Virtual Base classes
Virtual Base classes are related to multiple inheritance. To declare a virtual base class, add the C++ keyword virtual to the base class specifier. The keyword virtual prevents a base class from being duplicated within a derivation hierarchy.
Consider the situation with a base class, Parent; two derived classes, Child1 and Child2; and a fourth class, Grandchild, derived from both Child1 and Child2. In this arrangement, a problem can arise if a member function in the Grandchild class wants to access data or functions in the Parent class.
class Parent
{
protected :
int data ;
} ;
class Child1 : public Parent
{
} ;
class Child2 : public Parent
{
} ;
class Grandchild : public Child1, public Child2
{
public :
int readdata ( )
{
return data ;
}
} ;
A compiler error occurs when the readdata ( ) member function in Grandchild attempts to access data in Parent. When the Child1 and Child2 classes are derived from Parent, each inherits a copy of Parent. Each of the two child-classes contains its own copy of Parent's data. Now, when Grandchild refers to data, which of the two copies will it access? This situation is ambiguous, and that's what the compiler reports.
To eliminate the ambiguity, we make Child1 and Child2 into virtual base classes.
class Parent
{
protected :
int data ;
} ;
class Child1 : virtual public Parent
{
} ;
class Child2 : virtual public Parent
{
} ;
class Grandchild : public Child1, public Child2
{
public :
int readdata ( )
{
return data ;
}
} ;