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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 09:15:19 +0200
Organization: Scopus Network Technologies
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Victor Bazarov wrote:

[snip]

> 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
> --
> Please remove capital A's from my address when replying by mail

Thanks. You are right.

Here is convincing example using map.
We can see that
  * there is no problem with second.foo2();   // foo2() is non const
  * there is a problem with first.foo2();

===============================================================
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>
#include <map>

class AAA;

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

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

int main ()
{
map<AAA, AAA, less_AAA>::iterator iter_map;
  iter_map->second.foo1();
  iter_map->second.foo2();

  iter_map->first.foo1();
  iter_map->first.foo2(); // Line#26

set<AAA, less_AAA>::iterator iter_set;
  iter_set->foo1();
  iter_set->foo2();  // Line#30

  return 0;
}


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

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

%gpp ttt.c

ttt.c: In function `int main()':
ttt.c:26: passing `const AAA' as `this' argument of `void AAA::foo2()' discards
qualifiers
ttt.c:30: passing `const AAA' as `this' argument of `void AAA::foo2()' discards
qualifiers

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

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  Alex Vinokur
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