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Properly constrain detail::copy optimization #4474

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Merged
merged 1 commit into from
Jun 21, 2025
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Merged

Properly constrain detail::copy optimization #4474

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19 changes: 18 additions & 1 deletion include/fmt/base.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2032,6 +2032,17 @@ struct has_back_insert_iterator_container_append<
.append(std::declval<InputIt>(),
std::declval<InputIt>()))>> : std::true_type {};

template <typename OutputIt, typename InputIt, typename = void>
struct has_back_insert_iterator_container_insert_at_end : std::false_type {};

template <typename OutputIt, typename InputIt>
struct has_back_insert_iterator_container_insert_at_end<
OutputIt, InputIt,
void_t<decltype(get_container(std::declval<OutputIt>())
.insert(get_container(std::declval<OutputIt>()).end(),
std::declval<InputIt>(),
std::declval<InputIt>()))>> : std::true_type {};

// An optimized version of std::copy with the output value type (T).
template <typename T, typename InputIt, typename OutputIt,
FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value&&
Expand All @@ -2046,6 +2057,8 @@ FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
template <typename T, typename InputIt, typename OutputIt,
FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value &&
!has_back_insert_iterator_container_append<
OutputIt, InputIt>::value &&
has_back_insert_iterator_container_insert_at_end<
OutputIt, InputIt>::value)>
FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
-> OutputIt {
Expand All @@ -2055,7 +2068,11 @@ FMT_CONSTEXPR20 auto copy(InputIt begin, InputIt end, OutputIt out)
}

template <typename T, typename InputIt, typename OutputIt,
FMT_ENABLE_IF(!is_back_insert_iterator<OutputIt>::value)>
FMT_ENABLE_IF(!(is_back_insert_iterator<OutputIt>::value &&
(has_back_insert_iterator_container_append<
OutputIt, InputIt>::value ||
has_back_insert_iterator_container_insert_at_end<
OutputIt, InputIt>::value)))>
FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
while (begin != end) *out++ = static_cast<T>(*begin++);
return out;
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11 changes: 11 additions & 0 deletions test/base-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -279,6 +279,17 @@ TEST(base_test, is_back_insert_iterator) {
std::front_insert_iterator<std::string>>::value);
}

struct minimal_container {
using value_type = char;
void push_back(char) {}
};

TEST(base_test, copy) {
minimal_container c;
static constexpr char str[] = "a";
fmt::detail::copy<char>(str, str + 1, std::back_inserter(c));
}

TEST(base_test, get_buffer) {
mock_buffer<char> buffer;
void* buffer_ptr = &buffer;
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