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sd6.html
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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2024-09-24" />
<title>Feature-Test Macros and Policies</title>
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ul.task-list{list-style: none;}
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code.sourceCode > span { display: inline; }
</style>
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color : #4183C4;
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}
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h1#toctitle {
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#TOC li {
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h3.subtitle { margin-top: -15px; }
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code span.co { font-family: monospace; }
table tr {
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table {
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@media all {
body {
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body {
margin: 5em;
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}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
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<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Feature-Test Macros and
Policies</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>SD-FeatureTest</td>
</tr>
<tr>
<td>Date:</td>
<td>2024-09-24</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#explanation-and-rationale-for-the-approach" id="toc-explanation-and-rationale-for-the-approach"><span class="toc-section-number">1</span> Explanation and rationale for the
approach<span></span></a>
<ul>
<li><a href="#status-quo-before-feature-test-macros" id="toc-status-quo-before-feature-test-macros"><span class="toc-section-number">1.1</span> Status quo before feature-test
macros<span></span></a></li>
<li><a href="#characteristics-of-the-proposed-solution" id="toc-characteristics-of-the-proposed-solution"><span class="toc-section-number">1.2</span> Characteristics of the proposed
solution<span></span></a></li>
<li><a href="#examples" id="toc-examples"><span class="toc-section-number">1.3</span> Examples<span></span></a></li>
</ul></li>
<li><a href="#recommendations" id="toc-recommendations"><span class="toc-section-number">2</span> Recommendations<span></span></a>
<ul>
<li><a href="#introduction" id="toc-introduction"><span class="toc-section-number">2.1</span> Introduction<span></span></a></li>
<li><a href="#testing-for-the-presence-of-a-header-__has_include" id="toc-testing-for-the-presence-of-a-header-__has_include"><span class="toc-section-number">2.2</span> Testing for the presence of a
header: <code class="sourceCode cpp"><span class="ot">__has_include</span></code><span></span></a></li>
<li><a href="#testing-for-the-presence-of-an-attribute-__has_cpp_attribute" id="toc-testing-for-the-presence-of-an-attribute-__has_cpp_attribute"><span class="toc-section-number">2.3</span> Testing for the presence of an
attribute: <code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code><span></span></a></li>
</ul></li>
<li><a href="#policies" id="toc-policies"><span class="toc-section-number">3</span> Policies<span></span></a>
<ul>
<li><a href="#constexpr" id="toc-constexpr"><span class="toc-section-number">3.1</span>
<code class="sourceCode cpp"><span class="kw">constexpr</span></code><span></span></a></li>
<li><a href="#language-features-with-library-components" id="toc-language-features-with-library-components"><span class="toc-section-number">3.2</span> Language Features with Library
Components<span></span></a></li>
</ul></li>
<li><a href="#table-of-feature-test-macros" id="toc-table-of-feature-test-macros"><span class="toc-section-number">4</span> Table of Feature-Test
Macros<span></span></a>
<ul>
<li><a href="#language-feature-test-macros" id="toc-language-feature-test-macros"><span class="toc-section-number">4.1</span> Language Feature-Test
Macros<span></span></a></li>
<li><a href="#attribute-feature-test-macros" id="toc-attribute-feature-test-macros"><span class="toc-section-number">4.2</span> Attribute Feature-Test
Macros<span></span></a></li>
<li><a href="#library-feature-test-macros" id="toc-library-feature-test-macros"><span class="toc-section-number">4.3</span> Library Feature-Test
Macros<span></span></a></li>
</ul></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">5</span> References<span></span></a></li>
</ul>
</div>
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</style>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="explanation-and-rationale-for-the-approach"><span class="header-section-number">1</span> Explanation and rationale for the
approach<a href="#explanation-and-rationale-for-the-approach" class="self-link"></a></h1>
<p>The pace of innovation in the standardization of C++ makes long-term
stability of implementations unlikely. Features are added to the
language because programmers want to use those features. Features are
added to (the working draft of) the standard as the features become
well-specified. In many cases a feature is added to an implementation
well before or well after the standard officially introducing it is
approved.</p>
<p>This process makes it difficult for programmers who want to use a
feature to know whether it is available in any given implementation.
