Why have to use friend function

Question

I am trying to overload operator '=' and operator '<<' by the same method.

class Vect{
 public:
  //..
   Vect& operator=(const Vect& a);      
   ostream& operator<<(ostream& out, const Vect& vect);
  //..
 private:
  int *data;
  int size;
};

this work

Vect& Vect:: operator=(const Vect& a){
 //.. 
 //copy data operator
 for(int i = 0; i< size; i++){
  data[i] = a.data[i];
 }
 return *this;
}

however: this code cause error
[Error] 'std::ostream& Vect::operator<<(std::ostream&, const Vect&)' must take exactly one argument

ostream& Vect::operator<<(ostream& out, const Vect& vect){
 //.. print vect
}

I am reading "Data structure and algorithms in C++" book part (1.5.4). They said i have to use class friends for overloading '<<' operator because it is access private members data. I don't understand why. Overloading '=' operator i also access private member data without using "friend".

Aren't you curious about the error you got? Because it doesn't say anything about accessing private data members. (although that will likely also be a problem, once you fix the problem that's causing the current error). — Benjamin Lindley

Tony Delroy Tony Delroy · Accepted Answer · 2015-11-17T06:04:52

When you put a function declaration inside the class definition, it becomes by default a member function, so...

class Vect {
  public:
    ostream& operator<<(ostream& out, const Vect& vect);
};

...won't compile as it requests creation of a << function that takes too many arguments arguments: any member function operator<< is expected to use *this as the "left hand side" argument, and take one other argument to be the "right hand side".

You have two options:

Replace the above with friend ostream& operator<<(ostream& out, const Vect& vect);, which tells the compiler that the function is a friend of the surrounding class, but not a member thereof. As a non-member, the two arguments it operates on are out and vect - there is no *this object involved. That all works fine, and being a friend the definition also has access to private and protected member data in vect.
Move the operator<< declaration outside the class Vect definition; that also makes it a non-member function, but that doesn't make it a friend.

Why have to use friend function

2 Answers