Summary

Allocators are a fundamentally arcane topic in C++, mainly for historical reasons. Several different interfaces, with different obscure use-cases, are piled one on top of the other; all of them involve intense metaprogramming; and vendor support for many of these features, even relatively old C++11 features such as fancy pointers, is still lacking.

C++17 offers the standard library type std::pmr::memory_resource to clarify the existing distinction between memory resources (a.k.a. heaps) and allocators (a.k.a. handles to heaps). Memory resources provide allocate and deallocate methods; allocators provide those methods as well as construct and destroy.

If you implement your own allocator type A, it must be a template; its first template parameter should be the type T that it expects to allocate. Your allocator type A must also have a templated constructor to support "rebinding" from A<U> to A<T>. Just like any other kind of pointer, an allocator type must support the == and...