenation operator.
        // TODO(Zeke) change this to use relations.
      </CODE_SNIPPET>
      <p>
        If your <code>TODO</code> is of the form "At a future date do
        something" make sure that you either include a very specific
        date ("Fix by November 2005") or a very specific event
        ("Remove this code when all clients can handle XML responses.").
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Deprecation Comments">
    <SUMMARY>
      Mark deprecated interface points with <code>DEPRECATED</code> comments.
    </SUMMARY>
    <BODY>
      <p>
      You can mark an interface as deprecated by writing a comment containing
      the word <code>DEPRECATED</code> in all caps.  The comment goes either
      before the declaration of the interface or on the same line as the
      declaration.
      </p>
      
      <p>
      After the word <code>DEPRECATED</code>, write your name, e-mail address,
      or other identifier in parentheses.
      </p>
      <p>
      A deprecation comment must include simple, clear directions for people to
      fix their callsites.  In C++, you can implement a deprecated function as
      an inline function that calls the new interface point.
      </p>
      <p>
      Marking an interface point <code>DEPRECATED</code> will not magically
      cause any callsites to change.  If you want people to actually stop using
      the deprecated facility, you will have to fix the callsites yourself or
      recruit a crew to help you.
      </p>
      <p>
      New code should not contain calls to deprecated interface points.  Use
      the new interface point instead.  If you cannot understand the
      directions, find the person who created the deprecation and ask them for
      help using the new interface point.
      </p>
      
    </BODY>
  </STYLEPOINT>

</CATEGORY>

<CATEGORY title="Formatting">
  <p>
    Coding style and formatting are pretty arbitrary, but a
    
    project
    is much easier to follow if everyone uses the same style. Individuals
    may not agree with every aspect of the formatting rules, and some of
    the rules may take some getting used to, but it is important that all
    
    project contributors
    follow the style rules so that
    
    they
    can all read and understand everyone's code easily.
  </p>
  
  <p>
    To help you format code correctly, we've created a <A HREF="http://google-styleguide.googlecode.com/svn/trunk/google-c-style.el">settings
    file for emacs</A>.
  </p>

  <STYLEPOINT title="Line Length">
    <SUMMARY>
      Each line of text in your code should be at most 80 characters
      long.
    </SUMMARY>
    <BODY>
      
      <p>
        We recognize that this rule is controversial, but so much existing
        code already adheres to it, and we feel that consistency is
        important.
      </p>
      <PROS>
          Those who favor
          
          this rule argue
          that it is rude to force them to resize their windows and there
          is no need for anything longer.  Some folks are used to having
          several code windows side-by-side, and thus don't have room to
          widen their windows in any case.  People set up their work
          environment assuming a particular maximum window width, and 80
          columns has been the traditional standard.  Why change it?
      </PROS>
      <CONS>
          Proponents of change argue that a wider line can make code
          more readable.  The 80-column limit is an hidebound
          throwback to 1960s mainframes;
          
          modern equipment has
          wide screens that can easily show longer lines.
      </CONS>
      <DECISION>
        <p>
          
          80 characters is the maximum.
        </p>
        <p>
          Exception: if a comment line contains an example command or
          a literal URL longer than 80 characters, that line may be
          longer than 80 characters for ease of cut and paste.
        </p>
        <p>
          Exception: an <code>#include</code> statement with a long
          path may exceed 80 columns.  Try to avoid situations where this
          becomes necessary.
        </p>
        <p>
          Exception:  you needn't be concerned about
          <a href="#The__define_Guard">header guards</a>
          that exceed the maximum length.
          
        </p>
      </DECISION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Non-ASCII Characters">
    <SUMMARY>
      Non-ASCII characters should be rare, and must use UTF-8 formatting.
    </SUMMARY>
    <BODY>
      <p>
        You shouldn't hard-code user-facing text in source, even English,
        so use of non-ASCII characters should be rare.  However, in certain
        cases it is appropriate to include such words in your code.  For
        example, if your code parses data files from foreign sources,
        it may be appropriate to hard-code the non-ASCII string(s) used in
        those data files as delimiters.  More commonly, unittest code
        (which does not
        
        need to be localized) might contain non-ASCII strings.  In such
        cases, you should use UTF-8, since that is
        
