程序清单 18.2 Mammal13.cpp 的单继承问题使用纯虚函数解决

#include <iostream>

class Mammal
{
public:
    Mammal():age(1) { std::cout << "Mammal constructor ...\n"; }
    virtual ~Mammal() { std::cout << "Mammal destructor ...\n"; }
    virtual void speak() const { std::cout << "Mammal speak!\n"; }
protected:
    int age;
};

class Cat: public Mammal
{
public:
    Cat() { std::cout << "Cat constructor ...\n"; }
    ~Cat() { std::cout << "Cat destructor ...\n"; }
    void speak() const { std::cout << "Meow!\n"; }
    void purr() const { std::cout << "Rrrrrrrrrrr!\n"; }
};

class Dog: public Mammal
{
public:
    Dog() { std::cout << "Dog constructor ...\n"; }
    ~Dog() { std::cout << "Dog destructor ...\n"; }
    void speak() const { std::cout << "Woof!\n"; }
};

int main()
{
    const int numberMammals = 3;
    Mammal* zoo[numberMammals];
    Mammal* pMammal;
    int choice, i;
    for (i = 0; i < numberMammals; i++)
    {
        std::cout << "(1)Dog (2)Cat: ";
        std::cin >> choice;
        if (choice == 1)
            pMammal = new Dog;
        else
            pMammal = new Cat;

        zoo[i] = pMammal;
    }

    std::cout << "\n";

    for (i = 0; i < numberMammals; i++)
    {
        zoo[i]->speak();

        Cat *pRealCat =  dynamic_cast<Cat *> (zoo[i]);

        if (pRealCat)
            pRealCat->purr();
        else
            std::cout << "Uh oh, not a cat!\n";

        delete zoo[i];
        std::cout << "\n";
    }

    return 0;
}

此处想要对cat单独定义一个函数，对dog不能使用，但是pCat是mammal指针（多态的本质），故使用运算符转换为Cat指针。

对于这个问题也可以使用纯虚函数解决。如下，在Mammal中定义一个纯虚函数，使其成为ADT(抽象数据类型)，然后在dog和cat类中分别重写purr()函数。

#include <iostream>

class Mammal
{
public:
    Mammal():age(1) { std::cout << "Mammal constructor ...\n"; }
    virtual ~Mammal() { std::cout << "Mammal destructor ...\n"; }
    virtual void speak() const { std::cout << "Mammal speak!\n"; }
    virtual void purr() const=0;   //ADT
protected:
    int age;
};

class Cat: public Mammal
{
public:
    Cat() { std::cout << "Cat constructor ...\n"; }
    ~Cat() { std::cout << "Cat destructor ...\n"; }
    void speak() const { std::cout << "Meow!\n"; }
    void purr() const { std::cout << "Rrrrrrrrrrr!\n"; }
};

class Dog: public Mammal
{
public:
    Dog() { std::cout << "Dog constructor ...\n"; }
    ~Dog() { std::cout << "Dog destructor ...\n"; }
    void speak() const { std::cout << "Woof!\n"; }
    void purr() const {std::cout<<"Uh oh, not a cat!\n";}
};

int main()
{
    const int numberMammals = 3;
    Mammal* zoo[numberMammals];
    Mammal* pMammal;
    int choice, i;
    for (i = 0; i < numberMammals; i++)
    {
        std::cout << "(1)Dog (2)Cat: ";
        std::cin >> choice;
        if (choice == 1)
            pMammal = new Dog;
        else
            pMammal = new Cat;

        zoo[i] = pMammal;
    }

    std::cout << "\n";

    for (i = 0; i < numberMammals; i++)
    {
        zoo[i]->speak();

 //       Cat *pRealCat =  dynamic_cast<Cat *> (zoo[i]);

//        if (pRealCat)
  //          pRealCat->purr();
 //       else
 //         std::cout << "Uh oh, not a cat!\n";
        zoo[i]->purr();

        delete zoo[i];
        std::cout << "\n";
    }

    return 0;
}