继承与派生

内容：定义一个点类，包含X，Y的坐标数据成员，显示函数和计算面积的函数成员；以点为基类派生一个圆类，增加表示半径的数据成员，重载显示和计算面积的函数；定义一个直线类，以两个点类对象作数据成员，定义显示、求面积及长度函数。

-inheritance and derived elements : a definition point category, including X, Y coordinates of the data, calculations show function and the function area; point to the base class for a round like derivatives, the increase in the radius of the data, Heavy display and calculation of the area of function; definition of a linear type, points to two categories of data objects, definitions, seeking size and length function.