Can't initialize friend function operator <<

Question

I am having a problem with friend functions.

I figure this is the only part of the code needed.. My problem is with this function. It says the problem is with the first line, but I don't know how accurate that is.

friend ostream & operator << (ostream & b, Book & a)
    {
    b.setf(ios::fixed | ios::showpoint);
    b.precision(2);
    b << "Title     :  \"" << a.title << "\"\n"
    << "Author      : \"" << a.author << "\"\n"
    << "Price       : $" << a.price / 100.0 << endl
    << "Genre       : " <<a.genre << endl
    << "In stock? " << (a.status ? "yes" : "no") << endl
    << endl;
    return b;
    }

I get the errors : lab10.cpp:95: error: can't initialize friend function âoperator<<â

lab10.cpp:95: error: friend declaration not in class definition

Thanks in advance

Answer 1

You have to specify the function is friend of which class. You either put that function in the class declaration:

class Book{
...
  friend ostream & operator << (ostream & b, Book & a)
    {
    b.setf(ios::fixed | ios::showpoint);
    b.precision(2);
    b << "Title     :  \"" << a.title << "\"\n"
    << "Author      : \"" << a.author << "\"\n"
    << "Price       : $" << a.price / 100.0 << endl
    << "Genre       : " <<a.genre << endl
    << "In stock? " << (a.status ? "yes" : "no") << endl
    << endl;
    return b;
  }
};

The other way is to declare it as a friend inside the class, and define it in some other place:

class Book{
...
    friend ostream & operator << (ostream & b, Book & a);
};

...

// Notice, there is no "friend" in definition!
ostream & operator << (ostream & b, Book & a)
    {
    b.setf(ios::fixed | ios::showpoint);
    b.precision(2);
    b << "Title     :  \"" << a.title << "\"\n"
    << "Author      : \"" << a.author << "\"\n"
    << "Price       : $" << a.price / 100.0 << endl
    << "Genre       : " <<a.genre << endl
    << "In stock? " << (a.status ? "yes" : "no") << endl
    << endl;
    return b;
}

Answer 2

Do you have the friend function prototyped inside the class? You need to have something inside the class indicating this is a friend function. Like the line

  friend ostream& operator<<(...);

or something. Look up a complete example for overloading the insertion/extraction operators for more information.

Answer 3

#include <iostream>
#include <string>
using namespace std;

class Samp
{
public:
    int ID;
    string strName; 
    friend std::ostream& operator<<(std::ostream &os, const Samp& obj);
};
 std::ostream& operator<<(std::ostream &os, const Samp& obj)
    {
        os << obj.ID<< “ ” << obj.strName;
        return os;
    }

int main()
{
   Samp obj, obj1;
    obj.ID = 100;
    obj.strName = "Hello";
    obj1=obj;
    cout << obj <<endl<< obj1;

} 

OUTPUT: 100 Hello 100 Hello Press any key to continue…

This can be a friend function only because the object is on the rhs of << operator and argument cout is on the lhs. So this cant be a member function to the class it can only be a friend function.