unwrap-abstraction.cpp

// Having type T checks whether it is an instance of template U<V>.
// http://coliru.stacked-crooked.com/a/b5bcde32c99c7cab

#include <vector>
#include <type_traits>

template<class T, template<typename...> class U, class V = void>
struct is_instance_of : std::false_type
{};

template<template<typename...> class U, class... Args>
struct is_instance_of<U<Args...>, U, void> : std::true_type
{};

#if !defined( __GNUC__ )
template<class V, template<V...> class U, V... Args>
struct is_instance_of<U<Args...>, U, V> : std::true_type
{};
#endif

template<class V, class T, template<V...> class U>
struct is_integral_instance_of : std::false_type
{};

template<class V, template<V...> class U, V... i>
struct is_integral_instance_of<V, U<i...>, U> : std::true_type
{};



template<int... i>
struct S{};

int main()
{
    
    using T = std::vector<int>;
    using U = std::vector<int>;
    using V = S<10, 11, 12>;

    static_assert(is_instance_of<T, std::vector>::value, "NOK");
    static_assert(is_instance_of<U, std::vector>::value, "NOK");
    static_assert(is_integral_instance_of<int, V, S>::value, "NOK");

    return 0;
}