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From: Alex Vinokur <alexvn AT bigfoot DOT com>
Newsgroups: comp.os.msdos.djgpp,comp.lang.c++,alt.comp.lang.learn.c-c++
Subject: Re: Passing 'const' as 'this' discards qualifiers
Date: Wed, 05 Dec 2001 08:08:17 +0200
Organization: Scopus Network Technologies
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Karl Heinz Buchegger wrote:

> Alex Vinokur wrote:
> >
> > Karl Heinz Buchegger wrote:
> >
> > > Alex Vinokur wrote:
> > > >
> > >
> > > >   public:
> > > >     bool operator() (AAA& ins1, AAA& ins2) const {return false;}
> > >                                              *****
> > >
> >
> > operator() on my program doesn't contain const :
>
> I know, but it should. At least this is what your compilers
> error message means in plain english:
>
> > ttt.c:20: passing `const AAA' as `this' argument of `void AAA::foo1()'
> > discards qualifiers
>
> means
>
> "You try to call a non-const function for a const object"

Which object in my program is const?


>
>
> Oh sorry. Just figured out that the error message doesn't talk
> about the operator altogether. Instead you somewhere try
> to call a function fool(). But the cure is the same. Make
> foo() a const member function.

Thanks. If foo1() is const a compiler doesn't detect error.

But :
  1. Why does the compiler detect an error if foo2() is non const?
  2. What to do if foo2 must be non const?

===============================================================
Windows98
gpp : GNU C++ version 2.95.3 20010315/djgpp (release) (djgpp)
      compiled by GNU C version 2.95.3 20010315/djgpp (release).
===============================================================


//------------- C++ Code : BEGIN -------------
#include <set>

class AAA;

class less_AAA
{
public:
  bool operator()(AAA& ins1, AAA& ins2) {return false;}
};

class AAA
{
  public:
    void foo1 () const {};
    void foo2 () {};
};

int main ()
{
set<AAA, less_AAA>::iterator iter;
  iter->foo1();
  iter->foo2(); // Line#22

  return 0;
}


//-------------- C++ Code : END --------------

//------------- Compiling : BEGIN ------------

ttt.c: In function `int main()':
ttt.c:22: passing `const AAA' as `this' argument of `void AAA::foo2()'
discards qualifiers

//-------------- Compiling : END -------------


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  Alex Vinokur
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    mailto:alexvn AT dr DOT com
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