Chained conversion between classes without public inheritances

Question

Question

I have a series of ~10 template classes A, B, C, D, ...

I want to enable conversions from a class to previous classes in the series

Answer 1

Although @hlt's approach will do what you ask, without knowing more about the context, I'm wary about implementing the conversion in A. In the cases I can think of, A shouldn't be aware of B, C or D, so I'll suggest a different implementation.

You can create a variant of your test 3, where you implement one conversion operator per class, but where you also inherit a templated indirect conversion operator, like so:

#include 

template 
struct indirect_conversion {
    template >>
    operator T() {
        return static_cast(this)->operator T2();
    }
};

struct A {};

struct B : indirect_conversion {
    operator A();
};
struct C : indirect_conversion {
    operator B();
};
struct D : indirect_conversion {
    operator C();
};
A a = D();