How to get at a C++ Container<T>'s T if no Container::value_type is provided?

Question

It's rather common that container templates contain a value_type typedef. This makes it easy to create other templated code, most recently concepts, which are able to extract the T if only having been given Container and not Container<T>.

However, not all containers (or other templated classes) define such a value_type, especially older ones. Is it possible to get to the contained T even without it?

I know there are tricks like "if it is an iterator then .begin() should return a value of that type", but that does not help us to e.g. write a concept-requirement that checks whether a class's .begin() indeed follows the requirements that iterators have.

Answer 1

Here's a solution that's similar to Quimby's solution but using partial template specialization instead:

template<typename...>
struct inner_type_impl;

template<template<typename...> typename C, typename T, typename...Args>
struct inner_type_impl<C<T,Args...>>
{
    using type = T;
};

template<typename T>
using inner_type = typename inner_type_impl<T>::type;

Here's a demo that's borrowed from Quimby's solution.