普通函数作友元：声明普通函数distance用来计算两点距离，定义一个Circle类，两圆心的距离用普通函数distance来计算。这个函数需要访问Circle类的私有数据x和y，为此，将distance声明为Circle类的友元函数。主函数如下定义：-Common function for friend: Statement used to calculate the normal function of distance from two points define a Circle class, the distance between two center distance is calculated using an ordinary function. This function needs access to the Circle class private data x and y, this would distance the Circle class declaration for the friend function. The main function defined as follows: