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Extend memcpy fast path to sized sentinels (e.g. std::counted_iterator) (#5268)

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This commit is contained in:
Niels Lohmann
2026-07-12 09:16:06 +02:00
committed by GitHub
parent b2b47c69b1
commit c197feff81
11 changed files with 81 additions and 21 deletions
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include/nlohmann/detail/input/input_adapters.hpp
+13 -4
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@@ -220,20 +220,29 @@ class iterator_input_adapter
// whether IteratorType refers to a contiguous range and therefore supports
// a std::memcpy fast path (pointers always do; in C++20 we can also detect
// library iterators such as those of std::vector and std::string).
// The fast path also requires SentinelType == IteratorType so std::distance works.
// Computing the available element count needs either same-type iterators
// (plain std::distance) or, in C++20, a sized sentinel (std::ranges::distance),
// e.g. std::counted_iterator paired with std::default_sentinel_t.
static constexpr bool iterator_is_contiguous =
std::is_same<IteratorType, SentinelType>::value && (
#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
std::contiguous_iterator<IteratorType> ||
(std::is_same<IteratorType, SentinelType>::value || std::sized_sentinel_for<SentinelType, IteratorType>)
&& (std::contiguous_iterator<IteratorType> || std::is_pointer<IteratorType>::value);
#else
std::is_same<IteratorType, SentinelType>::value && std::is_pointer<IteratorType>::value;
#endif
std::is_pointer<IteratorType>::value);
// contiguous fast path: bulk copy the remaining range with std::memcpy
template<class T>
std::size_t get_elements_impl(T* dest, std::size_t count, std::true_type /*contiguous*/)
{
const std::size_t wanted = count * sizeof(T);
#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
// std::ranges::distance also supports sized sentinels of a different
// type (e.g. std::counted_iterator + std::default_sentinel_t)
const std::size_t available = static_cast<std::size_t>(std::ranges::distance(current, end)) * sizeof(char_type);
#else
const std::size_t available = static_cast<std::size_t>(std::distance(current, end)) * sizeof(char_type);
#endif
const std::size_t copied = (std::min)(wanted, available);
if (JSON_HEDLEY_LIKELY(copied != 0))
{