Access nested template types

Question

Let's say there is a class A and MyType

template<typename DataType>
class MyType {
...
}

template<typename MyType>
class A {
...
}

When I create an instance of A with A<MyType<int>> how can I access the template type int inside A?

Answer 1

Expose a type alias to the user:

template<typename DataType>
class MyType {
public:
    using InnerDataType = DataType;
};

template<typename MyType>
class A {
public:
    using InnerType = MyType;
};

Usage:

using MyA = A<MyType<int>>;
static_assert(std::is_same<
    typename MyA::InnerType::InnerDataType,
    int>{});

live example on wandbox