C++: How to require that one template type is derived from the other

Question

In a comparison operator:

template
bool operator==(Manager m1, Manager m2) {
    return m1.internal_field == m2         


        

Answer 1

template struct Derived_from {
 static void constraints(T* p) { B* pb = p; }
 Derived_from() { void(*p)(T*) = constraints; }
};

template
bool test(R1& r1) {
 Derived_from(); // accept if R1 is derived from R2
 return false;
}

class Base {
public:
 virtual ~Base() { }
};

class Derived : public Base {

};

class Other {

};

int _tmain(int argc, _TCHAR* argv[])
{
 Derived d;
 Other o;

 test(d); // OK
 test(o); // Fails in VC++ 2005

 return 0;
}

Credits go to http://www2.research.att.com/~bs/bs_faq2.html#constraints