Is it possible to infer second template argument type for this template class?-CodePudding

I have a template class A

template<template<class> class E, class N>
class A
{}

and I also have a template function that takes A as an argument:

template<class T>
auto make_something(const T& t){}

used like make_something(a) on an object a of A type.

I would like to be able to tell what is the type N from inside the make_something function. Is there any way of achieving this without changing the template argument of make_something?

CodePudding user response：

(In the following I've changed template<class> class E to template<class, class> class E because I used std::vector, which has two type template parameters, for E.)

I have no idea what you're doing, but to ask question about types, you need to write some traits.

I guess something like the following could be a starting point:

// to ask if something is an A at all
template<typename T>
struct IsAnA : std::false_type {};
template<template<class, class> class E, class N>
struct IsAnA<A<E, N>> : std::true_type {};

template<typename T>
constexpr bool is_an_A_v = IsAnA<T>::value;

// to query A's second template arg
template<typename T>
struct SecondTemplArgOfA {};
template<template<class, class> class E, class N>
struct SecondTemplArgOfA<A<E, N>> {
    using type = N;
};

template<typename T>
using second_templ_arg_of_A_t = typename SecondTemplArgOfA<T>::type;

// make use of it
template<class T>
auto make_something(const T& t){
    if constexpr (is_an_A_v<T>) {
        using its2ndTemplArg = second_templ_arg_of_A_t<T>;
        // so what?
    } else {
        // you've fed make_something with a non-A, so
        // you can't get its second template arg
    }
}

Demo.