        an encoding understood by most tools able
        to handle more than just ASCII.
        Hex encoding is also OK, and encouraged where it enhances
        readability &#8212; for example, <code>"\xEF\xBB\xBF"</code> is the
        Unicode zero-width no-break space character, which would be
        invisible if included in the source as straight UTF-8.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Spaces vs. Tabs">
    <SUMMARY>
      Use only spaces, and indent 2 spaces at a time.
    </SUMMARY>
    <BODY>
      <p>
        We use spaces for indentation. Do not use tabs in your code.
        You should set your editor to emit spaces when you hit the tab
        key.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Declarations and Definitions">
    <SUMMARY>
      Return type on the same line as function name, parameters on the
      same line if they fit.
    </SUMMARY>
    <BODY>
      <p>
        Functions look like this:
      </p>
      <CODE_SNIPPET>
        ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
          DoSomething();
          ...
        }
      </CODE_SNIPPET>
      <p>
        If you have too much text to fit on one line:
      </p>
      <CODE_SNIPPET>
        ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
                                                     Type par_name3) {
          DoSomething();
          ...
        }
      </CODE_SNIPPET>
      <p>
        or if you cannot fit even the first parameter:
      </p>
      <CODE_SNIPPET>
        ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
            Type par_name1,  // 4 space indent
            Type par_name2,
            Type par_name3) {
          DoSomething();  // 2 space indent
          ...
        }
      </CODE_SNIPPET>
      <p>
        Some points to note:
      </p>
      <ul>
        <li> The return type is always on the same line as the
             function name.
             </li>
        <li> The open parenthesis is always on the same line as the
             function name.
             </li>
        <li> There is never a space between the function name and the
             open parenthesis.
             </li>
        <li> There is never a space between the parentheses and the
             parameters.
             </li>
        <li> The open curly brace is always at the end of the same
             line as the last parameter.
             </li>
        <li> The close curly brace is either on the last line by itself
             or (if other style rules permit) on the same line as the
             open curly brace.
             </li>
        <li> There should be a space between the close parenthesis and
             the open curly brace.
             </li>
        <li> All parameters should be named, with identical names in the
             declaration and implementation.
             </li>
        <li> All parameters should be aligned if possible.
             </li>
        <li> Default indentation is 2 spaces.
             </li>
        <li> Wrapped parameters have a 4 space indent.
             </li>
      </ul>
      <p>
        If your function is <code>const</code>, the <code>const</code>
        keyword should be on the same line as the last parameter:
      </p>
      <CODE_SNIPPET>
        // Everything in this function signature fits on a single line
        ReturnType FunctionName(Type par) const {
          ...
        }

        // This function signature requires multiple lines, but
        // the const keyword is on the line with the last parameter.
        ReturnType ReallyLongFunctionName(Type par1,
                                          Type par2) const {
          ...
        }
      </CODE_SNIPPET>
      <p>
        If some parameters are unused, comment out the variable name in the
        function definition:
      </p>
      <CODE_SNIPPET>
        // Always have named parameters in interfaces.
        class Shape {
         public:
          virtual void Rotate(double radians) = 0;
        }

        // Always have named parameters in the declaration.
        class Circle : public Shape {
         public:
          virtual void Rotate(double radians);
        }

