From: Jack Klein <jackklein AT spamcop DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: [OT] About const int
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Date: Thu, 19 Jul 2001 04:48:14 GMT
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On Wed, 18 Jul 2001 21:41:11 +0530, Prashant Ramachandra
<rprash AT wilco-int DOT com> wrote in comp.os.msdos.djgpp:

> I have a question that's probably got nothing to with DJGPP, so please
> forgive me for this off-topic post.
> 
> I tried the program below and interestingly, it gives me...
> 
> "Constant is 10. It is actually 1"
> 
> Shouldn't gcc not make the assumption that the value of j *is* 10 and
> instead reference itfrom memory instead.
> 
> > Program:
> > 
> > #include <iostream.h>
> > 
> > class Test {
> > public:
> >   static const int i;
> > 
> >   void run () { cout << i << endl; }
> >   Test () { }
> > };
> > 
> > const int Test::i = 7;
> > 
> > const int j = 10;
> > 
> > int main ()
> > {
> >   Test t1;
> >   Test t2;
> > 
> >   int *k = (int *)&j;
> >   *k = 1;
> >   cout << "Constant is " << j << ". It is actually " << *k << endl;
> >   t1.run ();
> >   t2.run ();
> > }
> > 
> Thanks a lot for any explanations.
> 
> Regards,
> Prashant

1.  You should be using const_cast in C++ to remove the const
qualifier from a constant pointer.

2.  The minute you attempt to modify the value of an object defined
with a const qualifier you invoke undefined behavior.  You have no
recourse at all to the C++ standard for the result, the ANSI/ISO
standard specifically states that undefined behavior is that for which
the language standard imposes no requirements at all.

3.  What you have made here is no longer a C++ program.  If it printed
42 it would be just as correct as far as C++ is concerned.

-- 
Jack Klein
Home: http://JK-Technology.Com
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