Is there a way to prevent a class from being derived from twice using a static assert and type trait?

Question

I realize this is a contrived example, but I want a compile check to prevent this...

class A {};
class B : public A {};
class C : public A {};

class D : pub         


        

Answer 1

If you really want to, you an test both your base classes:

class A {};
class B : public A {};
class C : public A {};

class D : public B, public C
{
    static_assert(!(is_base_of::value && is_base_of::value),
                   "Invalid inheritance!");
};

Otherwise you can make the classes inherit virtually from A, so that there will still only be one instance of it in the derived class:

class A {};
class B : public virtual A {};
class C : public virtual A {};

class D : public B, public C
{
    // only one A here
};