Why does virtual inheritance need to be specified in the middle of a diamond hierarchy?

Question

I have diamond hierarchy of classes:

    A
  /   \\
 B     C
  \\   /
    D

To avoid two copies of A in D, we need to use virtual inheritance a

Answer 1

As A is the multiply-inherited class it is those that derive from it directly that have to do so virtual.

If you have a situation where B and C both derive from A and you want both in D and you can't use the diamond, then D can derive from just one of B and C, and have an instance of the other, through which it can forward functions.

workaround something like this:

class B : public A; // not your class, cannot change
class C : public A; // not your class, cannot change

class D : public B; // your class, implement the functions of B
class D2 : public C; // your class implement the functions of C

class D
{
   D2 d2;
};