Message-ID: <20010720002232.42201.qmail@web9906.mail.yahoo.com>
Date: Thu, 19 Jul 2001 17:22:32 -0700 (PDT)
From: Lets Go Canes <letsgonhlcanes AT yahoo DOT com>
Subject: Re: [OT] About const int 
To: djgpp AT delorie DOT com
In-Reply-To: <5BF60CD649EDD411A04600B0D049F53AFFA913@hydmail02.hyd.wilco-int.com>
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Hi all.

What you are doing is called "aliasing".  Many compilers would give you
the same result even if you removed the "const" qualifier.  Basically,
the compiler doesn't know that "k" is a pointer to the same memory
occupied by "j" (i.e., "k*" is an alias for "j"), and so if it already
has the value of "j" in a register, it will use it, even though you
just changed the value of "j" via the "k*" alias.  With most compilers,
you can use a command-line switch to warn it that you are using
aliases,
but then the compiler will assume that *all* memory references have to
go to memory instead of using a value cached in a register, and this
will slow your program.

Unless you have an exceptionally strong reason to do so, aliasing is
"a bad thing".  I rank it *well* below "goto" on the "bad things" list.


--- Prashant Ramachandra <rprash AT wilco-int DOT com> wrote:
> 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
> --
> Prashant TR <tr AT midpec DOT com>  Web: http://www.midpec.com/
> 
> "Those who do not understand Unix are condemned to reinvent it,
> poorly."
>      -- Henry Spencer
> 
> 
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=====
--------------
Lets Go Canes!

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