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What is a virtual base class?

Answer Posted / raj randhawa

Suppose you have two derived classes B and C that have a
common base class A, and you also have another class D that
inherits from B and C. You can declare the base class A as
virtual to ensure that B and C share the same subobject of
A.

In the following example, an object of class D has two
distinct subobjects of class L, one through class B1 and
another through class B2. You can use the keyword virtual
in front of the base class specifiers in the base lists of
classes B1 and B2 to indicate that only one subobject of
type L, shared by class B1 and class B2, exists.

For example:






class L { /* ... */ }; // indirect base class
class B1 : virtual public L { /* ... */ };
class B2 : virtual public L { /* ... */ };
class D : public B1, public B2 { /* ... */ }; // valid

Using the keyword virtual in this example ensures that an
object of class D inherits only one subobject of class L.

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