Tested aarch64-linux, pushed to trunk. -- >8 --
This was approved for C++23 at he June 2021 virtual meeting. This allows more efficient construction of std::pair members when {} is used as a constructor argument. The perfect forwarding constructor can be used, so that the member variables are constructed from forwarded arguments instead of being copy constructed from temporaries. libstdc++-v3/ChangeLog: PR libstdc++/105505 * include/bits/stl_pair.h (pair::pair(U1&&, U2&&)) [C++23]: Add default template arguments, as per P1951R1. * testsuite/20_util/pair/cons/default_tmpl_args.cc: New test. --- libstdc++-v3/include/bits/stl_pair.h | 8 ++++ .../20_util/pair/cons/default_tmpl_args.cc | 48 +++++++++++++++++++ 2 files changed, 56 insertions(+) create mode 100644 libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h index ce8826ae6a8..52f532f3c39 100644 --- a/libstdc++-v3/include/bits/stl_pair.h +++ b/libstdc++-v3/include/bits/stl_pair.h @@ -308,7 +308,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { } /// Constructor accepting two values of arbitrary types +#if __cplusplus > 202002L + template<typename _U1 = _T1, typename _U2 = _T2> +#else template<typename _U1, typename _U2> +#endif requires (_S_constructible<_U1, _U2>()) && (!_S_dangles<_U1, _U2>()) constexpr explicit(!_S_convertible<_U1, _U2>()) pair(_U1&& __x, _U2&& __y) @@ -316,7 +320,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y)) { } +#if __cplusplus > 202002L + template<typename _U1 = _T1, typename _U2 = _T2> +#else template<typename _U1, typename _U2> +#endif requires (_S_constructible<_U1, _U2>()) && (_S_dangles<_U1, _U2>()) constexpr explicit(!_S_convertible<_U1, _U2>()) pair(_U1&&, _U2&&) = delete; diff --git a/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc b/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc new file mode 100644 index 00000000000..5960bf72e21 --- /dev/null +++ b/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc @@ -0,0 +1,48 @@ +// { dg-do compile { target c++23 } } + +// P1951R1 Default Arguments for pair's Forwarding Constructor + +#include <utility> +#include <vector> +#include <string> +#include <testsuite_hooks.h> + +void +test_p1951r1_example() +{ + std::pair<std::string, std::vector<std::string>> p("hello", {}); +} + +struct Counter +{ + constexpr Counter() = default; + constexpr Counter(int) { } + constexpr Counter(const Counter& c) : copies(c.copies + 1), moves(c.moves) { } + constexpr Counter(Counter&& c) : copies(c.copies), moves(c.moves+1) { } + int copies = 0; + int moves = 0; +}; + +constexpr bool +test_count_copies() +{ + std::pair<Counter, Counter> p1(1, {}); + VERIFY( p1.first.copies == 0 && p1.second.copies == 0 ); + VERIFY( p1.first.moves == 0 && p1.second.moves == 1 ); + + std::pair<Counter, Counter> p2({}, 1); + VERIFY( p2.first.copies == 0 && p2.second.copies == 0 ); + VERIFY( p2.first.moves == 1 && p2.second.moves == 0 ); + + std::pair<Counter, Counter> p3({}, {}); + VERIFY( p3.first.copies == 0 && p3.second.copies == 0 ); + VERIFY( p3.first.moves == 1 && p3.second.moves == 1 ); + + return true; +} + +int main() +{ + test_p1951r1_example(); + static_assert( test_count_copies() ); +} -- 2.43.0