Deduct type of which other type is dependent

Question

I have this situation:

#include 

template
U f(T data) {
    return U();
}

int main() {
    std::vector

        

Answer 1

The issue here is std::vector doesn't have just one template parameter. It also has a parameter for the allocator type. To get around that you can add another template parameter, or just use a variadic template template parameter like

template<template<class...> class T, class U>
U f(T<U> data) {
    return U();
}

which will work with

return f<std::vector, int>(vec);

or even better

return f(vec);

Do note that this behavior has been changed in C++17. With DR: Matching of template template-arguments excludes compatible templates they relaxed the rules and

template<template<class> class T, class U>
U f(T<U> data) {
    return U();
}

will work with gcc in C++17 mode and clang in C++17 with -frelaxed-template-template-args enabled.