Doing a static_assert that a template type is another template

Question

How do I static_assert like this? Maybe Boost supports it if not C++ or new features in C++11?

template
struct foo {};

template

        

Answer 1

You could do something along these lines. Given a trait that can verify whether a class is an instantiation of a class template:

#include 

template class TT>
struct is_instantiation_of : std::false_type { };

template class TT>
struct is_instantiation_of, TT> : std::true_type { };

Use it as follows in your program:

template
struct foo {};

template
struct bar {
  static_assert(is_instantiation_of::value, "failure");
};

int main()
{
    bar b; // ERROR!
    bar> b; // OK!
}

If you want, you could generalize this to detect whether a class is an instance of a template with any number of (type) parameters, like so:

#include 

template class TT, typename T>
struct is_instantiation_of : std::false_type { };

template class TT, typename... Ts>
struct is_instantiation_of> : std::true_type { };

template
struct bar {
  static_assert(is_instantiation_of::value, "failure");
};

You would then use it this way in your program:

template
struct bar {
  static_assert(is_instantiation_of::value, "failure");
};

int main()
{
    bar b; // ERROR!
    bar> b; // OK!
}

Here is a live example.