友元函数用来访问类的非公有成员。友元函数不是类的成员函数。
A friend function is used for accessing the non-public members of a class.
要求：分别用成员函数和友元函数来求圆的面积和周长。
Requirements: member function respectively, and a friend function to find area and circumference of a circle.
要求：分别用成员函数和友元函数来求圆的面积和周长。
Requirements: member function respectively, and a friend function to find area and circumference of a circle.
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