How to check if an object is an instance of template class of multiple template arguments and that one of said arguments fulfills some condition?

user_18853370 0
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From How to check if an object is an instance of a template class of multiple template arguments in C++? I got the following trait to check whether a type is a particular template instantiation of a templated type of several template arguments:

template <typename T1, typename T2>
struct A
{
};

template <typename Type>
struct IsA: std::false_type
{
};

template <typename T1, typename T2>
struct IsA<A<T1, T2>> : std::true_type
{
};

How would one additionally add the condition that the second type T2 fulfills some other condition? I tried doing

template <typename Type>
struct IsA: std::false_type
{
};

template <typename T1, typename T2, std::enable_if_t<SomeCondition<T2>::value>>
struct IsA<A<T1, T2>> : std::true_type
{
};

but am getting the error

error: template parameters not deducible in partial specialization:
typetraits templates c++
1 Answers

You were on the right track:

#include <type_traits>
#include <iostream>

template <typename T1, typename T2>
struct A
{
};

template <typename Type, typename=void>
struct IsA: std::false_type
{
};

template <typename T1, typename T2>
struct IsA<A<T1, T2>, std::enable_if_t<std::is_same_v<T2, int>>>
    : std::true_type
{
};

int main()
{
    std::cout << IsA<int>::value << "\n";

    std::cout << IsA<A<char, char>>::value << "\n";

    std::cout << IsA<A<char, int>>::value << "\n";
    return 0;
}

In this trivial example the "some condition" is just a std::is_same_v<T2, int>, only for the sake of an example.

user_39433120
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