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 : public B, public C
{
    BOOST_STATIC_ASSERT((is_base_of_once<A,D>::value))
};

Answer 1

The following should work:

BOOST_STATIC_ASSERT(((A*)(D*)0 == 0)) 

If A exists twice, this should rise an ambiguity error, while otherwise the test will always succeed (because it compares two null pointers).