how to combine two classes that share the same base class in C++?

I give the following example to illustrate my question:

class Base
{
 public:
   virtual void do()=0;
}

class A: public Base
{
public:
  void do();
};

class B: public Base
{
 public:
  void do();
}

class AB: public Base
{ 
 public:
 void do()
  {
     a_.do();
     // if not succeed
     {
        b_.do();
     }
    }
private:
 A a_;
 B b_;

}

From the above codes, we can see that class A, B and AB come from the same Base class. class AB, however, needs to invoke both class A and class B. Here is my question: what's the potential problem with the class AB? Are there other alternatives?