        // Comment out unused named parameters in definitions.
        void Circle::Rotate(double /*radians*/) {}
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        // Bad - if someone wants to implement later, it's not clear what the
        // variable means.
        void Circle::Rotate(double) {}
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Function Calls">
    <SUMMARY>
      On one line if it fits; otherwise, wrap arguments at the
      parenthesis.
    </SUMMARY>
    <BODY>
      <p>
        Function calls have the following format:
      </p>
      <CODE_SNIPPET>
        bool retval = DoSomething(argument1, argument2, argument3);
      </CODE_SNIPPET>
      <p>
        If the arguments do not all fit on one line, they should be
        broken up onto multiple lines, with each subsequent line
        aligned with the first argument.  Do not add spaces after the
        open paren or before the close paren:
      </p>
      <CODE_SNIPPET>
        bool retval = DoSomething(averyveryveryverylongargument1,
                                  argument2, argument3);
      </CODE_SNIPPET>
      <p>
        If the function has many arguments, consider having one per
        line if this makes the code more readable:
      </p>
      <CODE_SNIPPET>
        bool retval = DoSomething(argument1,
                                  argument2,
                                  argument3,
                                  argument4);
      </CODE_SNIPPET>
      <p>
        If the function signature is so long that it cannot fit within
        the maximum <a href="#Line_Length">line length</a>, you may
        place all arguments on subsequent lines:
      </p>
      <CODE_SNIPPET>
        if (...) {
          ...
          ...
          if (...) {
            DoSomethingThatRequiresALongFunctionName(
                very_long_argument1,  // 4 space indent
                argument2,
                argument3,
                argument4);
          }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Conditionals">
    <SUMMARY>
      Prefer no spaces inside parentheses.  The <code>else</code>
      keyword belongs on a new line.
    </SUMMARY>
    <BODY>
      <p>
        There are two acceptable formats for a basic conditional
        statement. One includes spaces between the parentheses and the
        condition, and one does not.
      </p>
      <p>
        The most common form is without spaces.  Either is fine, but
        <em>be consistent</em>. If you are modifying a file, use the
        format that is already present. If you are writing new code,
        use the format that the other files in that directory or
        project use. If in doubt and you have no personal preference,
        do not add the spaces.
      </p>
      <CODE_SNIPPET>
        if (condition) {  // no spaces inside parentheses
          ...  // 2 space indent.
        } else if (...) {  // The else goes on the same line as the closing brace.
          ...
        } else {
          ...
        }
      </CODE_SNIPPET>
      <p>
        If you prefer you may add spaces inside the
        parentheses:
      </p>
      <CODE_SNIPPET>
        if ( condition ) {  // spaces inside parentheses - rare
          ...  // 2 space indent.
        } else {  // The else goes on the same line as the closing brace.
          ...
        }
      </CODE_SNIPPET>
      <p>
        Note that in all cases you must have a space between the
        <code>if</code> and the open parenthesis.  You must also have
        a space between the close parenthesis and the curly brace, if
        you're using one.
      </p>
      <BAD_CODE_SNIPPET>
        if(condition)     // Bad - space missing after IF.
        if (condition){   // Bad - space missing before {.
        if(condition){    // Doubly bad.
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        if (condition) {  // Good - proper space after IF and before {.
      </CODE_SNIPPET>
      <p>
        Short conditional statements may be written on one line if
        this enhances readability.  You may use this only when the
        line is brief and the statement does not use the
        <code>else</code> clause.
      </p>
      <CODE_SNIPPET>
        if (x == kFoo) return new Foo();
        if (x == kBar) return new Bar();
      </CODE_SNIPPET>
      <p>
        This is not allowed when the if statement has an
        <code>else</code>:
      </p>
      <BAD_CODE_SNIPPET>
        // Not allowed - IF statement on one line when there is an ELSE clause
        if (x) DoThis();
        else DoThat();
      </BAD_CODE_SNIPPET>
      <p>
        In general, curly braces are not required for single-line
        statements, but they are allowed if you like them;
        conditional or loop statements with complex conditions or
        statements may be more readable with curly braces. Some
        
        projects
        require that an <CODE>if</CODE> must always always have an
        accompanying brace.
      </p>
      <CODE_SNIPPET>
        if (condition)
          DoSomething();  // 2 space indent.

        if (condition) {
          DoSomething();  // 2 space indent.
        }
      </CODE_SNIPPET>
      <p>
        However, if one part of an <code>if</code>-<code>else</code>
        statement uses curly braces, the other part must too:
      </p>
      <BAD_CODE_SNIPPET>
        // Not allowed - curly on IF but not ELSE
        if (condition) {
          foo;
        } else
          bar;

        // Not allowed - curly on ELSE but not IF
        if (condition)
          foo;
        else {
          bar;
        }
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        // Curly braces around both IF and ELSE required because
        // one of the clauses used braces.
        if (condition) {
          foo;
        } else {
          bar;
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Loops and Switch Statements">
    <SUMMARY>
      Switch statements may use braces for blocks.  Empty loop bodies should use
      <code>{}</code> or <code>continue</code>.
    </SUMMARY>
    <BODY>
      <p>
        <code>case</code> blocks in <code>switch</code> statements can have
        curly braces or not, depending on your preference.  If you do
        include curly braces they should be placed as shown below.
      </p>
      <p>
        If not conditional on an enumerated value, switch statements
        should always have a <code>default</code> case (in the case of
        an enumerated value, the compiler will warn you if any values
        are not handled).  If the default case should never execute,
        simply
        <code>assert</code>:
      </p>
      
