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

In addition to templatetypedef answer, it may be pointed out that you also may wrap A into

class AVirt:virtual public A{}; 

and inherit other classes from it. You wil not need to mark explicitly other inheriances as virtual in this case