Does this class satisfy the Allocator requirement? - c++

Question

I made a custom allocator, but my code didn't compile on msvc and I'm not sure if my implementation satisfies the Allocator requirement (disregarding actual behavior of function implementations here). Here is a minimal example that reproduces the error on Visual Studio (16.11 P1 and 16.10):
#include <memory>
#include <vector>
template <typename T>
class Allocator
{
public:
using value_type = T;
[[nodiscard]]
T* allocate(std::size_t n)
{
return nullptr;
}
void deallocate(T* x, std::size_t n)
{
}
constexpr bool operator==(const Allocator& other) const noexcept
{
return true;
}
constexpr bool operator!=(const Allocator& other) const noexcept
{
return !(*this == other);
}
};
int main()
{
using Alloc = Allocator<int>;
using Vec = std::vector<int, Alloc>;
auto vec = Vec();
}
Godbolt isn't complaining for any major compiler but their msvc version is a little behind I think.
To me this looks like a compiler bug in msvc but I want to make sure before I open a ticket.
This is the compiler output:
Build started...
1>------ Build started: Project: Project1, Configuration: Debug x64 ------
1>main.cpp
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(714,27): error C2440: 'static_cast': cannot convert from 'Allocator<int>' to 'Allocator<_Newfirst>'
1> with
1> [
1> _Newfirst=std::_Container_proxy
1> ]
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(714,27): message : No constructor could take the source type, or constructor overload resolution was ambiguous
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(711): message : while compiling class template member function 'std::vector<int,Alloc>::~vector(void) noexcept'
1>C:\code\dumpster\Project1\Project1\main.cpp(36): message : see reference to function template instantiation 'std::vector<int,Alloc>::~vector(void) noexcept' being compiled
1>C:\code\dumpster\Project1\Project1\main.cpp(36): message : see reference to class template instantiation 'std::vector<int,Alloc>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(714,25): error C2530: '_Alproxy': references must be initialized
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(715,1): error C3536: '_Alproxy': cannot be used before it is initialized
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(715,9): error C2672: '_Delete_plain_internal': no matching overloaded function found
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(715,1): error C2893: Failed to specialize function template 'void std::_Delete_plain_internal(_Alloc &,_Alloc::value_type *const ) noexcept'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\xmemory(998): message : see declaration of 'std::_Delete_plain_internal'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(715,1): message : With the following template arguments:
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.29.30129\include\vector(715,1): message : '_Alloc=int'
1>Done building project "Project1.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
Edit
I opened a bug ticket for the msvc devs.

Answer 1

It does not.
An allocator rebound to a different value type must be constructible from the original allocator - this is the A a(b) row in the requirements you linked.
Your type fails that requirement.