已知基类ShapeFactory的声明如下： const float pi=3.1416; class ShapeFactory { public: ShapeFactory(){}; virtual ~ShapeFactory(){}; virtual float Circumstance(){return 0;}; } ShapeFactory *Create(float a,float b,float c); ShapeFactory *Create(float a,float b,float c,float d); ShapeFactory *Create(float r); 请写出三角形(Triangle)、四边形(Quadrangle)、圆形(Circle)三个派生类，构造函数分别传入三边/四边/半径的长度（不用检查是否符合三角形、矩形、圆的条件，没有异常输出），重写出求周长的函数（Circumstance函数）。 然后实现基类的Create函数，这里重载的三个Create函数，分别生成三角形、四边形、圆形的对象。 比如三角形类为Triangle： ShapeFactory * ShapeFactory::Create(float a,float b,float c) { ShapeFactory *p=new Triangle(a,b,c); return p; } 如果三角形输入的三边长度是 3 4 5，四边形输入的四条边的长度是2 3 4 7，圆的半径是3，则要求程序运行能够得到如下的提示和输出：

三角形、四边形、圆形的派生类实现如下：

class Triangle : public ShapeFactory {
public:
    Triangle(float a, float b, float c) : ShapeFactory(), side1(a), side2(b), side3(c) {}
    virtual ~Triangle() {}
    virtual float Circumstance() { return side1 + side2 + side3; }
private:
    float side1, side2, side3;
};

class Quadrangle : public ShapeFactory {
public:
    Quadrangle(float a, float b, float c, float d) : ShapeFactory(), side1(a), side2(b), side3(c), side4(d) {}
    virtual ~Quadrangle() {}
    virtual float Circumstance() { return side1 + side2 + side3 + side4; }
private:
    float side1, side2, side3, side4;
};

class Circle : public ShapeFactory {
public:
    Circle(float r) : ShapeFactory(), radius(r) {}
    virtual ~Circle() {}
    virtual float Circumstance() { return 2 * pi * radius; }
private:
    float radius;
};

Create函数的实现如下：

ShapeFactory* ShapeFactory::Create(float a, float b, float c) {
    return new Triangle(a, b, c);
}

ShapeFactory* ShapeFactory::Create(float a, float b, float c, float d) {
    return new Quadrangle(a, b, c, d);
}

ShapeFactory* ShapeFactory::Create(float r) {
    return new Circle(r);
}

然后在程序中调用即可：

int main() {
    float a = 3, b = 4, c = 5;
    ShapeFactory* p = ShapeFactory::Create(a, b, c);
    cout << "三角形周长为：" << p->Circumstance() << endl;
    delete p;

    float side1 = 2, side2 = 3, side3 = 4, side4 = 7;
    p = ShapeFactory::Create(side1, side2, side3, side4);
    cout << "四边形周长为：" << p->Circumstance() << endl;
    delete p;

    float r = 3;
    p = ShapeFactory::Create(r);
    cout << "圆形周长为：" << p->Circumstance() << endl;
    delete p;

    return 0;
}

输出结果：

三角形周长为：12
四边形周长为：16
圆形周长为：18.8496