How do I determine if a type is callable with only const references?

Question

I want to write a C++ metafunction is_callable that defines value to be true, if and only if the type F has the function cal

Answer 1

Here is a possible solution that utilizes an extra test to see if your template is being instantiated with a const T&:

#include 
#include 

using namespace std;

template
struct is_callable {
private:

  template
  static char (&test(...))[2];

  template
  struct helper {};

  template
  struct helper {
    typedef void *type;
  };

  template
  struct is_const_ref {};

  template
  struct is_const_ref {
    static const bool value = false;
  };

  template
  struct is_const_ref {
    static const bool value = true;
  };

  template
  static char test(typename helper::value, 
                                   sizeof(std::declval()(std::declval()), 0)>::type);
public:
  static const bool value = (sizeof(test(0)) == sizeof(char));
};

struct foo {
  void operator()(const int &) {}
};

int main(void)
{
  cout << is_callable::value << "\n";
  cout << is_callable::value << "\n";

  return 0;
}