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

A trait you want might look like this:

template <typename B, typename D>
struct is_base_of // check if B is a base of D
    typedef char yes[1];
    typedef char no[2];

    static yes& test(B*);
    static no& test(...);

    static D* get(void);

    static const bool value = sizeof(test(get()) == sizeof(yes);

Then you just need a static assert of some sort:

// really basic
template <bool>
struct static_assert;

template <>
struct static_assert<true> {}; // only true is defined

#define STATIC_ASSERT(x) static_assert<(x)>()

Then put the two together:

template<class R1, class R2>
bool operator==(Manager<R1> m1, Manager<R2> m2)
    STATIC_ASSERT(is_base_of<R1, R2>::value || is_base_of<R2, R1>::value);

    return p1.internal_field == p2.internal_field;

If one does not derive from the other, the function will not compile. (Your error will be similar to “static_assert<false> not defined”, and it will point to that line.)

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