c++ - inquiry

[Bernhard Guenther <bernhard at epgert dot com>]

Hello,

first of all, my best regards for the developers of gcc, it is a very
big and great piece of work, I appreciate it much and use it
frequently.

I am very curios over a piece of code I wrote recently.
In a slight variation it produces a compile error from g++ and I
have no idea if 

i) I do not understand c++ enough
ii) it is a bug in g++
iii) it is a design flaw in c++

so I decided to share this one with you (you can freely ignore it
without offending me).

Topic: overloading the == and != operators:

this code

#include <iostream>

// comment in to get compile error
#define BUG

struct t_settings {

        bool operator==(t_settings const &a){
                if (a.x != this->x) { return (false); }
                if (a.y != this->y) { return (false); }
                if (a.z != this->z) { return (false); }
                return (true);
        }
#ifndef BUG
        bool operator!=(t_settings const &a){
                return ( ! ( *this == a ) );
        }
#else   
        bool operator!=(t_settings const &a){
                return ( ! ( a == *this ) );
        }
#endif  
        int x;
        int y;
        int z;
};

int main (int argc, char **argv){

        t_settings mySettings;
        mySettings.x=1;
        mySettings.y=2;
        mySettings.z=3;

        t_settings hisSettings;
        hisSettings.x=1;
        hisSettings.y=2;
        hisSettings.z=4;

        if (mySettings != hisSettings){
                std::cout << "Settings not equal" << std::endl;
        } else {
                std::cout << "Settings equal" << std::endl;
        }
        
        return (0);

}

g++ (version 3 and 4 equally) produces:

In member function `bool t_settings::operator!=(const t_settings&)':
error: passing `const t_settings' as `this' argument of `bool t_settings::operator==(const t_settings&)' discards qualifiers

comment out #define BUG and it compiles.

So the order of a and *this are all that matters.

Sorry, but I have no idea why this is so.

Greetings

Bernhard Günther,

Bonn, Germany

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Bernhard Guenther                
<bernhard@guenther.pl>
<bernhard@epgert.com>
ICQ: 1632197, Jabber: tiger@jabber.fsinf.de 
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