c++ operator<< member of class

Question

I have a problem with the << operator in C++. I need this operation work:

  cout << comp2 << comp1;


class Compo
{
    string name;
    int power;
    string app;

public:
    Compo(string s, string a, int p):name(s), power(p),app(a){};
    //~Compo();    

     string GetAsString()const{
        ostringstream oss;    
        oss << name << " [ " << power << " ] desc: " << app << endl;        
        return oss.str();
    };


     string &operator<<(Compo& aux)const{               
        return aux.GetAsString();
    };
};

void main()
{
    Compo comp1("Pencil","Best",12);
    Compo comp2("Notes","Not Best",22);
    cout << comp2 << comp1;        
    cin.get();
}

i have erro on cout << comp2

the error description is,

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'Compo' (or there is no acceptable conversion)

and one warn alert,

returning address of local variable or temporary

ho i need change to this works.

You need to implement it as a free function. The link I provided when I voted to close this question tells you why,. — John Dibling, Jan 18 '14 at 20:01
@JohnKugelman: Maybe there is. I invite you to find a better one. — John Dibling, Jan 18 '14 at 20:02
<< and >> operators are implemented like two scrolls down in the provided link — prajmus, Jan 18 '14 at 20:03

πάντα ῥεῖ · Answer 1 · 2014-01-18T21:32:07.120

1

You'll need to declare a standalone output operator method to do this (class free function).

std::ostream& operator<<(std::ostream& os, const Compo& obj)
{
    obj.put(os);
    return os;
}

In class Compo have:

// ...
public:
    void put(std::ostream& os) const
    {
        // output member values here ...
    }
// ...

UPDATE:
Or alternatively (if you can't change Comp actually):

std::ostream& operator<<(std::ostream& os, const Compo& obj)
{
    os << obj.GetAsString();
    return os;
}

IMHO the 1st solution I've proposed here is the more flexible one, for fulfilling future requirements in the end. It might be used with implementations for std::ostream that don't handle stringyfied formats.

edited Jan 18 '14 at 21:32

answered Jan 18 '14 at 20:01

πάντα ῥεῖ

1
13
116
190

Put is the my GetAsString method! – Góis Jan 18 '14 at 20:08
@Góis _'Put is the my GetAsString method!'_ Not really (have a closer look to the method signatures!!), but you might use it to implement `put()` or the `operator<<()` methods. – πάντα ῥεῖ Jan 18 '14 at 20:11
@Góis I've put some updates to clarify ... – πάντα ῥεῖ Jan 18 '14 at 21:32

score 0 · Answer 2 · answered Jan 18 '14 at 20:11

The compiler can't find any operator<< which takes as a left argument std::ostream and as right argument Compo. You defined one which takes as a left argument a std::string and as the right argument Compo. Try:

class Compo{
//Stuff
public:
    friend ostream& operator<<(ostream& os, Compo& aux);
};
ostream& operator<<(ostream& o, const Compo& aux)
{
    o << name << " [ " << power << " ] desc: " << app << endl;
    return o;
}

score 0 · Accepted Answer · answered Jan 18 '14 at 20:14

0

You need something like this operator:

ostream& operator << (ostream& o, const Compo& c)
{
   return o << c.GetAsString();
}

answered Jan 18 '14 at 20:14

Igor

1,029
9
21

score 0 · Answer 4 · answered Jan 18 '14 at 20:24

0

#include <iostream>

using namespace std;

class Compo
{
    string name;
    int power;
    string app;

public:
    friend ostream& operator<< (ostream&, const Compo&);
     ....
     .....

};

ostream& operator<< (ostream& out, const Compo& c)
{
   out << c.name << "[" << c.power << " ] desc: " << c.app << endl;
}

answered Jan 18 '14 at 20:24

Ziyad Mestour

97
10

**NOTE:** The OP already stated, that it's not possible for him/her to use `friend` ... – πάντα ῥεῖ Jan 18 '14 at 21:19

c++ operator<< member of class

4 Answers4