Implementations rarely leap from one formal revision of the standard
directly to the next; the implementation process generally proceeds by
smaller steps. As a result, testing for a specific revision of the
standard (e.g. by examining the value of the
<code class="sourceCode cpp"><span class="ot">__cplusplus</span></code>
macro) often gives the wrong answer. Implementers generally don’t want
to appear to be claiming full conformance to a standard revision until
all of its features are implemented. That leaves programmers with no
portable way to determine which features are actually available to
them.</p>
<p>It is often possible for a program to determine, in a manner specific
to a single implementation, what features are supported by that
implementation; but the means are often poorly documented and ad hoc,
and sometimes complex – especially when the availability of a feature is
controlled by an invocation option. To make this determination for a
variety of implementations in a single source base is complex and
error-prone.</p>
<h2 data-number="1.1" id="status-quo-before-feature-test-macros"><span class="header-section-number">1.1</span> Status quo before feature-test
macros<a href="#status-quo-before-feature-test-macros" class="self-link"></a></h2>
<p>Here is some code that attempts to determine whether rvalue
references are available in the implementation in use:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifndef __USE_RVALUE_REFERENCES</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> <span class="pp">#if (</span><span class="ot">__GNUC__</span><span class="pp"> > 4 || </span><span class="ot">__GNUC__</span><span class="pp"> == 4 && </span><span class="ot">__GNUC_MINOR__</span><span class="pp"> >= 3) || \</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="pp"> _MSC_VER >= 1600</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> <span class="pp">#if __EDG_VERSION__ > 0</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> <span class="pp">#define __USE_RVALUE_REFERENCES </span><span class="op">(</span>__EDG_VERSION__<span class="pp"> </span><span class="op">>=</span><span class="pp"> </span><span class="dv">410</span><span class="op">)</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> <span class="pp">#else</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> <span class="pp">#define __USE_RVALUE_REFERENCES </span><span class="dv">1</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> <span class="pp">#endif</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> <span class="pp">#elif __clang__</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> <span class="pp">#define __USE_RVALUE_REFERENCES </span>__has_feature<span class="op">(</span>cxx_rvalue_references<span class="op">)</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a> <span class="pp">#else</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> <span class="pp">#define __USE_RVALUE_REFERENCES </span><span class="dv">0</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> <span class="pp">#endif</span></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
<p>First, the GNU and Microsoft version numbers are checked to see if
they are high enough. But then a check is made of the EDG version
number, since that front end also has compatibility modes for both those
compilers, and defines macros indicating (claimed) compatibility with
them. If the feature wasn’t implemented in the indicated EDG version, it
is assumed that the feature is not available – even though it is
possible for a customer of EDG to implement a feature before EDG
does.</p>
<p>Fortunately Clang has ways to test specifically for the presence of
specific features. But unfortunately, the function-call-like syntax used
for such tests won’t work with a standard preprocessor, so this fine new
feature winds up adding its own flavor of complexity to the mix.</p>
<p>Also note that this code is only the beginning of a real-world
solution. A complete solution would need to take into account more
compilers, and also command-line option settings specific to various
compilers.</p>
<h2 data-number="1.2" id="characteristics-of-the-proposed-solution"><span class="header-section-number">1.2</span> Characteristics of the proposed
solution<a href="#characteristics-of-the-proposed-solution" class="self-link"></a></h2>
<p>To preserve implementers’ freedom to add features in the order that
makes the most sense for themselves and their customers, implementers
should indicate the availability of each separate feature by adding a
definition of a macro with the name corresponding to that feature.</p>
<p>Important note: By recommending the use of these macros, WG21 is not
making any feature optional; the absence of a definition for the
relevant feature-test macro does not make an implementation that lacks a
feature conform to a standard that requires the feature. However, if
implementers and programmers follow these recommendations, portability
of code between real-world implementations should be improved.</p>
<p>To a first approximation, a feature is identified by the WG21 paper
in which it is specified, and by which it is introduced into the working
draft of the standard. Not every paper introduces a new feature worth a
feature-test macro, but every paper that is not just a collection of
issue resolutions is considered a candidate; exceptions are explicitly
justified.</p>
<p>For C++14, the feature-test macro name generally consists of some
combination of words from the title of the paper. In the future, it is
hoped that every paper will include its own recommendations concerning
feature-test macro names.</p>
<p>The value specified for a feature-test macro is based on the year and
month in which the feature is voted into the working draft. In a case
where a feature is subsequently changed in a significant way, but
arguably remains the same feature, the value of the macro is changed to
indicate the “revision level” of the specification of the feature.
