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

In this post I'm considering the problem of checking if a type exactly matches another, it's not exactly what is asked for, but it is simpler and I hope it can help to understand the applied template tricks.

As done in boost, template specializations can be adopted for that task, in fact you can define a template struct containing operations on a given type, and use nested template structs for that operations. In our case:

// Working on a specific type:
template 
struct is_type {
    // For all types T2!=T1 produce false:
    template 
    struct same_of { static const bool value = false; };
    // Specialization for type T2==T1 producing true:
    template <>
    struct same_of { static const bool value = true; };
};

Defining a macro allows to use it easily:

#define is_type_same(T1,T2) (is_type::same_of::value)

as follows:

template
bool operator==(Manager m1, Manager m2) {
    return is_type_same(R1,R2) && m1.internal_field == m2.internal_field;
}