template template parameter of unknown type

Question

Trying to extract template parameter value in the following code:

template<std::size_t SIZE>
class Foo {};

template <template<std::size_t> class T, std::size_t K>
auto extractSize(const T<K>&) {
    return K;
}

int main() {
    Foo<6> f1;
    Foo<13> f2;
    std::cout << extractSize(f1) << std::endl;
    std::cout << extractSize(f2) << std::endl;
}

(As an answer for the question: Extract C++ template parameters).

However, is there a way to do it without knowing the type of the template parameter. Something like (code below doesn't compile...):

template <template<class SIZE_TYPE> class T, SIZE_TYPE K>
auto extractSize(const T<K>&) {
    return K;
}

The compilation error on the above, is:

error: unknown type name 'SIZE_TYPE'
template <template<class SIZE_TYPE> class T, SIZE_TYPE K>
                                             ^

Answer 1

auto to the rescue!

template <template<auto> class T, auto K>
auto extractSize(const T<K>&) {
    return K;
}

That way the type of the value you passed in as template parameter is automatically inferred.