virtual class in C++

[deepacharya1]

Consider the case were there are 4 class A,B,C,D

B and C inherits A
and D inherits B and C

why do we define class A as virtual for B and C
Why not define Class B and C as virtual for Class D ?
Please answer :)

[Banfa]

A is defined a virtual because it is A that is the class you will end up with 2 copies of if you do not.

[Banfa]

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If B and C did not inherit A virtually then when D inherits B and C it ends up with 2 copies of A, 1 that B refers to and 1 that A refers to. This is normally not good as it leads to ambiguity so B and C inherit A virtually so that when D inherits B and C only 1 copy of A is used which is referred to by both B and C.