c++ - How does Stroustrup's Can_Copy template work?

Question

Stroustrup provides a Can_copy template. How does it work?

template<class T1, class T2> struct Can_copy {
    static void constraints(T1 a, T2 b) { T2 c = a; b = a; }
    Can_copy() { void(*p)(T1,T2) = constraints; }
};

In particular, why does he need the line void(*p)(T1,T2) = constraints; instead of an empty constructor? Are compilers allowed to produce only the functions a particular template instance uses as an optimisation?

Answer 1

It's because not used member functions in templates don't exist in generated code, so to check constrainst you would have to call constraints() explicitly somewhere.

This way code for constraints() is generated, so constraints are checked in compile time (and that is the purpose of Can_copy).