[committed] libstdc++: Implement C++23 P1951R1 (Default Args for pair's Forwarding Ctor) [PR105505]
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Commit Message
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
@@ -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;
new file mode 100644
@@ -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() );
+}