Multiple Inheritance

Diamond Problem

The diamond problem is where it may be ambiguous as to which parent class a particular feature is inherited from if more than one parent class implements said feature.

Resources

• https://www.makeuseof.com/what-is-diamond-problem-in-cpp/ (though seems to have some minor errors)

Remembering the name

It is called the “Diamond Problem” because the problem arises from a class hierarchy that is shaped like a diamond (see image below)!

From CS247

The Diamond Problem stems from a shared base class with Multiple Inheritance.

struct A {int a = 1;}
struct B: A{};
struct C: A{}; // Don't need to specify public - structs have public inheritance by default
struct D: B, C{};
D dobj;
dobj.a = 2; // Doesn't compile either

Because a D is-a B and a C, it ends up having 2 a fields - one from the B portion of the object and one from the C portion of the object.

Must instead disambiguate - which a field are we talking about? The one from the B portion or the one from the C portion?

dObj.B::a = 1 // setting different a fields in the D object
dObj.C::a = 2;

What if we wanted to disable this (somewhat strange) behaviour, and have only copy a single A in our hierarchy? Solution: Use Virtual Inheritance.

Fix to Diamond Problem

The fix to the diamond problem is to use Virtual Inheritance.