      <CODE_SNIPPET>
        switch (var) {
          case 0: {  // 2 space indent
            ...      // 4 space indent
            break;
          }
          case 1: {
            ...
            break;
          }
          default: {
            assert(false);
          }
        }
      </CODE_SNIPPET>
      <p>
        Empty loop bodies should use <code>{}</code> or
        <code>continue</code>, but not a single semicolon.
      </p>
      <CODE_SNIPPET>
        while (condition) {
          // Repeat test until it returns false.
        }
        for (int i = 0; i &lt; kSomeNumber; ++i) {}  // Good - empty body.
        while (condition) continue;  // Good - continue indicates no logic.
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        while (condition);  // Bad - looks like part of do/while loop.
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Pointer and Reference Expressions">
    <SUMMARY>
      No spaces around period or arrow.  Pointer operators do not have
      trailing spaces.
    </SUMMARY>
    <BODY>
      <p>
        The following are examples of correctly-formatted pointer and
        reference expressions:
      </p>
      <CODE_SNIPPET>
        x = *p;
        p = &amp;x;
        x = r.y;
        x = r-&gt;y;
      </CODE_SNIPPET>
      <p>
        Note that:
      </p>
      <ul>
        <li> There are no spaces around the period or arrow when
             accessing a member.
             </li>
        <li> Pointer operators have no space after the <code>*</code> or
             <code>&amp;</code>.
             </li>
      </ul>
      <p>
        When declaring a pointer variable or argument, you may place
        the asterisk adjacent to either the type or to the variable
        name:
      </p>
      <CODE_SNIPPET>
        // These are fine, space preceding.
        char *c;
        const string &amp;str;

        // These are fine, space following.
        char* c;    // but remember to do "char* c, *d, *e, ...;"!
        const string&amp; str;
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        char * c;  // Bad - spaces on both sides of *
        const string &amp; str;  // Bad - spaces on both sides of &amp;
      </BAD_CODE_SNIPPET>
      <p>
        You should do this consistently within a single
        file,
        so, when modifying an existing file, use the style in that
        file.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Boolean Expressions">
    <SUMMARY>
      When you have a boolean expression that is longer than the <a href="#Line_Length">standard line length</a>, be consistent in
      how you break up the lines.
    </SUMMARY>
    <BODY>
      <p>
        In this example, the logical AND operator is always at the end
        of the lines:
      </p>
      <CODE_SNIPPET>
        if (this_one_thing &gt; this_other_thing &amp;&amp;
            a_third_thing == a_fourth_thing &amp;&amp;
            yet_another &amp;&amp; last_one) {
          ...
        }
      </CODE_SNIPPET>
      <p>
        Note that when the code wraps in this example, both of
        the <code>&amp;&amp;</code> logical AND operators are at the
        end of the line.  This is more common in Google code, though
        wrapping all operators at the beginning of the line is also
        allowed.  Feel free to insert extra parentheses judiciously
        because they can be very helpful in increasing readability
        when used appropriately.  Also note that you should always use the
        punctuation operators, such as <code>&amp;&amp;</code> and
        <code>~</code>, rather than the word operators, such as <code>and</code>
        and <code>compl</code>.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Return Values">
    <SUMMARY>
      Do not needlessly surround the <code>return</code> expression with
      parentheses.
    </SUMMARY>
    <BODY>
      <p>
        Use parentheses in <code>return expr;</code> only where you would use
        them in <code>x = expr;</code>.
      </p>
      <CODE_SNIPPET>
        return result;                  // No parentheses in the simple case.
        return (some_long_condition &amp;&amp;  // Parentheses ok to make a complex
                another_condition);     //     expression more readable.
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        return (value);                // You wouldn't write var = (value);
        return(result);                // return is not a function!
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  
  <STYLEPOINT title="Variable and Array Initialization">
    <SUMMARY>
      Your choice of <code>=</code> or <code>()</code>.
    </SUMMARY>
    <BODY>
      <p>
        You may choose between <code>=</code> and <code>()</code>; the
        following are all correct:
      </p>
      <CODE_SNIPPET>
        int x = 3;
        int x(3);
        string name("Some Name");
        string name = "Some Name";
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Preprocessor Directives">
    <SUMMARY>
      The hash mark that starts a preprocessor directive should
      always be at the beginning of the line.
    </SUMMARY>
    <BODY>
      <p>
        Even when preprocessor directives are within the body of
        indented code, the directives should start at the beginning of
        the line.
      </p>
      <CODE_SNIPPET>
        // Good - directives at beginning of line
          if (lopsided_score) {
        #if DISASTER_PENDING      // Correct -- Starts at beginning of line
            DropEverything();
        # if NOTIFY               // OK but not required -- Spaces after #
            NotifyClient();
        # endif
        #endif
            BackToNormal();
          }
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        // Bad - indented directives
          if (lopsided_score) {
            #if DISASTER_PENDING  // Wrong!  The "#if" should be at beginning of line
            DropEverything();
            #endif                // Wrong!  Do not indent "#endif"
            BackToNormal();
          }
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Class Format">
    <SUMMARY>
      Sections in <code>public</code>, <code>protected</code> and
      <code>private</code> order, each indented one space.
    </SUMMARY>
    <BODY>
      <p>
        The basic format for a class declaration (lacking the
        comments, see <a HREF="#Class_Comments">Class Comments</a> for
        a discussion of what comments are needed) is:
      </p>
      <CODE_SNIPPET>
        class MyClass : public OtherClass {
         public:      // Note the 1 space indent!
          MyClass();  // Regular 2 space indent.
          explicit MyClass(int var);
          ~MyClass() {}

