* Use standard library helpers where available, with C++11 fallbacks
Add detail helpers that call the standard library function when the
library provides it (checked with the __cpp_lib_* feature-test macros, so
no new ABI-tag macros) and a C++11 implementation otherwise:
- detail::clamp (std::clamp): by value, so that static constexpr members
need no out-of-line definition before C++17; used in json_view's dump()
buffer sizing, where step() can now be a static constexpr member.
- detail::bit_cast (std::bit_cast): replaces dtoa_impl::reinterpret_bits
and the memcpy bit copies in the number parser, to_chars, the json_view
serializer, and binary_writer::to_char_type.
- detail::byteswap (std::byteswap): one implementation instead of the
binary writer's own byte_swap() overloads and the bswap chain in
to_chars' store_msb_first().
- count_leading_zeros/count_trailing_zeros: two 32-bit _BitScanReverse/
_BitScanForward calls for MSVC x86/ARM32 instead of the loop, and
std::countl_zero/countr_zero for compilers without builtins.
- conjunction, disjunction, negation, bool_constant, and uncvref_t map to
their std counterparts in C++17/C++20.
Builtins and intrinsics stay ahead of the <bit> functions, as MSVC's
std::countl_zero checks for LZCNT at run time unless AVX2 is enabled.
The generated code is unchanged with Clang (C++11/20/23, arm64 and
x86-64) and GCC 16 for parsing, dumping, and the binary formats. The
binary reader keeps its own byte_swap() and the writer its memcpy calls:
the smaller code from detail::byteswap/std::bit_cast there made GCC 16
inline get_number()/write_number() differently, which slowed reading by
10-18 % and writing by about 1 %.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Name the target byte order once (JSON_LITTLE_ENDIAN_TARGET, JSON_BIG_ENDIAN_TARGET)
to_chars.hpp and bit_ops.hpp repeated the long __BYTE_ORDER__/_MSC_VER
conditions; macro_scope.hpp now defines them once (undefined again in
macro_unscope.hpp).
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Explain why the serializer growth bound is not a clamp
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
3e031b06a5