Thread: Output overloading

I came up with a contradiction between the rules for overloading the output operator and what my code allowed me to do when I tried to provoque the error.

The class declares the operator unfriendly:

Code:

class Opers {
public:
/* ... */
    std::ostream& operator<<(std::ostream&, const Opers&);
private:
/*... */
};

The cpp file for the class goes on defining it:

Code:

#include "oper.h"
std::ostream& operator<<(std::ostream &os, const Opers &obj) {
    os << obj.val() << " - " << obj.text();
    return os;
}

No errors, no warnings and the test in main() outputs correctly. This shouldn't happen from what I've been learning. The operator, being declared a member of Oper, should have the implicit This as the first parameter...

What am I missing?

The operator, being declared a member of Oper, should have the implicit This as the first parameter...

Since operator<< is dyadic, an implicit this cannot be.

Have you actually tried to use the function?

My guess is that the operator<< member function is simply an error, but since it was never used, it never was a problem.

The actual operator<< free function works because it doesnt need to be a friend function. However, since it is declared in the source file, you would not be able to use it since a file that includes oper.h would not have its prototype.

I'm not sure if I understand Laserlight.

I have it prototyped in the class, much like I would have any other function member that is later defined on the cpp file.

The text in main is called with a simple std::cout << obj; (where obj is an object of Opers type). And it gave me the correct output.

Code:

class Opers {
public:
/* ... */
    std::ostream& operator<<(std::ostream&, const Opers&);
/*
 Error E2080: 'Opers::operator <<(std::ostream &,const Opers &)' 
 must be declared with one parameter
*/
private:
/*... */
};

Wait... I can see...

My function definition is also a declaration.
I didn't use the scope operator. I can understand what you mean by free function. The following does give me an error now.

Code:

#include "oper.h"
std::ostream& Opers::operator<<(std::ostream &os, const Opers &obj) {
    os << obj.val() << " - " << obj.text();
    return os;
}

Only question now is why did I have access to that function before?

I have it prototyped in the class, much like I would have any other function member that is later defined on the cpp file.

But there is no operator<< that takes three arguments. operator<< is dyadic.

The text in main is called with a simple std::cout << obj; (where obj is an object of Opers type). And it gave me the correct output.

Amazing.

Let's try a test:

1. oper.h

Code:

#ifndef OPER_H
#define OPER_H

#include <iosfwd>

class Opers {
public:
	std::ostream& operator<<(std::ostream&, const Opers&);
	int val() const { return 0; }
	int text() const { return 0; }
};

#endif

2. oper.cpp

Code:

#include "oper.h"

#include <ostream>

std::ostream& operator<<(std::ostream& os, const Opers& obj) {
	os << obj.val() << " - " << obj.text();
	return os;
}

3. main.cpp

Code:

#include <iostream>

#include "oper.h"

int main() {
	Opers test;
	std::cout << test << std::endl;
}

What do you get?

I believe this is just a separate function.

Code:

std::ostream& operator<<(std::ostream &os, const Opers &obj) {
    os << obj.val() << " - " << obj.text();
    return os;
}

Code:

#include <iostream>

class Opers {
};

std::ostream& operator<< (std::ostream &os, const Opers &obj) {
    os << "hello world";
    return os;
}

int main() {
   Opers op;
   std::cout << op << '\n';
    return 0;
}

/* my output
hello world
*/

Only question now is why did I have access to that function before?

You never did, since it was in a source file. Perhaps some build quirk? Did you remember to rebuild all to check?

Ah! The Rebuild All did it. Thanks.

Now I get the proper errors. Trying to declare operator<< with more than one argument inside the class and a no match error from trying to access the operator<< from main.

My first time where this strategy to provoque errors to better understand what I'm learning, backfires on me.


Thanks both again

Trying not to start another thread on a related issue...

It seems I'm not being able to understand correctly how to use the overload input operator.
The following isn't working. The error states a no match for operator>> on the line colored red.

Code:

------- Opers.h ------
class Opers {
public:
/* ... */
    friend std::istream& operator>>(std::istream&, const Opers&);
private:
    int val_;
    std::string str_;
};

------- Opers.cpp ------
/* ... */
std::istream& operator>>(std::istream &is, const Opers &obj) {
    is >> obj.val_ >> obj.str_;
    if(!is)
        obj = Opers();
    return is;
}

The idea is to read two values from the stream. What am I missing?

Terribly sorry. The second parameter can't be const. The error message was too cryptic for me to look at the obvious.