抽象类之间的组成

Question

Is it possible to build a relationship of composition (not aggregation) between two abstract classes?

I don't think so because I cannot instantiate an abstract class. But maybe there's a simple way to do it.

Answer 1

Yes:

class A
{
    virtual ~A() = 0;
};


class B
{
    virtual ~B() = 0;
    A* mA;
};

You are allowed to have pointers to abstract types.

Answer 2

You can use a pointer or reference to the other class. For example:

class A {
public:
  virtual void m() = 0;
  virtual ~A() { }
};

class B {
public:
  virtual void m() = 0;
  virtual ~A() { }

private:
  boost::scoped_ptr<A> a;
};

In C++11 you can use std::unique_ptr instead. Note that this makes B noncopyable—that is somewhat common for classes using subtype polymorphism.