How to tell if template type is an instance of a template class?

Question

I have a function that takes a template type to determine a return value. Is there any way to tell at compile time if the template type is some instantiation of a template c

Answer 1

Here's an option:

#include 
#include 
#include 

template  class>
struct is_instance : public std::false_type {};

template  class U>
struct is_instance, U> : public std::true_type {};

template 
class Second 
{};

int main()
{
    using A = Second;
    using B = Second;
    using C = float;
    std::cout << is_instance{} << '\n'; // prints 1
    std::cout << is_instance{} << '\n'; // prints 1
    std::cout << is_instance{} << '\n'; // prints 0
}

It's basically specializing the is_instance struct for types that are instantiations of a template.