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c++templatestype-traits
Kent Knox
Kent Knox

Reputation: 348

C++ type trait to check if a type is an instance of template class?

I would like to be able to deduce whether a given type is a template type. I've looked through boost's type trait classes, but can not find is_* traits related to templates: http://www.boost.org/doc/libs/1_52_0/libs/type_traits/doc/html/index.html

What would be even more interesting is if there were ways at compile time to determine properties of the template parameters, such as how many template parameters or if parameters are template template parameters.

Upvotes: 4

Views: 3358

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477228

Here's a partial solution:

#include <iostream>
#include <type_traits>

template <typename> struct is_template : std::false_type {};

template <template <typename...> class Tmpl, typename ...Args>
struct is_template<Tmpl<Args...>> : std::true_type {};


template <typename> struct Foo {};

int main()
{
  std::cout << is_template<int>::value << std::endl;
  std::cout << is_template<Foo<char>>::value << std::endl;
}

The problem is that a template can have an arbitrary structure, so it needn't just consist of type parameters. You can't exhaustively enumerate all kinds of template arguments.

However, pursuing this approach for a minute, an arugment counter is readily made:

template <typename> struct nargs : std::integral_constant<unsigned int, 0> { };

template <template <typename...> class Tmpl, typename ...Args>
struct nargs<Tmpl<Args...> : std::integral_constant<unsigned int, sizeof...(Args)> { };

Upvotes: 8

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