However, in most cases it is expected that the presence of a feature can
be determined by the presence of any non-zero macro value; for
example:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> use_empty_base_opt <span class="op">:</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> std<span class="op">::</span>integral_constant<span class="op"><</span><span class="dt">bool</span>,</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> std<span class="op">::</span>is_empty<span class="op"><</span>T<span class="op">>::</span>value</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_lib_is_final</span></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a> <span class="op">&&</span> <span class="op">!</span>std<span class="op">::</span>is_final<span class="op"><</span>T<span class="op">>::</span>value</span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a> <span class="op">></span></span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a><span class="op">{</span> <span class="op">}</span>;</span></code></pre></div>
<p>To avoid the user’s namespace, names of macros for language features
are prefixed by <code class="sourceCode cpp">__cpp_</code>; for library
features, by <code class="sourceCode cpp">__cpp_lib_</code>. A library
feature that doesn’t introduce a new header is expected to be defined by
the header(s) that implement the feature.</p>
<h2 data-number="1.3" id="examples"><span class="header-section-number">1.3</span> Examples<a href="#examples" class="self-link"></a></h2>
<p>Selecting a more efficient compile-time implementation based on the
availability of a feature:</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_variadic_using</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="co">// can use the compile-time efficient, flat inheritance</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> <span class="op">...</span>T<span class="op">></span> <span class="kw">struct</span> Callable <span class="op">:</span> T<span class="op">...</span> <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="kw">using</span> T<span class="op">::</span><span class="kw">operator</span><span class="op">()</span> <span class="op">...</span>;</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a><span class="co">// fall-back to linear recursion for older compilers</span></span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> <span class="op">...</span>T<span class="op">></span> <span class="kw">struct</span> Callable;</span>
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T, <span class="kw">typename</span> <span class="op">...</span>U<span class="op">></span></span>
<span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> Callable<span class="op"><</span>T, U<span class="op">...></span> <span class="op">:</span> T, Callable<span class="op"><</span>U<span class="op">...></span> <span class="op">{</span></span>
<span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a> <span class="kw">using</span> T<span class="op">::</span><span class="kw">operator</span><span class="op">()</span>;</span>
<span id="cb3-12"><a href="#cb3-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">using</span> Callable<span class="op"><</span>U<span class="op">...>::</span><span class="kw">operator</span><span class="op">()</span>;</span>
<span id="cb3-13"><a href="#cb3-13" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb3-14"><a href="#cb3-14" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T<span class="op">></span> <span class="kw">struct</span> Callable<span class="op"><</span>T<span class="op">></span> <span class="op">:</span> T <span class="op">{</span></span>
<span id="cb3-15"><a href="#cb3-15" aria-hidden="true" tabindex="-1"></a> <span class="kw">using</span> T<span class="op">::</span><span class="kw">operator</span><span class="op">()</span>;</span>
<span id="cb3-16"><a href="#cb3-16" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb3-17"><a href="#cb3-17" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><></span> <span class="kw">struct</span> Callable<span class="op"><></span> <span class="op">{}</span>;</span>
