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How to overload << operator for a class?

Posted on 2007-03-23
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Last Modified: 2010-04-01
Hi,

How do we overload the stream operator for a class again? I want to do this:

class Something {
    int a;
    double d;
    void OverloadedStreamOperator <<()
    {
        ????
    }
};

int main()
{
    Something s;
    s.a = 44;
    s.d = 34.34;
    cout << s << endl;
}

Thanks
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Question by:DJ_AM_Juicebox
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9 Comments
 
LVL 53

Expert Comment

by:Infinity08
ID: 18782467
 ostream& operator<< (ostream& os, const Something& s)
  {
    return os << s.a << " " << s.d << endl;
  }
0
 

Author Comment

by:DJ_AM_Juicebox
ID: 18782538
I tried adding that as a member of my class but get this compiler error:

     binary 'operator <<' has too many parameters
0
 
LVL 86

Expert Comment

by:jkr
ID: 18782537
Note that this is not a member operator, but an operator at global scope, so to make that work you'll usually have to make it a 'friend' of your class to use for non-public members also, i.e.

 ostream& operator<< (ostream& os, const Something& s)
  {
    return os << s.a << " " << s.d << endl;
  }
class Something {

friend ostream& operator<< (ostream&, const Something&);

    int a;
    double d;

};
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Author Comment

by:DJ_AM_Juicebox
ID: 18782552
is there any way to make it a member of the class? Otherwise I'll have all these friends for all my classes at global scope?
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LVL 53

Expert Comment

by:Infinity08
ID: 18782563
Or completely with your class :

class Something {
    int a;
    double d;
  public :
    friend istream& operator>>(istream& is, Something& s);
    friend ostream& operator<<(ostream& os, const Something& s);
};

istream& operator>>(istream& is, Something& s) {
  is >> s.a;
  is >> s.d;
  return is;
}
ostream& operator<<(ostream& os, const Something& s) {
  os << "a = " << s.a << endl;
  os << "d = " << s.d << endl;
  return os;
}
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LVL 86

Expert Comment

by:jkr
ID: 18782586
>>is there any way to make it a member of the class?

No, the '<<' operator has a 'left to right' association. You can make a

Something s;

s << "whatever";

a member, but not a

ostream os;

os << s;
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LVL 86

Accepted Solution

by:
jkr earned 2000 total points
ID: 18782660
BTW, you could also add a 'write()' method like

class Something {
    int a;
    double d;
    ostream& write(ostream& os) const
    {
        os << a;
        os << d;

        return os;
    }
};

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Author Comment

by:DJ_AM_Juicebox
ID: 18782690
yeah i ended up just doing a write() function. How sad!

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LVL 86

Expert Comment

by:jkr
ID: 18782711
I'd actually have chosen the 'friend' approach. It is just a C++ concept that your friends have access to your privates (pardon the pun ;o)
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