Explanation of the relationship between C ++ and youyuan

In C ++, in some cases, it is convenient to allow a specific non-member function to access a private member of a class while still blocking normal access. For example, an overloaded operator, such as an input or output operator, often needs a private data member of the operator class. These operators cannot be members of a class. However, although they are not members of a class, they are still the "interface components" of the class ". Youyuan allows a class to grant access to its non-public members to a specified function or class. The statement of youyuan starts with the keyword "friend. It can only appear within the class definition. A friend statement can appear anywhere in the class: A friend is not a member of the class that grants a friend relationship, so they are not affected by Access Control in part of the statement. The following are all the conclusions and restrictions on user Meta in C ++, and examples are given for each detail: 1. There are three types of user meta relationships: 1.1. A non-member function is reset () declared as a friend function of the example Class, so that this non-member function can be a private member of the example class. [Cpp] class example; // The class example must be declared in the forward direction. Otherwise, the following function declaration will report the error void reset (example & e); class example {public: friend void reset (class example & e); private: int n ;}; // The function definition must be placed behind the class example, otherwise, the variable n void reset (example & e) {e. n = 0;} 1.2, declares the class man as a woman-like user Meta class, so that you can use the class man object to declare the private member of the woman class. [Cpp] class woman; // class man {public: void disp (woman & w); void reset (woman & w) ;}; class woman {public: friend class man; // set man to woman's friend class, so that any member function of the man object can access woman's private member private: string name ;}; void man :: disp (woman & w) {cout <w. name <endl;} void man: reset (woman & w) {w. name. clear ();} 1.3: declares a member function of class Y as a friend function of Class X, so that you can use this member function of class Y to access a private member of class X. [Cpp] class woman; // class man {public: void disp (woman & w); void reset (woman & w) ;}; class woman {public: friend void man: disp (woman & w); // set one of man's member functions disp () to woman's friend function, you can use this function to access the private member of the woman object. private: string name ;}; void man: disp (woman & w) {cout <w. name <endl;} // The reset () member function of man is not a woman function, so it cannot access its private member/* void man: reset (woman & w) {w. name. clear ();} */2,