          void SomeFunction();
          void SomeFunctionThatDoesNothing() {
          }

          void set_some_var(int var) { some_var_ = var; }
          int some_var() const { return some_var_; }

         private:
          bool SomeInternalFunction();

          int some_var_;
          int some_other_var_;
          DISALLOW_COPY_AND_ASSIGN(MyClass);
        };
      </CODE_SNIPPET>
      <p>
        Things to note:
      </p>
      <ul>
        <li> Any base class name should be on the same line as the
             subclass name, subject to the 80-column limit.
             </li>
        <li> The <code>public:</code>, <code>protected:</code>, and
             <code>private:</code> keywords should be indented one
             space.
             </li>
        <li> Except for the first instance, these keywords should be preceded
             by a blank line. This rule is optional in small classes.
             </li>
        <li> Do not leave a blank line after these keywords.
             </li>
        <li> The <code>public</code> section should be first, followed by
             the <code>protected</code> and finally the
             <code>private</code> section.
             </li>
        <li> See <a HREF="#Declaration_Order">Declaration Order</a> for
             rules on ordering declarations within each of these sections.
             </li>
      </ul>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Constructor Initializer Lists">
    <SUMMARY>
      Constructor initializer lists can be all on one line or with
      subsequent lines indented four spaces.
    </SUMMARY>
    <BODY>
      <p>
        There are two acceptable formats for initializer lists:
      </p>
      <CODE_SNIPPET>
        // When it all fits on one line:
        MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}
      </CODE_SNIPPET>
      <p>
        or
      </p>
      <CODE_SNIPPET>
        // When it requires multiple lines, indent 4 spaces, putting the colon on
        // the first initializer line:
        MyClass::MyClass(int var)
            : some_var_(var),             // 4 space indent
              some_other_var_(var + 1) {  // lined up
          ...
          DoSomething();
          ...
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Namespace Formatting">
    <SUMMARY>
      The contents of namespaces are not indented.
    </SUMMARY>
    <BODY>
      <p>
        <a href="#Namespaces">Namespaces</a> do not add an extra level of
        indentation. For example, use:
      </p>
      <CODE_SNIPPET>
        namespace {

        void foo() {  // Correct.  No extra indentation within namespace.
          ...
        }

        }  // namespace
      </CODE_SNIPPET>
      <p>
        Do not indent within a namespace:
      </p>
      <BAD_CODE_SNIPPET>
        namespace {

          // Wrong.  Indented when it should not be.
          void foo() {
            ...
          }

        }  // namespace
      </BAD_CODE_SNIPPET>
      <p>
        When declaring nested namespaces, put each namespace on its own line.
      </p>
      <CODE_SNIPPET>
        namespace foo {
        namespace bar {
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Horizontal Whitespace">
    <SUMMARY>
      Use of horizontal whitespace depends on location.  Never put trailing
      whitespace at the end of a line.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="General">
        <CODE_SNIPPET>
        void f(bool b) {  // Open braces should always have a space before them.
          ...
        int i = 0;  // Semicolons usually have no space before them.
        int x[] = { 0 };  // Spaces inside braces for array initialization are
        int x[] = {0};    // optional.  If you use them, put them on both sides!
        // Spaces around the colon in inheritance and initializer lists.
        class Foo : public Bar {
         public:
          // For inline function implementations, put spaces between the braces
          // and the implementation itself.
          Foo(int b) : Bar(), baz_(b) {}  // No spaces inside empty braces.
          void Reset() { baz_ = 0; }  // Spaces separating braces from implementation.
          ...
        </CODE_SNIPPET>
        <p>
          Adding trailing whitespace can cause extra work for others editing
          the same file, when they merge, as can removing existing trailing
          whitespace.  So:  Don't introduce trailing whitespace.  Remove it
          if you're already changing that line, or do it in a separate
          clean-up
          