<span id="cb3-18"><a href="#cb3-18" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<p>Likewise</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_fold_expressions</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span><span class="op">...</span> T<span class="op">></span></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> sum<span class="op">(</span>T<span class="op">...</span> args<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> <span class="op">(</span>args <span class="op">+</span> <span class="op">...)</span>; <span class="op">}</span></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> sum<span class="op">()</span> <span class="op">{</span> <span class="cf">return</span> <span class="dv">0</span>; <span class="op">}</span></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> sum<span class="op">(</span>T t<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> t; <span class="op">}</span></span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">(</span><span class="kw">typename</span> T, <span class="kw">typename</span><span class="op">...</span> Ts<span class="op">)</span></span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> sum<span class="op">(</span>T t, Ts<span class="op">...</span> ts<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> t <span class="op">+</span> sum<span class="op">(</span>ts<span class="op">...)</span>; <span class="op">}</span></span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<p>Selecting a more efficient run-time implementation based on the
availability of a feature:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> update<span class="op">(</span>std<span class="op">::</span>set<span class="op"><</span>X<span class="op">>&</span> set, <span class="kw">const</span> X<span class="op">&</span> elem, <span class="dt">int</span> val<span class="op">)</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> pos <span class="op">=</span> set<span class="op">.</span>find<span class="op">(</span>elem<span class="op">)</span>;</span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span>pos <span class="op">==</span> set<span class="op">.</span>end<span class="op">())</span></span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span>;</span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_lib_node_extract</span></span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> next <span class="op">=</span> std<span class="op">::</span>next<span class="op">(</span>pos<span class="op">)</span>;</span>
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a> <span class="kw">auto</span> x <span class="op">=</span> set<span class="op">.</span>extract<span class="op">(</span>pos<span class="op">)</span>;</span>
<span id="cb5-9"><a href="#cb5-9" aria-hidden="true" tabindex="-1"></a> x<span class="op">.</span>value<span class="op">().</span>update<span class="op">(</span>val<span class="op">)</span>;</span>
<span id="cb5-10"><a href="#cb5-10" aria-hidden="true" tabindex="-1"></a> set<span class="op">.</span>insert<span class="op">(</span>next, std<span class="op">::</span>move<span class="op">(</span>x<span class="op">))</span>;</span>
<span id="cb5-11"><a href="#cb5-11" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb5-12"><a href="#cb5-12" aria-hidden="true" tabindex="-1"></a> X tmp <span class="op">=</span> <span class="op">*</span>pos;</span>
<span id="cb5-13"><a href="#cb5-13" aria-hidden="true" tabindex="-1"></a> pos <span class="op">=</span> set<span class="op">.</span>erase<span class="op">(</span>pos<span class="op">)</span>;</span>
<span id="cb5-14"><a href="#cb5-14" aria-hidden="true" tabindex="-1"></a> tmp<span class="op">.</span>update<span class="op">(</span>val<span class="op">)</span>;</span>
<span id="cb5-15"><a href="#cb5-15" aria-hidden="true" tabindex="-1"></a> set<span class="op">.</span>insert<span class="op">(</span>pos, std<span class="op">::</span>move<span class="op">(</span>tmp<span class="op">))</span>;</span>
<span id="cb5-16"><a href="#cb5-16" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb5-17"><a href="#cb5-17" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>In some cases, the value of a feature-test macro can change over time
as the underlying feature changes. To make it easier to follow the
evolution of each feature, the tables in this document are grouped by
macro name - with a row for each possible value and the proposal(s)
associated with it.</p>
<p>Conditionally implementing a feature, based on <a href="#__cpp_static_assert"><code class="sourceCode cpp">__cpp_static_assert</code></a>.</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_static_assert</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="pp"># if __cpp_static_assert > 201400</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="pp"># define Static_Assert</span><span class="op">(</span>cond<span class="op">)</span><span class="pp"> </span><span class="kw">static_assert</span><span class="op">(</span>cond<span class="op">)</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="pp"># else</span></span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a><span class="pp"># define Static_Assert</span><span class="op">(</span>cond<span class="op">)</span><span class="pp"> </span><span class="kw">static_assert</span><span class="op">(</span>cond,<span class="pp"> #</span>cond<span class="op">)</span></span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a><span class="pp"># endif</span></span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a><span class="pp"># define Static_Assert_Msg</span><span class="op">(</span>cond,<span class="pp"> </span>msg<span class="op">)</span><span class="pp"> </span><span class="kw">static_assert</span><span class="op">(</span>cond,<span class="pp"> </span>msg<span class="op">)</span></span>
