Friend function in C ++

The friend function in C ++ of a class is defined outside of that class scope, but it has access to all private and protected members of that class. Even if the prototypes for friend functions appear in the class definition, friend functions are not member functions.

A friend can be a function, a function pattern, or a member function, or a class or a class template, in this case, the entire class and all its members are friends.

To declare a function of friend function of a class, precede that function prototype in class definition with friend keyword in C ++, as follows:

 class Box { double  chieurong ; public : double  chieudai ; friend void  inChieuRong ( Box  box  ); void  setChieuRong ( double  rong  ); }; 

To declare all member functions of the LopMai class as friend type of LopMot class, put a following declaration in the definition of LopMot class:

 friend class LopHai ; 

You consider the following program:

 #include using namespace  std ; class Box { double  chieurong ; public : friend void  inChieuRong ( Box  box  ); void  setChieuRong ( double  rong  ); }; // phan dinh nghia ham thanh vien void Box :: setChieuRong ( double  rong  ) {  chieurong  =  rong ; } // Ghi chu: inChieuRong() khong la ham thanh vien cua bat cu class nao. void  inChieuRong ( Box  box  ) { /* Boi vi, ham inChieuRong() la ham friend cua Box, do vay no co the truc tiep truy cap bat cu thanh vien nao cua class nay */  cout  << "Chieu rong cua box la: " <<  box . chieurong  << endl ; } // ham main cua chuong trinh int  main ( ) { Box  box ; // thiet lap chieurong cua box, khong su dung ham thanh vien  box . setChieuRong ( 25.3 ); // su dung ham friend de in chieu rong.  inChieuRong (  box  ); return 0 ; } 

Compiling and running the above C ++ program will produce the following results:

According to Tutorialspoint

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