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's something I hacked up which may or may not be what you need; it does seem to give true (false) for (const) int &...

#include 

template 
struct Callable
{
private:
  typedef char                      yes;
  typedef struct { char array[2]; } no;

  template 
  static yes test(decltype(std::declval()(std::declval())) *);

  template 
  static no test(...);

public:
  static bool const value = sizeof(test(nullptr)) == sizeof(yes);
};

struct Foo
{
  int operator()(int &) { return 1; }
  // int operator()(int const &) const { return 2; } // enable and compare
};

#include 
int main()
{
  std::cout << "Foo(const int &): " << Callable::value << std::endl
            << "Foo(int &):       " << Callable::value << std::endl
    ;
}