C++ operation overloading question

[rekahsoft]

Hi all i have been haviung an issue with operating overloading. I have been reading my C++ book and have came accross code that does not work. Here is an example://test.h
#ifndef __TEST_H
#include <string>
class Test {
friend bool operator==(const Test&, const Test&);
public:
bool operator==(const Test&, const Test&);
protected:
std::string test;
};
#endif

//test.cc
#include "test.h"
bool Test::operator==(const Test& a, const Test& b) {
return a.test == b.test;
}When i try to compile this with g++ i get the following error:bool Test::operator==(const Test&, const Test&) must take exactly one argumentThen when i define the example like so://test.h
#ifndef __TEST_H
#include <string>
class Test {
friend bool operator==(const Test&);
public:
bool operator==(const Test&);
protected:
std::string test;
};
#endif

//test.cc
#include "test.h"
bool Test::operator==(const Test& a) {
return this->test == b.test;
} i get the following error:bool Test::operator==(const Test&) must take exaclty two argumentThen when i define the class without the friend declaration it compiles. Can anybody explane what is happening here? This has thoroughly confused me. Thanks

[WW]

It appears that you have not copied your example and error message
correctly in the second case. In test.cc, change b.test to a.test.
When test.cc is compiled, the error is
bool Test::operater==(const Test&) must take exactly two arguments

I think you are putting in the friend declaration unnecessarily.
Since operator== is a member function, it should not be declared a
friend. In fact, this line
friend bool operator==(cont Test&);
seems to be telling the compiler that you will also define (somewhere
else) a different, non-member operator== function. And, apparently,
a non-member operator== function must take exactly two arguments
(which makes sense).

So your code should really be

//test.h
#ifndef __TEST_H
#include <string>
class Test {
public:
bool operator==(const Test&);
protected:
std::string test;
};
#endif

//test.cc
#include "test.h"
bool Test::operator==(const Test& a) {
return this->test == a.test;
}

with no friends declared.

[rekahsoft]

Thanks for the explanation, i had been using the the way that works (shown by your example) but just did not understand why, it makes more sence now but i do still not tottaly understand. I have fixed my original post so that it makes sence. Thanks

[kvorion]

Boy its been a long time since I used C++. In your case you are trying to overload the == operator in two ways:

1) As a member function
public:
bool operator==(const Test&, const Test&);

and

2) As a friend function

friend bool operator==(const Test&, const Test&);

I reckon that the first definition is what is causing you the error. See, the == operator is a binary operator, and therefore it takes only ONE object as an argument IF it is defined as a member function

So your first definition must be like this:

bool Test:: operator==(const Test& a)
{
return (a.test == this.test)
}


My advice is that avoid using a friend function for overloading if you can achieve the same using a member function. In case you must use the friend function to do you job, remember that a friend function takes TWO arguments for a binary operator.

So you can have this:


bool operator == (const Test& a, Test &b)
{
return (a.test == b.test)
}



BOTH of these will work, but I don know if you should use both of them in the same program cos I dont know if the compiler will be able to identify which one it wants to use.
One more thing, make sure that your == operator can actually compare two strings if this program should work :)

[rekahsoft]

Thanks :D i understand now :)