          operation (preferably when no-one else
          is working on the file).
        </p>
      </SUBSECTION>
      <SUBSECTION title="Loops and Conditionals">
        <CODE_SNIPPET>
        if (b) {          // Space after the keyword in conditions and loops.
        } else {          // Spaces around else.
        }
        while (test) {}   // There is usually no space inside parentheses.
        switch (i) {
        for (int i = 0; i &lt; 5; ++i) {
        switch ( i ) {    // Loops and conditions may have spaces inside
        if ( test ) {     // parentheses, but this is rare.  Be consistent.
        for ( int i = 0; i &lt; 5; ++i ) {
        for ( ; i &lt; 5 ; ++i) {  // For loops always have a space after the
          ...                   // semicolon, and may have a space before the
                                // semicolon.
        switch (i) {
          case 1:         // No space before colon in a switch case.
            ...
          case 2: break;  // Use a space after a colon if there's code after it.
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="Operators">
        <CODE_SNIPPET>
        x = 0;              // Assignment operators always have spaces around
                            // them.
        x = -5;             // No spaces separating unary operators and their
        ++x;                // arguments.
        if (x &amp;&amp; !y)
          ...
        v = w * x + y / z;  // Binary operators usually have spaces around them,
        v = w*x + y/z;      // but it's okay to remove spaces around factors.
        v = w * (x + z);    // Parentheses should have no spaces inside them.
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="Templates and Casts">
        <CODE_SNIPPET>
        vector&lt;string&gt; x;           // No spaces inside the angle
        y = static_cast&lt;char*&gt;(x);  // brackets (&lt; and &gt;), before
                                    // &lt;, or between &gt;( in a cast.
        vector&lt;char *&gt; x;           // Spaces between type and pointer are
                                    // okay, but be consistent.
        set&lt;list&lt;string&gt; &gt; x;       // C++ requires a space in &gt; &gt;.
        set&lt; list&lt;string&gt; &gt; x;      // You may optionally use
                                    // symmetric spacing in &lt; &lt;.
        </CODE_SNIPPET>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>


  <STYLEPOINT title="Vertical Whitespace">
    <SUMMARY>
      Minimize use of vertical whitespace.
    </SUMMARY>
    <BODY>
      <p>
        This is more a principle than a rule: don't use blank lines
        when you don't have to.  In particular, don't put more than
        one or two blank lines between functions, resist starting
        functions with a blank line, don't end functions with a blank
        line, and be discriminating with your use of blank lines
        inside functions.
      </p>
      <p>
        The basic principle is: The more code that fits on one screen,
        the easier it is to follow and understand the control flow of
        the program.  Of course, readability can suffer from code
        being too dense as well as too spread out, so use your
        judgement.  But in general, minimize use of vertical
        whitespace.
      </p>
      <p>
        Some rules of thumb to help when blank lines may be useful:
      </p>
      <ul>
        <li> Blank lines at the beginning or end of a function very
             rarely help readability.
             </li>
        <li> Blank lines inside a chain of if-else blocks may well
             help readability.
             </li>
      </ul>
    </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Exceptions to the Rules">
  <p>
    The coding conventions described above are mandatory.  However,
    like all good rules, these sometimes have exceptions, which we
    discuss here.
  </p>

  
  <STYLEPOINT title="Existing Non-conformant Code">
    <SUMMARY>
      You may diverge from the rules when dealing with code that does not
      conform to this style guide.
    </SUMMARY>
    <BODY>
      <p>
        If you find yourself modifying code that was written to
        specifications other than those presented by this guide, you may
        have to diverge from these rules in order to stay consistent with
        the local conventions in that code.  If you are in doubt about
        how to do this, ask the original author or the person currently
        responsible for the code.  Remember that <em>consistency</em>
        includes local consistency, too.
      </p>
    </BODY>
  </STYLEPOINT>

