std::scoped_allocator_adaptor

The std::scoped_allocator_adaptor class template is an allocator which can be used with multilevel containers (vector of sets of lists of tuples of maps, etc). It is instantiated with one outer allocator type OuterAlloc and zero or more inner allocator types InnerAlloc.... A container constructed directly with a scoped_allocator_adaptor uses OuterAlloc to allocate its elements, but if an element is itself a container, it uses the first inner allocator. The elements of that container, if they are themselves containers, use the second inner allocator, etc. If there are more levels to the container than there are inner allocators, the last inner allocator is reused for all further nested containers.

The purpose of this adaptor is to correctly initialize stateful allocators in nested containers, such as when all levels of a nested container must be placed in the same shared memory segment. The adaptor's constructor takes the arguments for all allocators in the list, and each nested container obtains its allocator's state from the adaptor as needed.

For the purpose of scoped_allocator_adaptor, if the next inner allocator is A, any class T for which std::uses_allocator<T,A>::value == true participates in the recursion as if it was a container. Additionally, std::pair is treated as such a container by specific overloads of scoped_allocator_adaptor::construct.

Typical implementation holds an instance of a std::scoped_allocator_adaptor<InnerAllocs...> as a member object.

Note that std::pmr::polymorphic_allocators propagate to nested containers following uses-allocator construction and do not need (and do not work with) std::scoped_allocator_adaptor.