什么是协议类

An alternative to the Handle class approach is to make Person a special kind of abstract base class called a Protocol class. By
definition, a Protocol class has no implementation; its raison d'être is to specify an interface for derived classes (see Item 36).
As a result, it typically has no data members, no constructors, a virtual destructor (see Item 14), and a set of pure virtual
functions that specify the interface. A Protocol class for Person might look like this:
class Person {
public:
  virtual ~Person();
  virtual string name() const = 0;
  virtual string birthDate() const = 0;
  virtual string address() const = 0;
  virtual string nationality() const = 0;
};

简而言之，就是一种特殊的抽象类，特殊在哪里？抽象类可以有实现的部分，但是协议类完全没有实现，相当于java接口。（说抽象类相当于接口有点牵强，但说协议类相当于接口勉强可以接受，呵呵。）