  
  <STYLEPOINT title="Windows Code">
    <SUMMARY>
      
      Windows programmers have developed their own set of coding
      conventions, mainly derived from the conventions in Windows headers
      and other Microsoft code.  We want to make it easy for anyone to
      understand your code, so we have a single set of guidelines for
      everyone writing C++ on any platform.
    </SUMMARY>
    <BODY>
      <p>
        It is worth reiterating a few of the guidelines that you might
        forget if you are used to the prevalent Windows style:
      </p>
      <ul>
        <li> Do not use Hungarian notation (for example, naming an
             integer <code>iNum</code>). Use the Google naming conventions,
             including the <code>.cc</code> extension for source files.
             </li>
        <li> Windows defines many of its own synonyms for primitive
             types, such as <code>DWORD</code>, <code>HANDLE</code>, etc.
             It is perfectly acceptable, and encouraged, that you use these
             types when calling Windows API functions. Even so, keep as
             close as you can to the underlying C++ types. For example, use
             <code>const TCHAR *</code> instead of <code>LPCTSTR</code>.
             </li>
        <li> When compiling with Microsoft Visual C++, set the
             compiler to warning level 3 or higher, and treat all
             warnings as errors.
             </li>
        <li> Do not use <code>#pragma once</code>; instead use the
             standard Google include guards.  The path in the include
             guards should be relative to the top of your project
             tree.
             </li>
        <li> In fact, do not use any nonstandard extensions, like
             <code>#pragma</code> and <code>__declspec</code>, unless you
             absolutely must.  Using <code>__declspec(dllimport)</code> and
             <code>__declspec(dllexport)</code> is allowed; however, you
             must use them through macros such as <code>DLLIMPORT</code>
             and <code>DLLEXPORT</code>, so that someone can easily disable
             the extensions if they share the code.
             </li>
      </ul>
      <p>
        However, there are just a few rules that we occasionally need
        to break on Windows:
      </p>
      <ul>
        <li> Normally we <a HREF="#Multiple_Inheritance">forbid
             the use of multiple implementation inheritance</a>; however,
             it is required when using COM and some ATL/WTL
             classes. You may use multiple implementation inheritance
             to implement COM or ATL/WTL classes and interfaces.
             </li>
        <li> Although you should not use exceptions in your own code,
             they are used extensively in the ATL and some STLs,
             including the one that comes with Visual C++. When using
             the ATL, you should define <code>_ATL_NO_EXCEPTIONS</code> to
             disable exceptions. You should investigate whether you can
             also disable exceptions in your STL, but if not, it is OK to
             turn on exceptions in the compiler. (Note that this is
             only to get the STL to compile. You should still not
             write exception handling code yourself.)
             </li>
        <li> The usual way of working with precompiled headers is to
             include a header file at the top of each source file,
             typically with a name like <code>StdAfx.h</code> or
             <code>precompile.h</code>. To make your code easier to share
             with other projects, avoid including this file explicitly
             (except in <code>precompile.cc</code>), and use the
             <code>/FI</code> compiler option to include the file
             automatically.
             </li>
        <li> Resource headers, which are usually named
             <code>resource.h</code> and contain only macros, do not need
             to conform to these style guidelines.
             </li>
      </ul>
    </BODY>
  </STYLEPOINT>

  
</CATEGORY>

<PARTING_WORDS>
  <p>
    Use common sense and <em>BE CONSISTENT</em>.
  </p>
  <p>
    If you are editing code, take a few minutes to look at the
    code around you and determine its style. If they use spaces
    around their <code>if</code> clauses, you should, too. If
    their comments have little boxes of stars around them, make
    your comments have little boxes of stars around them too.
  </p>
  <p>
    The point of having style guidelines is to have a common
    vocabulary of coding so people can concentrate on what you are
    saying, rather than on how you are saying it.  We present
    global style rules here so people know the vocabulary. But
    local style is also important.  If code you add to a file
    looks drastically different from the existing code around it,
    the discontinuity throws readers out of their rhythm when they
    go to read it. Try to avoid this.
  </p>
  
  <p>
    OK, enough writing about writing code; the code itself is much
    more interesting. Have fun!
  </p>
</PARTING_WORDS>

<HR/>

<p align="right">
Revision 3.199
</p>


<address>
Benjy Weinberger<br/>
Craig Silverstein<br/>
Gregory Eitzmann<br/>
Mark Mentovai<br/>
Tashana Landray
</address>

</GUIDE>