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb6-9"><a href="#cb6-9" aria-hidden="true" tabindex="-1"></a><span class="pp"># define Static_Assert</span><span class="op">(</span>cond<span class="op">)</span></span>
<span id="cb6-10"><a href="#cb6-10" aria-hidden="true" tabindex="-1"></a><span class="pp"># define Static_Assert_Msg</span><span class="op">(</span>cond,<span class="pp"> </span>msg<span class="op">)</span></span>
<span id="cb6-11"><a href="#cb6-11" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<p>Attributes can also change semantics over time, which is why the
<code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
facility described below evaluates to a value rather than simply
<code class="sourceCode cpp"><span class="dv">1</span></code> or
<code class="sourceCode cpp"><span class="dv">0</span></code>. This
allows a user to conditionally provide a version of nodiscard based on
<a href="#nodiscard"><code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span><span class="op">(</span>nodiscard<span class="op">)</span></code></a>:</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#if </span><span class="ot">__has_cpp_attribute</span><span class="pp">(nodiscard) >= 201907</span></span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// nodiscard has a reason and can</span></span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> <span class="co">// be applied to constructors</span></span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD</span><span class="op">(</span>msg<span class="op">)</span><span class="pp"> </span><span class="op">[[</span><span class="at">nodiscard</span><span class="op">(</span><span class="at">msg</span><span class="op">)]]</span></span>
<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD_CTOR</span><span class="op">(</span>msg<span class="op">)</span><span class="pp"> </span><span class="op">[[</span><span class="at">nodiscard</span><span class="op">(</span><span class="at">msg</span><span class="op">)]]</span></span>
<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a><span class="pp">#elif </span><span class="ot">__has_cpp_attribute</span><span class="pp">(nodiscard) >= 201603</span></span>
<span id="cb7-7"><a href="#cb7-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// nodiscard doesn't have a reason, nor can</span></span>
<span id="cb7-8"><a href="#cb7-8" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD</span><span class="op">(</span>msg<span class="op">)</span><span class="pp"> </span><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></span>
<span id="cb7-9"><a href="#cb7-9" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD_CTOR</span><span class="op">(</span>msg<span class="op">)</span></span>
<span id="cb7-10"><a href="#cb7-10" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb7-11"><a href="#cb7-11" aria-hidden="true" tabindex="-1"></a> <span class="co">// nodiscard doesn't exist at all yet</span></span>
<span id="cb7-12"><a href="#cb7-12" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD</span><span class="op">(</span>msg<span class="op">)</span></span>
<span id="cb7-13"><a href="#cb7-13" aria-hidden="true" tabindex="-1"></a><span class="pp"># define NODISCARD_CTOR</span><span class="op">(</span>msg<span class="op">)</span></span>
<span id="cb7-14"><a href="#cb7-14" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="recommendations"><span class="header-section-number">2</span>
Recommendations<a href="#recommendations" class="self-link"></a></h1>
<h2 data-number="2.1" id="introduction"><span class="header-section-number">2.1</span> Introduction<a href="#introduction" class="self-link"></a></h2>
<p>For the sake of improved portability between partial implementations
of various C++ standards, WG21 (the ISO technical committee for the C++
programming language) recommends that implementers and programmers
follow the guidelines in this document concerning feature-test
macros.</p>
<p>Implementers who provide a new standard feature should define a macro
with the recommended name and value, in the same circumstances under
