git
/
gcc
mirror of git://gcc.gnu.org/git/gcc.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

libstdc++: Fix std::get<T> for std::pair with reference members [PR121745] 

Make the std::get<T> overloads for rvalues use std::forward<T>(p.first)
not std::move(p.first), so that lvalue reference members are not
incorrectly converted to rvalues.

It might appear that std::move(p).first would also work, but the
language rules say that for std::pair<T&&, U> that would produce T&
rather than the expected T&& (see the discussion in P2445R1 §8.2).

Additional tests are added to verify all combinations of reference
members, value categories, and const-qualification.

libstdc++-v3/ChangeLog:

	PR libstdc++/121745
	* include/bits/stl_pair.h (get): Use forward instead of move in
	std::get<T> overloads for rvalue pairs.
	* testsuite/20_util/pair/astuple/get_by_type.cc: Check all value
	categories and cv-qualification.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely 2025-09-01 18:12:27 +01:00 committed by Jonathan Wakely
parent dd6fe9f557
commit c8a24f60b6
No known key found for this signature in database
2 changed files with 56 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
libstdc++-v3/include/bits/stl_pair.h
View File

@ -1315,12 +1315,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template <typename _Tp, typename _Up>
constexpr _Tp&&
get(pair<_Tp, _Up>&& __p) noexcept
{ return std::move(__p.first); }
{ return std::forward<_Tp>(__p.first); }
template <typename _Tp, typename _Up>
constexpr const _Tp&&
get(const pair<_Tp, _Up>&& __p) noexcept
{ return std::move(__p.first); }
{ return std::forward<const _Tp>(__p.first); }
template <typename _Tp, typename _Up>
constexpr _Tp&
@ -1335,12 +1335,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template <typename _Tp, typename _Up>
constexpr _Tp&&
get(pair<_Up, _Tp>&& __p) noexcept
{ return std::move(__p.second); }
{ return std::forward<_Tp>(__p.second); }
template <typename _Tp, typename _Up>
constexpr const _Tp&&
get(const pair<_Up, _Tp>&& __p) noexcept
{ return std::move(__p.second); }
{ return std::forward<const _Tp>(__p.second); }
#endif // __glibcxx_tuples_by_type

52
libstdc++-v3/testsuite/20_util/pair/astuple/get_by_type.cc
View File

@ -33,3 +33,55 @@ void test01()
const int&& cpsecond __attribute__((unused)) =
std::get<int>(std::move(cp));
}
// PR libstdc++/121745 return of get(pair<_Up, _Tp>&& __p) may be ill-formed
void
test_pr121745(std::pair<float&, int&> p)
{
float& pfirst = std::get<float&>(std::move(p));
int& psecond = std::get<int&>(std::move(p));
const auto& p2 = p;
float& p2first = std::get<float&>(std::move(p2));
int& p2second = std::get<int&>(std::move(p2));
}
template<typename T, typename Pair>
using get_t = decltype(std::get<T>(std::declval<Pair>()));
// Check that get<T>(Pair) returns Ret
template<typename T, typename Pair, typename Ret>
constexpr bool verify = std::is_same<get_t<T, Pair>, Ret>::value;
template<typename T1, typename T2>
void
check()
{
// Overloads for accessing first member
static_assert( verify<T1, std::pair<T1, T2>&, T1&>,
"T1& get(pair<T1, T2>&)" );
static_assert( verify<T1, const std::pair<T1, T2>&, const T1&>,
"const T1& get(const pair<T1, T2>&)" );
static_assert( verify<T1, std::pair<T1, T2>&&, T1&&>,
"T1&& get(pair<T1, T2>&&)" );
static_assert( verify<T1, const std::pair<T1, T2>&&, const T1&&>,
"const T1&& get(const pair<T1, T2>&&)" );
// Overloads for accessing second member
static_assert( verify<T2, std::pair<T1, T2>&, T2&>,
"T2& get(pair<T1, T2>&)" );
static_assert( verify<T2, const std::pair<T1, T2>&, const T2&>,
"const T2& get(const pair<T1, T2>&)" );
static_assert( verify<T2, std::pair<T1, T2>&&, T2&&>,
"T2&& get(pair<T1, T2>&&)" );
static_assert( verify<T2, const std::pair<T1, T2>&&, const T2&&>,
"const T2&& get(const pair<T1, T2>&&)" );
}
void
test_all()
{
check<float, int>();
check<float&, int&>();
check<float&&, int&&>();
}