get decltype of template argument

Question

I often want to get the decltype of a class template argument in order to use it further, like in a loop which I have stripped and simplified to show my problem:

template <typename T>
class Foo {
public:
    T type; //This is my hack to get decltype of T
};

template <typename T>
class Bar {
public:

};

int main() {
    for(auto& foo : fs) {
        //Is there some way to get the decltype of the template without having to refer to some arbitrary T member of Foo?
        auto bar = someFunction<decltype(foo.type)>();
    }
}

Is there a way to get the decltype of the template argument without doing this hack? If not, what is the best workaround to get the decltype of such a value?

Answer 1

You may create a type_traits:

template <typename C>
struct get_template_type;

template <template <typename > class C, typename T>
struct get_template_type<C<T>>
{
    using type = T;
};

and then you use it (assuming C = Foo<T>, you will have T)

typename get_template_type<C>::type

Live example

EDIT: For classes with multiple template parameter, to retrieve the first one:

template <template <typename > class C, typename T, typename ... Ts>
struct get_template_type<C<T, Ts...>>
{
    using type = T;
};