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From: "Victor Bazarov" <vAbazarov AT dAnai DOT com>
Newsgroups: comp.os.msdos.djgpp,comp.lang.c++,alt.comp.lang.learn.c-c++
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Subject: Re: Passing 'const' as 'this' discards qualifiers
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Message-ID: <WgjP7.548$cH6.58104@rwcrnsc52>
Date: Wed, 05 Dec 2001 06:37:42 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

"Alex Vinokur" <alexvn AT bigfoot DOT com> wrote in message
news:3C0DB9D0 DOT 6217E7AD AT bigfoot DOT com...
>
>
> 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?


My guess would be that since 'set' is an associative container,
dereferencing an iterator returns a constant object (each set
is an ordered collection, what good would non-const object ref
do if you could arbitrarily change it?)

>   2. What to do if foo2 must be non const?

Tough luck.  You are not supposed to change the object once they
have been inserted into an associative container.  That would
potentially change their relation and violate their ordering.

Victor
--
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