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Only compile the simdutf backend from C++17 on

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simdutf.h rejects anything below C++17 with an #error, so defining
JSON_USE_SIMDUTF in a C++11 or C++14 translation unit did not fail with a
message about simdutf being unavailable - it failed to compile at all, taking
the library's C++11 support with it. Nothing caught this because no build ever
compiled that path.

Gate the include and both uses on JSON_HAS_CPP_17, the same way number_parse.hpp
gates std::from_chars. Below C++17 the macro now has no effect and the scalar
validator runs; it accepts and rejects exactly the same input, so the macro is
safe to set project-wide even when some translation units use an older standard.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MXDi7NTMKAmoArUZSKMc4T
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 14:59:30 +00:00
committed by Claude
co-authored by Claude
parent 78e2a87ab1
commit feed1a48a4
3 changed files with 40 additions and 21 deletions
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include/nlohmann/detail/input/string_scan.hpp
+16 -10
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@@ -12,17 +12,23 @@
#include <cstdint> // uint64_t
#include <cstring> // memcpy
#if defined(JSON_USE_SIMDUTF)
// Optional SIMD backend for bulk UTF-8 validation. This is an opt-in
// external dependency: nlohmann/json itself stays header-only and the C++11
// scalar validator below is always available; defining JSON_USE_SIMDUTF
// additionally requires the simdutf headers on the include path and linking
// the simdutf library. See string_bulk_run().
#include <nlohmann/detail/macro_scope.hpp>
// Optional SIMD backend for bulk UTF-8 validation. This is an opt-in external
// dependency: nlohmann/json itself stays header-only and the C++11 scalar
// validator below is always available; defining JSON_USE_SIMDUTF additionally
// requires the simdutf headers on the include path and linking the simdutf
// library. See string_bulk_run().
//
// simdutf.h itself requires C++17 - it rejects older standards with an #error -
// so the backend is only compiled in from C++17 on. Below that the macro has no
// effect and the scalar validator is used; it accepts and rejects exactly the
// same input, so only throughput differs. macro_scope.hpp is included above to
// have JSON_HAS_CPP_17 available for this test.
#if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
#include <simdutf.h>
#endif
#include <nlohmann/detail/macro_scope.hpp>
// This file contains the byte-level string-scanning helpers used by the lexer's
// contiguous fast path. They operate purely on raw bytes (no dependency on the
// lexer's template parameters) so they are free functions, keeping the lexer
@@ -174,7 +180,7 @@ inline std::size_t scalar_string_bulk_run(const unsigned char* data, std::size_t
return pos;
}
#if defined(JSON_USE_SIMDUTF)
#if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
// Index of the first quote/escape/control byte in [data, data+n) (non-ASCII
// bytes are *not* stops here - the whole run is handed to simdutf), or n.
inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t n) noexcept
@@ -221,7 +227,7 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
// produces the precise diagnostic. Without it, the pure scalar path is used.
inline std::size_t string_bulk_run(const unsigned char* data, std::size_t n) noexcept
{
#if defined(JSON_USE_SIMDUTF)
#if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
const std::size_t run = find_string_delimiter(data, n);
if (run != 0 && simdutf::validate_utf8(reinterpret_cast<const char*>(data), run))
{