amorfo77/json
1
0
Fork 0
mirror of https://github.com/nlohmann/json.git synced 2026-10-03 13:10:33 +00:00
Code Issues Packages Projects Releases Wiki Activity

Prefer strtod_l over libc++'s std::from_chars

Browse Source 
libc++'s std::from_chars for float and double is 1.3x to 2.8x slower
per number than Apple's strtod_l, and it was tried before Clinger's
fast path. With Apple clang in C++17 mode, parsing random doubles took
1.75x as long as on develop, short numbers such as 123.45 1.3x, and
mesh.json 1.2x.

Use libc++'s std::from_chars only where the C library has no strtod_l.
On Apple platforms, floats are now converted by Clinger's fast path and
strtod_l, which is 0.90x to 1.01x the time of develop.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 10:12:50 +02:00
parent 436bfb1358
commit 42e3489abd
3 changed files with 87 additions and 83 deletions
Download Patch File Download Diff File Expand all files Collapse all files
single_include/nlohmann/json.hpp
+41 -39
View File
@@ -8505,45 +8505,6 @@ NLOHMANN_JSON_NAMESPACE_END
// #include <nlohmann/detail/macro_scope.hpp>
// std::from_chars lives in <charconv>, but being in C++17 mode does not
// guarantee the header exists: GCC 7 sets __cplusplus to C++17 yet ships no
// <charconv> (added in GCC 8; floating-point support in GCC 11). Guard the
// include with __has_include so such toolchains fall back to the scalar path.
#if defined(JSON_HAS_CPP_17) && defined(__has_include)
#if __has_include(<charconv>)
#include <charconv> // from_chars
#include <system_error> // errc
// std::from_chars is used for floating-point numbers
// - for float, double, and long double if __cpp_lib_to_chars announces
// complete support (only checked in C++17 or later: some standard
// libraries, e.g. libstdc++ 15, define it even in C++14 mode, where
// <charconv> is not included);
// - for float and double with libc++ 20 or later, which does not define
// __cpp_lib_to_chars because long double is missing. On Apple
// platforms, the implementation is part of the system's libc++ and
// only available when deploying to macOS/iOS 26 or later; for older
// deployment targets, _LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT
// is 0, and the fallbacks below are used.
#if defined(__cpp_lib_to_chars)
#define JSON_HAS_FLOAT_FROM_CHARS 1
#define JSON_HAS_LONG_DOUBLE_FROM_CHARS 1
#elif defined(_LIBCPP_VERSION) && defined(_LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT)
#if _LIBCPP_VERSION >= 200000 && _LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT
#define JSON_HAS_FLOAT_FROM_CHARS 1
#endif
#endif
#endif
#endif
#ifndef JSON_HAS_FLOAT_FROM_CHARS
#define JSON_HAS_FLOAT_FROM_CHARS 0
#endif
#ifndef JSON_HAS_LONG_DOUBLE_FROM_CHARS
#define JSON_HAS_LONG_DOUBLE_FROM_CHARS 0
#endif
// strtof_l/strtod_l/strtold_l convert with a given locale object instead of the
// global C locale. They are not part of ISO C or C++, so they are only used where
// the C library is known to declare them: Microsoft's UCRT (as _strtod_l etc.),
@@ -8562,6 +8523,47 @@ NLOHMANN_JSON_NAMESPACE_END
#define JSON_HAS_C_LOCALE_STRTOD 0
#endif
// std::from_chars lives in <charconv>, but being in C++17 mode does not
// guarantee the header exists: GCC 7 sets __cplusplus to C++17 yet ships no
// <charconv> (added in GCC 8; floating-point support in GCC 11). Guard the
// include with __has_include so such toolchains fall back to the scalar path.
#if defined(JSON_HAS_CPP_17) && defined(__has_include)
#if __has_include(<charconv>)
#include <charconv> // from_chars
#include <system_error> // errc
// std::from_chars is used for floating-point numbers
// - for float, double, and long double if __cpp_lib_to_chars announces
// complete support (only checked in C++17 or later: some standard
// libraries, e.g. libstdc++ 15, define it even in C++14 mode, where
// <charconv> is not included);
// - for float and double with libc++ 20 or later, which does not define
// __cpp_lib_to_chars because long double is missing, but only where
// the C library offers no strtod_l: libc++'s implementation is slower
// than Apple's strtod_l (by 1.3x to 2.8x per number), and it would be
// tried before Clinger's fast path. On Apple platforms, it is also only
// available when deploying to macOS/iOS 26 or later; for older
// deployment targets, _LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT
// is 0.
#if defined(__cpp_lib_to_chars)
#define JSON_HAS_FLOAT_FROM_CHARS 1
#define JSON_HAS_LONG_DOUBLE_FROM_CHARS 1
#elif !JSON_HAS_C_LOCALE_STRTOD && defined(_LIBCPP_VERSION) && defined(_LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT)
#if _LIBCPP_VERSION >= 200000 && _LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT
#define JSON_HAS_FLOAT_FROM_CHARS 1
#endif
#endif
#endif
#endif
#ifndef JSON_HAS_FLOAT_FROM_CHARS
#define JSON_HAS_FLOAT_FROM_CHARS 0
#endif
#ifndef JSON_HAS_LONG_DOUBLE_FROM_CHARS
#define JSON_HAS_LONG_DOUBLE_FROM_CHARS 0
#endif
// This file contains the value-conversion helpers used by the lexer to turn an
// already-validated number token into a value, where possible without the
// locale/errno overhead of std::strtoull/std::strtod. They are free functions so