[C++]純虚関数を継承

// LISTING 12.10 - Deriving Abstract Classes from Other Abstract Classes
#include <iostream>
using namespace std;

enum COLOR { Red, Green, Blue, Yellow, White, Black, Brown } ;

class Animal // common base to both Mammal and Fish
{
public:
    Animal(int);
    virtual ~Animal() { cout << "Animal destructor...
"; }
    virtual int GetAge() const { return itsAge; }
    virtual void SetAge(int age) { itsAge = age; }
    virtual void Sleep() const = 0;
    virtual void Eat() const = 0;
    virtual void Reproduce() const = 0;
    virtual void Move() const = 0;
    virtual void Speak() const = 0;
private:
    int itsAge;
};

Animal::Animal(int age):
itsAge(age)
{
    cout << "Animal constructor...
";
}

class Mammal : public Animal
{
public:
    Mammal(int age):Animal(age)
    { cout << "Mammal constructor...
";}
    virtual ~Mammal() { cout << "Mammal destructor...
";}
    virtual void Reproduce() const
    { cout << "Mammal reproduction depicted...
"; }
};

class Fish : public Animal
{
public:
    Fish(int age):Animal(age)
    { cout << "Fish constructor...
";}
    virtual ~Fish() {cout << "Fish destructor...
"; }
    virtual void Sleep() const { cout << "fish snoring...
"; }
    virtual void Eat() const { cout << "fish feeding...
"; }
    virtual void Reproduce() const
    { cout << "fish laying eggs...
"; }
    virtual void Move() const
    { cout << "fish swimming...
"; }
    virtual void Speak() const { }
};

class Horse : public Mammal
{
public:
    Horse(int age, COLOR color ):
      Mammal(age), itsColor(color)
      { cout << "Horse constructor...
"; }
      virtual ~Horse() { cout << "Horse destructor...
"; }
      virtual void Speak()const { cout << "Whinny!... 
"; }
      virtual COLOR GetItsColor() const { return itsColor; }
      virtual void Sleep() const
      { cout << "Horse snoring...
"; }
      virtual void Eat() const { cout << "Horse feeding...
"; }
      virtual void Move() const { cout << "Horse running...
";}

protected:
    COLOR itsColor;
};

class Dog : public Mammal
{
public:
    Dog(int age, COLOR color ):
      Mammal(age), itsColor(color)
      { cout << "Dog constructor...
"; }
      virtual ~Dog() { cout << "Dog destructor...
"; }
      virtual void Speak()const { cout << "Whoof!... 
"; }
      virtual void Sleep() const { cout << "Dog snoring...
"; }
      virtual void Eat() const { cout << "Dog eating...
"; }
      virtual void Move() const { cout << "Dog running...
"; }
      virtual void Reproduce() const
      { cout << "Dogs reproducing...
"; }

protected:
    COLOR itsColor;
};

int main()
{
    Animal *pAnimal=0;
    int choice;
    bool fQuit = false;

    while (fQuit == false)
    {
        cout << "(1)Dog (2)Horse (3)Fish (0)Quit: ";
        cin >> choice;

        switch (choice)
        {
        case 1: pAnimal = new Dog(5,Brown);
            break;
        case 2: pAnimal = new Horse(4,Black);
            break;
        case 3: pAnimal = new Fish (5);
            break;
        default: fQuit = true;
            break;
        }
        if (fQuit == false)
        {
            pAnimal->Speak();
            pAnimal->Eat();
            pAnimal->Reproduce();
            pAnimal->Move();
            pAnimal->Sleep();
            delete pAnimal;
            cout << endl;
        }
    }
    return 0;
}

ここでは、哺乳類が動物を継承する、繁殖するReproduce()を非純虚関数とし、その他は依然として純虚関数を保持しているのを見るが、哺乳類の繁殖方式は同じであるため、繁殖方式を非純虚関数とすべきであり、他の関数は異なる可能性がある.Mammalには純粋な虚関数が含まれているため,これは依然として抽象クラスであり,インスタンス化できない.
 
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