fig07_11.cpp

《c＋＋大学教程》源码

// Fig. 7.11: fig07_11.cpp  
// Friends can access private members of a class.
#include <iostream>

using std::cout;
using std::endl;

// Count class definition 
// (note that there is no friendship declaration)
class Count {
   friend void setX( Count &, int ); // friend declaration

public:

   // constructor
   Count()
      : x( 0 )  // initialize x to 0
   { 
      // empty body
   
   } // end constructor Count

   // output x 
   void print() const       
   { 
      cout << x << endl; 

   } // end function print

private:
   int x;  // data member

}; // end class Count

// function setX can modify private data of Count 
// because setX is declared as a friend of Count  
void setX( Count &c, int val )
{
   c.x = val;  // legal: setX is a friend of Count

} // end function setX

int main()
{
   Count counter;       // create Count object

   cout << "counter.x after instantiation: ";
   counter.print();

   setX( counter, 8 );  // set x with a friend

   cout << "counter.x after call to setX friend function: ";
   counter.print();

   return 0;

} // end main

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