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Section: 20.2.7.2 [allocator.uses.construction] Status: New Submitter: Jiang An Opened: 2022-02-16 Last modified: 2022-03-04
Priority: 2
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Discussion:
It seems unclear whether cv-qualified pair specializations are considered as specializations of pair in 20.2.7.2 [allocator.uses.construction].
Currently MSVC STL only considered cv-unqualified pair types as such specializations, while libstdc++ accept both cv-unqualified and const-qualified pair types as such specializations. The resolution of LWG 3525 uses remove_cv_t, which possibly imply that the specialization of pair may be cv-qualified. The difference can be observed via the following program:
#include <utility>
#include <memory>
#include <vector>
#include <cassert>
template<class T>
class payload_ator {
int payload{};
public:
payload_ator() = default;
constexpr explicit payload_ator(int n) noexcept : payload{n} {}
template<class U>
constexpr explicit payload_ator(payload_ator<U> a) noexcept : payload{a.payload} {}
friend bool operator==(payload_ator, payload_ator) = default;
template<class U>
friend constexpr bool operator==(payload_ator x, payload_ator<U> y) noexcept
{
return x.payload == y.payload;
}
using value_type = T;
constexpr T* allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }
constexpr void deallocate(T* p, std::size_t n) { return std::allocator<T>{}.deallocate(p, n); }
constexpr int get_payload() const noexcept { return payload; }
};
bool test()
{
constexpr int in_v = 42;
using my_pair_t = std::pair<int, std::vector<int, payload_ator<int>>>;
auto out_v = std::make_obj_using_allocator<const my_pair_t>(payload_ator<int>{in_v}).second.get_allocator().get_payload();
return in_v == out_v;
}
int main()
{
assert(test()); // passes only if a const-qualified pair specialization is considered as a pair specialization
}
[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll.
Proposed resolution: