When do I have to use a friend function rather than a member function?
当我需要使用友元函数而不是成员函数?。
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.
要求：分别用成员函数和友元函数来求圆的面积和周长。
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