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What is a pure virtual function?

A pure virtual function is a function that must be overridden in a derived class and need not be defined. A virtual function is declared to be "pure" using the curious "=0" syntax. For example:

	class Base {
	public:
		void f1();		// not virtual
		virtual void f2();	// virtual, not pure
		virtual void f3() = 0;	// pure virtual
	};

	Base b;	// error: pure virtual f3 not overridden

Here, Base is an abstract class (because it has a pure virtual function), so no objects of class Base can be directly created: Base is (explicitly) meant to be a base class. For example:

	class Derived : public Base {
		// no f1: fine
		// no f2: fine, we inherit Base::f2
		void f3();
	};

	Derived d;	// ok: Derived::f3 overrides Base::f3

Abstract classes are immensely useful for defining interfaces. In fact, a class with only pure virtual functions is often called an interface.

You can define a pure virtual function:

	Base::f3() { /* ... */ }

This is very occasionally useful (to provide some simple common implementation detail for derived classes), but Base::f3() must still be overridden in some derived class.

If you don't override a pure virtual function in a derived class, that derived class becomes abstract:

	class D2 : public Base {
		// no f1: fine
		// no f2: fine, we inherit Base::f2
		// no f3: fine, but D2 is therefore still abstract
	};

	D2 d;	// error: pure virtual Base::f3 not overridden

 
转载自：http://www.research.att.com/~bs/bs_faq2.html#pure-virtual
                之  Bjarne Stroustrup's C++ Style and Technique FAQ