c++抽象数据类型

#include <iostream>

class Shape
{
public:
    Shape() {}
    virtual ~Shape() {}
    virtual long getArea() { return -1; } // error
    virtual long getPerim() { return -1; }
    virtual void draw() {}
};

class Circle : public Shape
{
public:
    Circle(int newRadius):radius(newRadius) {}
    ~Circle() {}
    long getArea() { return 3 * radius * radius; }
    long getPerim() { return 9 * radius; }
    void draw();
private:
    int radius;
    int circumference;
};

void Circle::draw()
{
    std::cout << "Circle drawing routine here!\n";
}

class Rectangle : public Shape
{
public:
    Rectangle(int newLen, int newWidth):
        length(newLen), width(newWidth) {}
    virtual ~Rectangle() {}
    virtual long getArea() { return length * width; }
    virtual long getPerim() { return 2 * length + 2 * width; }
    virtual int getLength() { return length; }
    virtual int getWidth() { return width; }
    virtual void draw();
private:
    int width;
    int length;
};

void Rectangle::draw()
{
    for (int i = 0; i < length; i++)
    {
        for (int j = 0; j < width; j++)
            std::cout << "x ";

        std::cout << "\n";
    }
}

class Square : public Rectangle
{
public:
    Square(int len);
    Square(int len, int width);
    ~Square() {}
    long getPerim() { return 4 * getLength(); }
};

Square::Square(int newLen):
    Rectangle(newLen, newLen)
{}

Square::Square(int newLen, int newWidth):
    Rectangle(newLen, newWidth)
{
    if (getLength() != getWidth())
        std::cout << "Error, not a square ... a rectangle?\n";
}

int main()
{
    int choice;
    bool fQuit = false;
    Shape * sp;

    while (1)
    {
        std::cout << "(1) Circle (2) Rectangle (3) Square (0) Quit: ";
        std::cin >> choice;

        switch (choice)
        {
        case 1:
            sp = new Circle(5);
            break;
        case 2:
            sp = new Rectangle(4, 6);
            break;
        case 3:
            sp = new Square(5);
            break;
        default:
            fQuit = true;
            break;
        }
        if (fQuit)
            break;

        sp->draw();
        std::cout << "\n";
    }
    return 0;
}

shape是一种抽象数据类型（ADT）,只为其派生类提供接口，如果试图实例化Shape对象，将很麻烦，也许应该使其不可能。

抽象数据类型表示一种概念（如形状），而不是具体的对象（如圆）。在c++中，ADT只能用于其他类的基类，而不能创建其实，因此，如果将Shape声明为ADT就无法创建Shape实例了。

18.2纯虚函数

C++通过提供纯虚函数来支持创建抽象数据类型。纯虚函数是必须在派生类中重写的虚函数。通过将虚函数初始化为 0 来将其声明为纯虚的。

任何包含一个或多个纯虚函数的类都是ADT不能将其实例化，试图这样做将导致编译错误，将纯虚函数放在类中向其客户指出以下两点。

1.不要创建这个类的对象，应该使其派生。

2.务必重写这个类继承的纯虚函数。

在ADT派生而来的类中，继承的纯虚函数仍然是纯虚的，要实例化的话，必须要对派生类中每个纯虚函数，否则派生类也将是ADT。

ADT不能实例化如下：

#include <iostream>
using namespace std;

class Shape
{
public:
    Shape() {}
    virtual ~Shape() {}
    virtual long getArea() { return -1; } // error
    //virtual long getArea()=0;


    virtual long getPerim() { return -1; }
    //virtual long getPerim()=0;



    virtual void draw() {}
    //virtual void draw()=0;


};

class Circle : public Shape
{
public:
    Circle(int newRadius):radius(newRadius) {cout<<"circle"<<endl;}
    ~Circle() {}
    long getArea() { return 3 * radius * radius; }
    long getPerim() { return 9 * radius; }
    void draw();
private:
    int radius;
    int circumference;
};

void Circle::draw()
{
    std::cout << "Circle drawing routine here!\n";
}

class Rectangle : public Shape
{
public:
    Rectangle(int newLen, int newWidth):
        length(newLen), width(newWidth) {}
    virtual ~Rectangle() {}
    virtual long getArea() { return length * width; }
    virtual long getPerim() { return 2 * length + 2 * width; }
    virtual int getLength() { return length; }
    virtual int getWidth() { return width; }
    virtual void draw();
private:
    int width;
    int length;
};

void Rectangle::draw()
{
    for (int i = 0; i < length; i++)
    {
        for (int j = 0; j < width; j++)
            std::cout << "x ";

        std::cout << "\n";
    }
}

class Square : public Rectangle
{
public:
    Square(int len);
    Square(int len, int width);
    ~Square() {}
    long getPerim() { return 4 * getLength(); }
};

Square::Square(int newLen):
    Rectangle(newLen, newLen)
{}

Square::Square(int newLen, int newWidth):
    Rectangle(newLen, newWidth)
{
    if (getLength() != getWidth())
        std::cout << "Error, not a square ... a rectangle?\n";
}

int main()
{
    int choice;
    bool fQuit = false;
    Shape * sp;

   // Shape *ptr=new Shape;    //ci chu shi bu neng chuang jian shape dui xiang de 

    while (1)
    {
        std::cout << "(1) Circle (2) Rectangle (3) Square (0) Quit: ";
        std::cin >> choice;

        switch (choice)
        {
        case 1:
            sp = new Circle(5);
            break;
        case 2:
            sp = new Rectangle(4, 6);
            break;
        case 3:
            sp = new Square(5);
            break;
        default:
            fQuit = true;
            break;
        }
        if (fQuit)
            break;

        sp->draw();
        std::cout << "\n";
    }
    return 0;
}