8.7 程序举例_ss[0] = new rect(10,10); ss[1] = new rect(8, 12); -优快云博客

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8.7 程序举例

本节通过两个例子展示了多态性和虚函数的实际应用。

例8.18 计算几种几何图形的面积之和

这个例子计算了五种几何图形（矩形、三角形、圆形、梯形、正方形）的面积之和。以下是程序的实现：

#include <iostream>
using namespace std;

class Shape {
public:
    virtual double Area() const = 0;
};

class Triangle : public Shape {
public:
    Triangle(double h, double w) : H(h), W(w) {}
    double Area() const override {
        return H * W * 0.5;
    }

private:
    double H, W;
};

class Rectangle : public Shape {
public:
    Rectangle(double h, double w) : H(h), W(w) {}
    double Area() const override {
        return H * W;
    }

private:
    double H, W;
};

class Circle : public Shape {
public:
    Circle(double r) : radius(r) {}
    double Area() const override {
        return radius * radius * 3.1415;
    }

private:
    double radius;
};

class Trapezoid : public Shape {
public:
    Trapezoid(double top, double bottom, double high)
        : T(top), B(bottom), H(high) {}
    double Area() const override {
        return (T + B) * H * 0.5;
    }

private:
    double T, B, H;
};

class Square : public Shape {
public:
    Square(double side) : S(side) {}
    double Area() const override {
        return S * S;
    }

private:
    double S;
};

class Application {
public:
    double Compute(Shape* s[], int n) const {
        double sum = 0;
        for (int i = 0; i < n; ++i)
            sum += s[i]->Area();
        return sum;
    }
};

class MyProgram : public Application {
public:
    MyProgram() {
        s = new Shape*[5];
        s[0] = new Triangle(3.0, 4.0);
        s[1] = new Rectangle(6.0, 8.0);
        s[2] = new Circle(6.5);
        s[3] = new Trapezoid(10.0, 8.0, 5.0);
        s[4] = new Square(6.7);
    }
    ~MyProgram() {
        for (int i = 0; i < 5; ++i)
            delete s[i];
        delete[] s;
    }
    double Run() {
        return Compute(s, 5);
    }

private:
    Shape** s;
};

int main() {
    cout << "Area's sum = " << MyProgram().Run() << endl;
    return 0;
}

说明

抽象类 Shape：定义了一个纯虚函数 Area，作为所有几何图形的基类。
五个派生类：Triangle、Rectangle、Circle、Trapezoid 和 Square，每个类都实现了 Area 函数。
Application 类：包含一个计算面积和的函数 Compute。
MyProgram 类：继承自 Application，在构造函数中创建了五个几何图形对象，在析构函数中删除这些对象。

程序的输出结果为：

mathematica

复制代码

Area's sum = 276.618

这种结构使得添加新图形非常方便，只需增加一个继承自 Shape 的派生类并实现 Area 函数即可。

例8.19 宠物管理系统

该例子展示了一个宠物管理系统，假设一个人拥有20个宠物窝，一半用于养猫，另一半用于养狗。程序定义了一个动物基类 Animal，以及 Cat 和 Dog 两个派生类，并实现了虚函数 WhoAmI。

#include <iostream>
#include <cstring>
using namespace std;

class Animal {
public:
    Animal() : name(nullptr) {}
    Animal(const char* n) : name(strdup(n)) {}
    virtual ~Animal() { delete[] name; }
    virtual void WhoAmI() { cout << "generic animal." << endl; }

protected:
    char* name;
};

class Cat : public Animal {
public:
    Cat() : Animal() {}
    Cat(const char* n) : Animal(n) {}
    void WhoAmI() override {
        cout << "I am a cat named " << name << endl;
    }
};

class Dog : public Animal {
public:
    Dog() : Animal() {}
    Dog(const char* n) : Animal(n) {}
    void WhoAmI() override {
        cout << "I am a dog named " << name << endl;
    }
};

class Kennel {
public:
    Kennel(unsigned max);
    ~Kennel();
    unsigned Accept(Animal* d);
    Animal* Release(unsigned pen);
    void ListAnimals();

private:
    unsigned MaxAnimals, NumAnimals;
    Animal** Residents;
};

Kennel::Kennel(unsigned max) : MaxAnimals(max), NumAnimals(0) {
    Residents = new Animal*[MaxAnimals];
    for (unsigned i = 0; i < MaxAnimals; ++i)
        Residents[i] = nullptr;
}

Kennel::~Kennel() {
    for (unsigned i = 0; i < MaxAnimals; ++i)
        delete Residents[i];
    delete[] Residents;
}

unsigned Kennel::Accept(Animal* d) {
    if (NumAnimals == MaxAnimals)
        return 0;
    for (unsigned i = 0; i < MaxAnimals; ++i) {
        if (Residents[i] == nullptr) {
            Residents[i] = d;
            ++NumAnimals;
            return i + 1;
        }
    }
    return 0;
}

Animal* Kennel::Release(unsigned pen) {
    if (pen > MaxAnimals || Residents[pen - 1] == nullptr)
        return nullptr;
    Animal* temp = Residents[pen - 1];
    Residents[pen - 1] = nullptr;
    --NumAnimals;
    return temp;
}

void Kennel::ListAnimals() {
    for (unsigned i = 0; i < MaxAnimals; ++i) {
        if (Residents[i] != nullptr) {
            cout << "The animal in pen " << i + 1 << " says:" << endl;
            Residents[i]->WhoAmI();
        }
    }
}

Dog d1("Rover"), d2("Spot"), d3("Chip"), d4("Buddy"), d5("Butch");
Cat c1("Tinkerbell"), c2("Inky"), c3("Fluffy"), c4("Princess"), c5("Sylvester");

int main() {
    Kennel K(20);
    K.Accept(&d1);
    unsigned c2pen = K.Accept(&c2);
    K.Accept(&d3);
    K.Accept(&c1);
    unsigned d4pen = K.Accept(&d4);
    K.Accept(&d5);
    K.Accept(&c5);
    K.Release(c2pen);
    K.Accept(&c4);
    K.Accept(&c3);
    K.Release(d4pen);
    K.Accept(&d2);
    K.ListAnimals();
    return 0;
}

说明

基类 Animal：定义了虚函数 WhoAmI，用于输出动物的身份信息。
派生类 Cat 和 Dog：分别实现了 WhoAmI 函数，输出各自的身份信息。
Kennel 类：管理动物的数组，提供接收、释放和列出动物的方法。

程序的输出结果如下：

The animal in pen 1 says: I am a dog named Rover
The animal in pen 2 says: I am a cat named Princess
The animal in pen 3 says: I am a dog named Chip
The animal in pen 4 says: I am a cat named Tinkerbell
The animal in pen 5 says: I am a dog named Spot
The animal in pen 6 says: I am a dog named Butch
The animal in pen 7 says: I am a cat named Sylvester
The animal in pen 8 says: I am a cat named Fluffy

这个例子展示了虚函数的使用，使得对不同类型的动物有统一的操作接口，方便地进行多态操作。