How to extract type list from tuple for struct/class

May 8, 2022

I want to use a static method in a class that gets a type list in the form std::tuple<T1, T2, T3,...>. Instead of working with std::tuple<...> I want to have <...>.
How to implement example struct x resulting in Ts == <T1, T2, T3,...>

template<template <typename...> typename TL, typename... Ts>
struct x {
    static void test() {
        // expecting Ts == <int, char, float, double> (without std::tuple<>)
        std::cout << __PRETTY_FUNCTION__ << '\n';
    }
};

using types = std::tuple<int, char, float, double>;
x<types>::test();

See example on godbolt

>Solution :

It seems to me that you’re looking for template specialization.

Something as

// declaration (not definition) for a template struct x
// receiving a single template parameter 
template <typename>
struct x;

// definition for a x specialization when the template
// parameter is in the form TL<Ts...>
template<template<typename...> typename TL, typename... Ts>
struct x<TL<Ts...>> {
    static constexpr void test() {
        // you can use Ts... (and also TL, if you want) here
        std::cout << __PRETTY_FUNCTION__ << ' ' << sizeof...(Ts) << '\n';            
    }
};