which the feature is available (for example, taking into account
relevant command-line options), to indicate the presence of support for
that feature.</p>
<p>Programmers who wish to determine whether a feature is available in
an implementation should base that determination on the state of the
macro with the recommended name. (The absence of a tested feature may
result in a program with decreased functionality, or the relevant
functionality may be provided in a different way. A program that
strictly depends on support for a feature can just try to use the
feature unconditionally; presumably, on an implementation lacking
necessary support, translation will fail. Therefore, if the most useful
purpose for a feature-test macro would be to control the inclusion of a
#error directive if the feature is unavailable, that is considered
inadequate justification for the macro. Note that the usefulness of a
test macro for a feature is completely independent of the usefulness of
the feature itself.)</p>
<h2 data-number="2.2" id="testing-for-the-presence-of-a-header-__has_include"><span class="header-section-number">2.2</span> Testing for the presence of a
header: <code class="sourceCode cpp"><span class="ot">__has_include</span></code><a href="#testing-for-the-presence-of-a-header-__has_include" class="self-link"></a></h2>
<p>It is impossible for a C++ program to directly, reliably, and
portably determine whether or not a library header is available for
inclusion. Conditionally including a header requires the use of a
configuration macro, whose setting can be determined by a
configuration-test process at build time (reliable, but less portable),
or by some other means (often not reliable or portable).</p>
<p>To solve this general problem, WG21 recommends that programmers use
the <code class="sourceCode cpp"><span class="ot">__has_include</span></code>
feature.</p>
<h3 data-number="2.2.1" id="syntax"><span class="header-section-number">2.2.1</span> Syntax<a href="#syntax" class="self-link"></a></h3>
<blockquote>
<div class="line-block"><em>h-preprocessing-token</em>:<br />
any <em>preprocessing-token</em> other than
<code class="sourceCode cpp"><span class="op">></span></code><br />
<br />
<em>h-pp-tokens</em>:<br />
<em>h-preprocessing-token</em><br />
<em>h-pp-tokens</em> <em>h-preprocessing-token</em></div>
</blockquote>
<blockquote>
<div class="line-block"><em>has-include-expression</em>:<br />
<code class="sourceCode cpp"><span class="ot">__has_include</span></code>
<code class="sourceCode cpp"><span class="op">(</span></code>
<em>header-name</em>
<code class="sourceCode cpp"><span class="op">)</span></code><br />
<code class="sourceCode cpp"><span class="ot">__has_include</span></code>
<code class="sourceCode cpp"><span class="op">(</span></code>
<em>string-literal</em>
<code class="sourceCode cpp"><span class="op">)</span></code><br />
<code class="sourceCode cpp"><span class="ot">__has_include</span></code>
<code class="sourceCode cpp"><span class="op">(</span></code>
<code class="sourceCode cpp"><span class="op"><</span></code>
<em>h-pp-tokens</em>
<code class="sourceCode cpp"><span class="op">></span></code>
<code class="sourceCode cpp"><span class="op">)</span></code></div>
</blockquote>
<h3 data-number="2.2.2" id="semantics"><span class="header-section-number">2.2.2</span> Semantics<a href="#semantics" class="self-link"></a></h3>
<p>In the first form of the <em>has-include-expression</em>, the
parenthesized <em>header-name</em> token is not subject to macro
expansion. The second and third forms are considered only if the first
form does not match, and the preprocessing tokens are processed just as
in normal text.</p>
<p>A <em>has-include-expression</em> shall appear only in the
controlling constant expression of a <code class="sourceCode cpp"><span class="pp">#</span><span class="er">if</span></code>
or <code class="sourceCode cpp"><span class="pp">#</span><span class="er">elif</span></code>
directive ([cpp.cond] 16.1). Prior to the evaluation of such an
expression, the source file identified by the parenthesized
preprocessing token sequence in each contained
<em>has-include-expression</em> is searched for as if that preprocessing
token sequence were the <em>pp-tokens</em> in a
<code class="sourceCode cpp"><span class="pp">#include</span></code>
directive, except that no further macro expansion is performed. If such
a directive would not satisfy the syntactic requirements of a
<code class="sourceCode cpp"><span class="pp">#include</span></code>
directive, the program is ill-formed. The
<em>has-include-expression</em> is replaced by the <em>pp-number</em>
<code class="sourceCode cpp"><span class="dv">1</span></code> if the
