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I have some issues with map/unordered_map and custom allocator my allocator looks like this, according to documentation of visual studio. I derived my allocator from base type to ensure all template types like allocator::value_type is set correctly.

template <class T>
class std_allocator : public std::allocator<T> {
   public:
    std_allocator() noexcept;
    std_allocator(const std_allocator& aOther) noexcept;
    template <class O>
    std_allocator(const std_allocator<O>& aOther) noexcept;

   public:
    void deallocate(T* const aPtr, const size_t aCount);
    T* allocate(const size_t aCount);
};

now i defined a my unordered map:

class Test {
   private:
    std::unordered_map<const SomeObject*,
                       void*,
                       std::hash<const SomeObject*>,
                       std::equal_to<const SomeObject*>,
                       std_allocator<std::pair<const SomeObject*, void*>>>
        mData;
};

no i got the following compiler error: :\Development\Microsoft\Visual Studio 2019\VC\Tools\MSVC\14.28.29333\include\list(784,49): error C2338: list<T, Allocator> requires that Allocator's value_type match T (See N4659 26.2.1 [container.requirements.general]/16 allocator_type) Either fix the allocator value_type or define _ENFORCE_MATCHING_ALLOCATORS=0 to suppress this diagnostic.

from the unodered_map header the template looks like this

template <class _Kty, class _Ty, class _Hasher = hash<_Kty>, class _Keyeq = equal_to<_Kty>,
    class _Alloc = allocator<pair<const _Kty, _Ty>>>

from my point of view it looks correct. I also tried using a key without "const" except for the pair definition in allocator. the error says that I may disable the error by defining a constant but I guess that would be not a good idea. can some give some advice here?

Cheers

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  • I am not sure that you have defined all the needed member functions, I seem to remember there need to be a lot more. Commented Nov 14, 2020 at 19:44
  • According to the c++17 documentation you just need allocate and deallocate plus several c'tors. All other functions are marked as deprecared and will be removed in c++20. I use the same allocator in vectors and it works without issues Commented Nov 14, 2020 at 19:52

1 Answer 1

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The key detail:

class _Alloc = allocator<pair<const _Kty, _Ty>>>

The const part is the key. The key itself must be constant, which is not the same as a pointer to a constant object. There's a difference between a pointer to a constant object, and constant pointer to a (possibly const) object.

Your map definitely appears to be keyed by a pointer to a constant object. gcc 10 compiles this:

#include <memory>
#include <unordered_map>

template <class T>
class std_allocator : public std::allocator<T> {
   public:
    std_allocator() noexcept;
    std_allocator(const std_allocator& aOther) noexcept;
    template <class O>
    std_allocator(const std_allocator<O>& aOther) noexcept;

   public:
    void deallocate(T* const aPtr, const size_t aCount);
    T* allocate(const size_t aCount);
};

class SomeObject {};

class Test {
   private:
    std::unordered_map<const SomeObject*,
                       void*,
                       std::hash<const SomeObject*>,
                       std::equal_to<const SomeObject*>,
                       std_allocator<std::pair<const SomeObject* const, void*>>>
        mData;
};
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1 Comment

yes I see the const is key here. putting an additonal const to make the object const and not the pointer works. thanks

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