CODEWITHSUNDEEP
c++templatessfinae
Jared Hoberock
Jared Hoberock

Reputation: 11416

How to check a type for a nested template?

I'd like to create a type trait which detects the existence of a nested class template within some other type of interest.

For example, suppose I want to create the type trait has_foo, which detects the presence of a nested template of one parameter named foo within some type T:

#include <cassert>

template<class T>
struct has_foo
{
  // XXX what goes here?
};

struct with_foo
{
  template<class T> struct foo {};
};

struct without_foo {};

int main()
{
  assert(has_foo<with_foo>::value);
  assert(!has_foo<without_foo>::value);

  return 0;
}

What is the best way to implement has_foo?

Upvotes: 2

Views: 565

Answers (1)

Columbo
Columbo

Reputation: 61009

template <template<class> class> using void_templ = void;

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

template <typename T>
struct has_foo<T, void_templ<T::template foo>>  : std::true_type {};

Demo. GCC will allow any type here; That is a bug. To fix that make void_templ a class template

template <template<class> class>
struct void_templ {using type = void;};

Demo.

Upvotes: 6

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