Feature/dragonbox floats

CMakeLists.txt

@@ -244,6 +244,7 @@ set(FlatBuffers_Tests_SRCS
   tests/alignment_test.cpp
   tests/64bit/offset64_test.h
   tests/64bit/offset64_test.cpp
+  tests/float_to_string_test.cpp
   include/flatbuffers/code_generators.h
   src/code_generators.cpp
 )
@@ -543,7 +544,11 @@ if(FLATBUFFERS_BUILD_TESTS)
   )
 
   # Have tests load data from the source directory, not the build directory.
-  add_definitions(-DFLATBUFFERS_TEST_PATH_PREFIX=${CMAKE_CURRENT_SOURCE_DIR}/)
+  target_compile_definitions(flattests 
+    PRIVATE
+      -DFLATBUFFERS_TEST_PATH_PREFIX=${CMAKE_CURRENT_SOURCE_DIR}/
+      # -DFLATBUFFERS_PREFER_OLD_FLOATTOSTRING
+  )
 
   # The flattest target needs some generated files
   SET(FLATC_OPT_COMP --cpp --gen-compare --gen-mutable --gen-object-api --reflect-names)

include/flatbuffers/util.h

@@ -155,33 +155,46 @@ inline std::string NumToString<char>(char t) {
   return NumToString(static_cast<int>(t));
 }
 
+// definitions in util.cpp
+template <typename Float>
+std::string FloatToStringImpl(Float value);
+
 // Special versions for floats/doubles.
 template <typename T>
 std::string FloatToString(T t, int precision) {
   // clang-format off
 
   #ifndef FLATBUFFERS_PREFER_PRINTF
-    // to_string() prints different numbers of digits for floats depending on
-    // platform and isn't available on Android, so we use stringstream
-    std::stringstream ss;
-    // Use std::fixed to suppress scientific notation.
-    ss << std::fixed;
-    // Default precision is 6, we want that to be higher for doubles.
-    ss << std::setprecision(precision);
-    ss << t;
-    auto s = ss.str();
+    #ifndef FLATBUFFERS_PREFER_OLD_FLOATTOSTRING
+      (void)precision;  // unused, can't use [[maybe_unused]]
+      return FloatToStringImpl(t);
+    #else  // FLATBUFFERS_PREFER_OLD_FLOATTOSTRING
+      // to_string() prints different numbers of digits for floats depending on
+      // platform and isn't available on Android, so we use stringstream
+      std::stringstream ss;
+      // Use std::fixed to suppress scientific notation.
+      ss << std::fixed;
+      // Default precision is 6, we want that to be higher for doubles.
+      ss << std::setprecision(precision);
+      ss << t;
+      auto s = ss.str();
+    #endif // FLATBUFFERS_PREFER_OLD_FLOATTOSTRING
   #else // FLATBUFFERS_PREFER_PRINTF
     auto v = static_cast<double>(t);
     auto s = NumToStringImplWrapper(v, "%0.*f", precision);
   #endif // FLATBUFFERS_PREFER_PRINTF
+
+  #if defined(FLATBUFFERS_PREFER_OLD_FLOATTOSTRING) || defined(FLATBUFFERS_PREFER_PRINTF)
+    // Sadly, std::fixed turns "1" into "1.00000", so here we undo that.
+    auto p = s.find_last_not_of('0');
+    if (p != std::string::npos) {
+      // Strip trailing zeroes. If it is a whole number, keep one zero.
+      s.resize(p + (s[p] == '.' ? 2 : 1));
+    }
+
+    return s;
+  #endif
   // clang-format on
-  // Sadly, std::fixed turns "1" into "1.00000", so here we undo that.
-  auto p = s.find_last_not_of('0');
-  if (p != std::string::npos) {
-    // Strip trailing zeroes. If it is a whole number, keep one zero.
-    s.resize(p + (s[p] == '.' ? 2 : 1));
-  }
-  return s;
 }
 
 template <>

src/BUILD.bazel

@@ -44,6 +44,7 @@ cc_library(
         "idl_parser.cpp",
         "reflection.cpp",
         "util.cpp",
+        "dragonbox.h"
     ],
     hdrs = [
         "//:public_headers",
@@ -141,6 +142,7 @@ cc_library(
         "idl_namer.h",
         "namer.h",
         "util.cpp",
+        "dragonbox.h",
     ],
     hdrs = [
         "//:flatc_headers",