search for the source file succeeds, and by the <em>pp-number</em>
<code class="sourceCode cpp"><span class="dv">0</span></code> if the
search fails.</p>
<p>The <code class="sourceCode cpp"><span class="pp">#</span><span class="er">ifdef</span></code>
and <code class="sourceCode cpp"><span class="pp">#</span><span class="er">ifndef</span></code>
directives, and the defined conditional inclusion operator, shall treat
<code class="sourceCode cpp"><span class="ot">__has_include</span></code> as
if it were the name of a defined macro. The identifier <code class="sourceCode cpp"><span class="ot">__has_include</span></code>
shall not appear in any context not mentioned in this section.</p>
<h3 data-number="2.2.3" id="example"><span class="header-section-number">2.2.3</span> Example<a href="#example" class="self-link"></a></h3>
<p>This demonstrates a way to use a library optional facility only if it
is available. Note that having <code class="sourceCode cpp"><span class="ot">__has_include</span><span class="op">(<</span>optional<span class="op">>)</span></code>
succeed is insufficient since on many toolchains, headers may exist in
installations but have their contents guarded based on compile flags.
For example, the following:</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifdef __has_include</span></span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a><span class="pp">#if </span><span class="ot">__has_include</span><span class="pp">(<optional>)</span></span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im"><optional></span></span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a>std<span class="op">::</span>optional<span class="op"><</span><span class="dt">int</span><span class="op">></span> o;</span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb8-7"><a href="#cb8-7" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">(){</span> <span class="op">}</span></span></code></pre></div>
<p>will still fail to compile with <code class="sourceCode cpp">g<span class="op">++</span> <span class="op">-</span>std<span class="op">=</span>c<span class="op">++</span><span class="dv">14</span></code>
(using libstdc++).</p>
<p>Hence, we need to do:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifdef __has_include</span></span>
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a><span class="pp"># if </span><span class="ot">__has_include</span><span class="pp">(<optional>)</span></span>
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a><span class="pp"># include </span><span class="im"><optional></span></span>
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a><span class="pp"># if __cpp_lib_optional >= 201606</span></span>
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a><span class="pp"># define have_optional </span><span class="dv">1</span></span>
<span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a><span class="pp"># endif</span></span>
<span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a><span class="pp"># elif </span><span class="ot">__has_include</span><span class="pp">(<experimental/optional>)</span></span>
<span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a><span class="pp"># include </span><span class="im"><experimental/optional></span></span>
<span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a><span class="pp"># if __cpp_lib_experimental_optional >= 201400</span></span>
<span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a><span class="pp"># define have_optional </span><span class="dv">1</span></span>
<span id="cb9-11"><a href="#cb9-11" aria-hidden="true" tabindex="-1"></a><span class="pp"># define experimental_optional </span><span class="dv">1</span></span>
<span id="cb9-12"><a href="#cb9-12" aria-hidden="true" tabindex="-1"></a><span class="pp"># endif</span></span>
<span id="cb9-13"><a href="#cb9-13" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb9-14"><a href="#cb9-14" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb9-15"><a href="#cb9-15" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifndef have_optional</span></span>
<span id="cb9-16"><a href="#cb9-16" aria-hidden="true" tabindex="-1"></a><span class="pp"># define have_optional </span><span class="dv">0</span></span>
<span id="cb9-17"><a href="#cb9-17" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
<p>Additionally, the <code class="sourceCode cpp"><span class="op"><</span>version<span class="op">></span></code>
header <span class="citation" data-cites="P0754R2">[<a href="https://wg21.link/p0754r2" role="doc-biblioref">P0754R2</a>]</span> is a light-weight header that
defines all the standard library feature-test macros. An alternate
