Using Virtual Base Classes to Avoid Ambiguity

Let’s start this with the following example program:


  // this program contains errors
  #include<iostream.h>

  // base class
  class base
  {
  public:
    int a;
  };

  // derived class (1)
  class derived1:public base
  {
  public:
    int b;
  };

  // derived class (2)
  class derived2:public base
  {
  public:
    int c;
  };

  // derived class (3)
  class derived3:public derived1, public derived2
  {
  public:
    int d;
  };

  // main
  void main()
  {
    derived3 ob;

    // this is ambiguous
    // since two copies of
    // base is present in
    // the class derived3
    // one from derived1 &
    // the other from derived2
    ob.a=10;
  }

The program above contains an error (as stated) in the following line:

ob.a=10;

Since the base class is inherited by the two classes (derived1 and derived2),
which are again inherited (both together) by another class derived3, there are
two copies of the base class (and hence two copies of variable ‘a’
too).

Therefore when we refer to the variable ‘a’ with respect to the
derived3 class then as there are two copies of it, it becomes ambiguous or confusing
for the compiler to select one from the two and hence it is reported as an ERROR.

This is very common error which creeps in when you write long and complex program
consisting of classes some of which are hierarchically inherited.

So how can we prevent this from happening?

Actually, we can prevent this by using the scope resolution operator but this
is not a permanent solution besides having other flaws. Then what else can we
do?

Yeah, you guessed it right; Virtual Base Class is he answer.

Now, have a look at the same program with slight modification (as to apply
the virtual base class method), now it’s perfectly all right!


  // Virtual base classes
  #include<iostream.h>

  // base class
  class base
  {
  public:
    int a;
  };

  // derived class (1)
  // notice the use of virtual keyword
  class derived1:virtual public base
  {
  public:
    int b;
  };

  // derived class (2)
  // notice the use of virtual keyword
  class derived2:virtual public base
  {
  public:
    int c;
  };

  // derived class (3)
  class derived3:public derived1, public derived2
  {
  public:
    int d;
  };

  // main
  void main()
  {
    derived3 ob;

    // now its O. K.
    ob.a=10;
  }

As you can see, we’re using virtual keyword to derive the classes (derived1
and derived2) from the base class.

This prevents other inheriting classes (which inherits more than one such derived
classes) to have only one copy of the base class and hence ruling out chances
of ambiguity.

Please note that the derived classes (derived1 and derived2 individually) still
contains one-one copy of base class, it is only when they are inherited is the
virtual nature comes into action.

We conclude this article with the following example program to illustrate all
that we have discussed so far in this article.


  // Virtual base classes
  #include<iostream.h>

  // base class
  class base
  {
  public:
    int a;
  };

  // derived class (1)
  // notice the use of virtual keyword
  class derived1:virtual public base
  {
  public:
    int b;
  };

  // derived class (2)
  // notice the use of virtual keyword
  class derived2:virtual public base
  {
  public:
    int c;
  };

  // derived class (3)
  class derived3:public derived1, public derived2
  {
  public:
    int d;
  };

  // main
  void main()
  {
    derived1 ob1;
    derived2 ob2;
    derived3 ob3;

    // derived classes still
    // contain one-one copy 
    // of the base class
    ob1.a=10;
    ob2.a=20;

    // but the class derived
    // form the classes of 
    // the above two objects
    // contains only one copy
    // of the base class
    ob3.a=100;
  }