C++/Class/Friend

Define friend function for

<source lang="cpp">

1. include <iostream>
2. include <cstring>
3. include <cstdlib>

using namespace std; class StringClass {

 char *p;
 int len;

public:

 StringClass(char *ptr);
 ~StringClass() {
    delete [] p;
 }
 friend ostream &operator<<(ostream &stream, StringClass &ob);

}; StringClass::StringClass(char *ptr) {

 len = strlen(ptr)+1;
 p = new char [len];
 if(!p) {
   cout << "Allocation error\n";
   exit(1);
 }
 strcpy(p, ptr);

} ostream &operator<<(ostream &stream, StringClass &ob) {

 stream << ob.p;
 
 return stream;

} int main() {

 StringClass stringObject1("www.java2s.com"), stringObject2("www.java2s.com");
 cout << stringObject1;
 cout << endl << stringObject2 << endl;
 return 0;

}

 </source>

friend class for each other

<source lang="cpp">

1. include <iostream>
2. include <string.h>

using namespace std; class Curly {

public:
  Curly(char *msg) { strcpy(message, msg); };
  void show_message(void) { cout << message << endl; };
  friend class Moe;
  void show_moe(class Moe moe);
private:
  char message[256];
};

class Moe {

public:
  Moe(char *msg) { strcpy(message, msg); };
  void show_message(void) { cout << message << endl; };
  friend class Curly;
  void show_curly(class Curly curly);
private:
  char message[256];
};

void Curly::show_moe(class Moe moe) { cout << moe.message << endl; }; void Moe::show_curly(class Curly curly) { cout << curly.message << endl; }; int main(void) {

  class Moe moe("nuck...");
  class Curly curly("whoop...");

  moe.show_message();
  moe.show_curly(curly);
  curly.show_message();
  curly.show_moe(moe);

}

 </source>

Friend function Demo

<source lang="cpp">

1. include <iostream>

using namespace std; class myclass {

 int num;

public:

 myclass(int x) { 
    num = x; 
 }
 friend int isneg(myclass ob);

}; int isneg(myclass ob) {

 return (ob.num < 0) ? 1 : 0;

} int main() {

 myclass a(-1), b(2);
 cout << isneg(a) << " " << isneg(b);
 cout << endl;
 return 0;

}

 </source>

Friends can access private members of a class.

<source lang="cpp">

1. include <iostream>

using std::cout; using std::endl; class Count {

  friend void setX( Count &, int ); 

public:

  Count() { x = 0; }                
  void print() const { cout << x << endl; }

private:

  int x;

}; void setX( Count &c, int val ) {

  c.x = val;  // legal: setX is a friend of Count

} int main() {

  Count counter;
  cout << "counter.x after instantiation: ";
  counter.print();
  cout << "counter.x after call to setX friend function: ";
  setX( counter, 8 );  // set x with a friend
  counter.print();
  return 0;

}

 </source>

Make the inserter into a friend function

<source lang="cpp">

1. include <iostream>
2. include <cstring>

using namespace std; class PhoneNumber {

 char name[80];
 int areaCode;
 int prefix;
 int num;

public:

 PhoneNumber(char *n, int a, int p, int nm)
 {
   strcpy(name, n);
   areaCode = a;
   prefix = p;
   num = nm;
 }
 friend ostream &operator<<(ostream &stream, PhoneNumber o);

}; // Display name and phone number. ostream &operator<<(ostream &stream, PhoneNumber o) {

 stream << o.name << " ";
 stream << "(" << o.areaCode << ") ";
 stream << o.prefix << "-" << o.num << "\n";
 return stream; // must return stream

} int main() {

 PhoneNumber a("T", 111, 555, 1234);
 PhoneNumber b("A", 312, 555, 5768);
 PhoneNumber c("T", 212, 555, 9991);
 cout << a << b << c;
 return 0;

}

 </source>

Use friend function to access the non-public member variable

<source lang="cpp">

1. include <iostream>

using namespace std; class MyClass {

 int a, b;
 public:
   friend int sum(MyClass x);
   void set_ab(int i, int j);

}; void MyClass::set_ab(int i, int j) {

 a = i;
 b = j;

} int sum(MyClass object) {

 return object.a + object.b;

} int main(void) {

 MyClass integer;
 cout << "Adding 3 and 4:" << endl;
 integer.set_ab(3,4);
 cout << sum(integer);

}

 </source>