implementation could be:</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifndef __has_include</span></span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="pp"># define __has_include</span><span class="op">(</span>x<span class="op">)</span><span class="pp"> </span><span class="dv">0</span></span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a><span class="pp">#if </span><span class="ot">__has_include</span><span class="pp">(<version>)</span></span>
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a><span class="pp"># include </span><span class="im"><version></span></span>
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a><span class="pp">#elif </span><span class="ot">__has_include</span><span class="pp">(<optional>)</span></span>
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a><span class="pp"># include </span><span class="im"><optional></span></span>
<span id="cb10-9"><a href="#cb10-9" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb10-10"><a href="#cb10-10" aria-hidden="true" tabindex="-1"></a><span class="pp">#if __cpp_lib_optional >= 201606</span></span>
<span id="cb10-11"><a href="#cb10-11" aria-hidden="true" tabindex="-1"></a><span class="pp"># define have_optional </span><span class="dv">1</span></span>
<span id="cb10-12"><a href="#cb10-12" aria-hidden="true" tabindex="-1"></a><span class="pp">#else</span></span>
<span id="cb10-13"><a href="#cb10-13" aria-hidden="true" tabindex="-1"></a><span class="pp"># define have_optional </span><span class="dv">0</span></span>
<span id="cb10-14"><a href="#cb10-14" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
<h2 data-number="2.3" id="testing-for-the-presence-of-an-attribute-__has_cpp_attribute"><span class="header-section-number">2.3</span> Testing for the presence of an
attribute: <code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code><a href="#testing-for-the-presence-of-an-attribute-__has_cpp_attribute" class="self-link"></a></h2>
<p>A C++ program cannot directly, reliably, and portably determine
whether or not a standard or vendor-specific attribute is available for
use. Testing for attribute support generally requires complex macro
logic, as illustrated above for language features in general.</p>
<p>To solve this general problem, WG21 recommends that programmers use
the <code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
feature.</p>
<h3 data-number="2.3.1" id="syntax-1"><span class="header-section-number">2.3.1</span> Syntax<a href="#syntax-1" class="self-link"></a></h3>
<blockquote>
<div class="line-block"><em>has-attribute-expression</em>:<br />
<code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
<code class="sourceCode cpp"><span class="op">(</span></code>
<em>attribute-token</em>
<code class="sourceCode cpp"><span class="op">)</span></code></div>
</blockquote>
<h3 data-number="2.3.2" id="semantics-1"><span class="header-section-number">2.3.2</span> Semantics<a href="#semantics-1" class="self-link"></a></h3>
<p>A <em>has-attribute-expression</em> shall appear only in the
controlling constant expression of a <code class="sourceCode cpp"><span class="pp">#</span><span class="er">if</span></code>
or <code class="sourceCode cpp"><span class="pp">#</span><span class="er">elif</span></code>
directive ([cpp.cond] 16.1). The <em>has-attribute-expression</em> is
replaced by a <em>non-zero</em> <em>pp-number</em> if the implementation
supports an attribute with the specified name, and by the
<em>pp-number</em>
<code class="sourceCode cpp"><span class="dv">0</span></code>
otherwise.</p>
<p>For a standard attribute, the value of the <code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
macro is based on the year and month in which the attribute was voted
into the working draft. In the case where the attribute is
vendor-specific, the value is implementation-defined. However, in most
cases it is expected that the availability of an attribute can be
detected by any non-zero result.</p>
<p>The <code class="sourceCode cpp"><span class="pp">#</span><span class="er">ifdef</span></code>
and <code class="sourceCode cpp"><span class="pp">#</span><span class="er">ifndef</span></code>
directives, and the defined conditional inclusion operator, shall treat
<code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
as if it were the name of a defined macro. The identifier <code class="sourceCode cpp"><span class="ot">__has_cpp_attribute</span></code>
shall not appear in any context not mentioned in this section.</p>
<h3 data-number="2.3.3" id="example-1"><span class="header-section-number">2.3.3</span> Example<a href="#example-1" class="self-link"></a></h3>
<p>This demonstrates a way to use the attribute <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>