c++ is_derived implementation

gistfile1.txt

is_derived  r: 1
is_derived  x: 1
is_derived  y: 0

is_derived.cpp

#include <iostream>
#include <string>
#include <exception>
#include <stdexcept>

using namespace std;

struct yes_size { char c[256]; };
yes_size dispatch_exception(exception const*);
struct no_size {};
no_size dispatch_exception(void const*);

template <class T>
struct is_derived {
  enum e { value = sizeof(dispatch_exception(static_cast<T*>(NULL))) == sizeof(yes_size) };
};

class X : public runtime_error {};

class Y {};

int main()
{
  cout << "is_derived  r: " << is_derived<runtime_error>::value << endl;
  cout << "is_derived  x: " << is_derived<X>::value << endl;
  cout << "is_derived  y: " << is_derived<Y>::value << endl;

  return 0;
}