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Class A
{
Activator.CreateInstance(Type.GetType("ClassB"));
}

If you are talking about C#

1) If you inherit ClassB, it can replace composition.
2) Another way to do it in java is using reflection.

Java reflection will work in this case:

B b =(B) Class.forName("B").newInstance();

how about multiple inheritance using virtual base class in c++ ?
can anyone talk about the object layout for the following hierarchy:
class A : virtual public C { }
class B : virtual public C { }
class D: public A, public B { }

The question should be:
classB *b=new classB();

another way is

classB b;

Use dependency injection:

class A {
   B b;

   A(B b) {
      this.b = b;
   }
}

spring dependency injection