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

You can use boost's typetraits (is_base_of), and boost's enable_if.

#include 
#include 

template 
struct has_derived_base_relationship :
    boost::integral_constant<
        bool, boost::is_base_of::value || boost::is_base_of::value 
    >
{};

template
typename boost::enable_if, bool>::type 
operator==(Manager m1, Manager m2) {
    return p1.internal_field == p2.internal_field;
}

On the other hand, why would operator== usage have more value with types of the same inheritance tree? Wouldn't it have to use double dispatch to achieve meaningful results?