.. _ch12-xt4:

习题4
=======


定义抽象基类Shape
-------------------

.. code-block:: c++ 

   #include <iostream>
   using namespace std;

   class Shape
   {
   public:
        virtual float area() const =0;
   }; 

   class Circle:public Shape
   {
   public:
        Circle(float r):radius(r){}
        virtual float area() const
        {
            return 3.14 * radius * radius;
        }

    protected:
        float radius;
    };

派生类Circle(圆形)
--------------------

.. code-block:: c++ 

   class Circle:public Shape
   {
   public:
        Circle(float r):radius(r){}
        virtual float area() const
        {
            return 3.14 * radius * radius;
        }

   protected:
        float radius;
   };
                            
派生类Rectangle(矩形)
----------------------

.. code-block:: c++ 

   class Rectangle: public Shape
   {
   public:
        Rectangle(float h, float w): height(h), width(w){}
        virtual float area() const
        {
            return height * width;
        }

    protected:
        float height;
        float width;
    };

派生类Triangle（三角形）
-------------------------

.. code-block:: c++ 

   class Triangle: public Shape
   {
   public:
        Triangle(float h, float l): height(h), lenght(l){}
        virtual float area() const
        {
            return 0.5 * height * lenght;
        }

   protected:
        float height;
        float lenght;
   };

输出面积的函数printArea
-------------------------

.. code-block:: c++ 

   void printArea(const Shape &s) 
   {
    cout << s.area()<<endl;
   }


写main函数
------------

.. code-block:: c++ 

   int main()
   {
       Circle circle(2.4);
       cout << "圆的面积是: ";
       printArea(circle);

       Rectangle rectangle(1.3, 1.4);
       cout <<"矩形的面积是： ";
       printArea(rectangle);

       Triangle tri(2.4, 5.9);
       cout << "三角形的面积是：";
       printArea(tri);

       return 0;

   }