C + + Learning notes-friend

C + + Controls access to the private part of the class object, and the private or protected members of the class cannot be accessed directly outside. Generally, public-class methods provide a unique access path. Sometimes this restriction is too restrictive to be appropriate for a particular programming problem. So C + + provides a friend of this form, by making a function or class A friend of Class A, you can give the function or class A member function with the same access rights.

There are 3 types of friends:

(1) Friend function

(2) Friend class

(3) Friend member function

1, friend function

Put its prototype in the class declaration and add the Friend keyword

Such as:

Class time{public://friend function friend time operator* (double N, const time & T);//overloaded << operator friend Std::ostream & Oper ator<< (Std::ostream & OS, const time & T);p rivate:int m_hours;int m_minutes;};

Attention:

(1) operator* () is declared in a class declaration, but it is not a class member function, so when defining a function, you cannot use the time:: qualifier.

(2) operator* () is not a member function, but has the same access rights as member functions.

In summary, the friend function of a class is a non-member function, and its access rights are the same as member functions.

2, Friend class

You can use a class as a friend, and all methods of a friend class can access the private and protected members of the original class.

The following friend class Remote; Declare remote as a friend of the TV class:

#ifndef tv_h_#define tv_h_//TV class Tv{public:friend class remote;//declares that the Remote class is a friend of the TV class, All methods of the remote class can access the private and protected members of the TV enum{off,on};enum{minval,maxval = 20}; Tv (int s = off,int MC = +), ~tv (void), void On_off () {state = (state = = on? off:on); BOOL Is_on () const{return state = = on;}; BOOL Vol_up (); bool Vol_down (); void channel_up (); void Channel_down (); void Show_settings () const;private:int State;int Volume;int Maxchannel;int channel;};/ /Remote Control class Remote{public:bool VOL_UP (TV & TV) {return tv.vol_up ();}; BOOL Vol_down (TV & TV) {return Tv.vol_down ();}; void On_off (TV & TV) {Tv.on_off ();}; void CHANNEL_UP (TV & TV) {tv.channel_up ();}; void Channel_down (TV & TV) {Tv.channel_down ();}; void Set_channel (TV & TV, int ch) {Tv.channel = ch;};/ /friend can access the private member of the original class}; #endif

a friend declaration can be in the public, private, or protected section, where it does not matter.

3, friend member function

In the remote class, only the Set_channel (TV & TV, int ch) method accesses the private members of the TV class directly, so you can choose to make this method a friend of the class without having to make the entire remote class A friend. However, this must be done carefully by arranging the order of declarations and definitions.

To make Remote::set_channel () a friend of the TV class, the non-method is to declare it as a friend in the TV class:

Class tv{   friend void Remote::set_channel (TV & TV, int ch);};

C + + Learning notes-friend