Question

How do I extract the template type from a class:

for example, I have a class like:

template <typename T, typename T2 = def>
class A
{
    typedef T type;
    typedef T2 type2;
    //other stuff
}

And I want to use type2 in other templates:

template <typename G>
foo(A<G> a)
{
    //This doesn't work:
    std::vector<a::type2> vec;

    //Neither does this:
    std::vector<a->type2> vec;

    //or this:
    std::vector<typename a::type2> vec;

}

So how do I figure out what type2 is for the instance a (can a have a value for type2 that isn’t the default)?

Answer 1

This should work:

std::vector<typename A<G>::type2> vec;

Reason: std::vector expects a complete type as its argument, and just A is a template, but A<G> becomes a complete type. From your example, I have mentioned A<G>, but it can be A<int>, A<char> anything.