Why is this partial template specialization failing?

March 31, 2022

Here is my code.

#include <iostream>

template<class> struct IsInteger;
template<class> struct IsInteger { using value = std::false_type; };
template<> struct IsInteger<int> { using value = std::true_type; };

int main()
{
   std::cout << std::boolalpha << 
   IsInteger<5>::value::value << '\n';
}

Above code results in an error saying

Source.cpp(9,36): error C2974: 'IsInteger': invalid template argument for '<unnamed-symbol>', type expected

Source.cpp(9,50): error C2955: 'IsInteger': use of class template requires template argument list

I don’t understand why the compiler doesn’t pick
template<> struct IsInteger<int> { using value = std::true_type; };
in this case. Why does it result in an error?

>Solution :

You need to use your trait as IsInteger<int> instead of IsInteger<5>.

Also, the idiomatic way to use std::true_type and std::false_type in cases like this is to inherit from them, instead of aliasing them as value:

template<class> struct IsInteger : std::false_type {};
template<> struct IsInteger<int> : std::true_type {};

int main()
{
   std::cout << std::boolalpha << IsInteger<int>::value << '\n';
}