Friends have access to the private parts

A friend can be  a function, a method, or an entire class (all its methods)

Friends have good uses:
    Simulating package visibility
    Increasing encapsulation

Friends have bad uses:
    Indiscrimate violation of encapsulation
    Performance hacks

(See template notes for template friends)

A Friend of a Friend is not a Friend

Example: implementing an output stream insertion operator

#include <iostream.h>
#include <string.h>

const int MAX_CHAR = 10;

class BankAccount {
 friend ostream& operator<<(ostream& o, BankAccount &b);
public:
        BankAccount ( char* name ="unknown", char * mr = "Miss", float y=0.0)
        {
          title = mr;
          strcpy(owner, name);
          balance = y;
        }

private:
     char * title;
        char owner[MAX_CHAR];
        float balance;
};
 

ostream& operator<< (ostream& o, BankAccount &b)
{
 o <<"who:"<< b.title << " " << b.owner<<" how much="<< b.balance<<"\n";
 o << endl;
 return o;
}

void main(){
 BankAccount acc("Lou","Mr", 100);
 cout << acc;
}

#include <iostream.h>
#include <string.h>

const int MAX_CHAR = 10;

class BankAccount {
public:
        BankAccount ( char* name ="unknown", char * mr = "Miss", float y=0.0)
        {
          title = mr;
          strcpy(owner, name);
          balance = y;
        }
        void print(ostream& o)
        {  o <<"who:"<< title << " " << owner<<" how much="<< balance<<"\n"; }

private:
     char * title;
        char owner[MAX_CHAR];
        float balance;
};

//notice use of references -to be discussed later
ostream& operator<< (ostream& o, BankAccount &b)  //notice pass by reference
{
 b.print(o);
 o << endl;
 return o;
}

void main(){
 BankAccount acc("Lou","Mr", 100);
 cout << acc;
}

Example: BankAccount with friend class Bank

#include <iostream.h>
#include <string.h>

const int MAX_CHAR = 10;

class BankAccount {
 friend ostream& operator<<(ostream& o, BankAccount &b);
 friend  class Bank;
public:
        BankAccount ( char* name ="unknown", char * mr = "Miss", float y=0.0)
        {
          title = mr;
          strcpy(owner, name);
          balance = y;
        }

private:
     char * title;
        char owner[MAX_CHAR];
        float balance;
};

ostream& operator<< (ostream& o, BankAccount &b)
{
 o <<"who:"<< b.title << " " << b.owner<<" how much="<< b.balance<<"\n";
 o << endl;
 return o;
}
 
 

class Bank{
  public:
 void deposit(BankAccount & b, float amount){b.balance += amount;}
 void withdraw(BankAccount & b, float amount){b.balance -= amount;}
};
 
 
 

void main(){
 BankAccount acc("Lou","Mr", 100);
 Bank theBank;
 theBank.deposit(acc,500);
 cout << acc;
}