prog output?
vijay
Code:
#include "iostream.h"
void main(void)
{
const int i=10;
int *p;
p=const_cast<int*>(&i);
*p=111;
cout<<i<<" "<<*p;
}
output:
10 111
Can you plaese tell me why the value of i and *p is different??
Thanks and regards,
Vijay
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
Ulrich Eckhardt
> #include "iostream.h"
If it was a system header, it would have to be <iostream.h>. Further, even
that would be an ancient (pre-standard) iostream header. Check the book
you are learning from with the reviews at http://accu.org and dump the
book if it isn't good (or the teacher, in case you're learning at a
school).
> void main(void)
main() returns an int. Always.
> {
> const int i=10;
> int *p;
> p=const_cast<int*>(&i);
> *p=111;
Using const_cast to remove the const from a pointer/reference that refers
to a const object (i.e. when the const in the pointer/ref was not added
lateron) invokes undefined behaviour. In other words, your program is
broken and could output whatever it wants.
[...]
> output:
>
> 10 111
>
>
> Can you plaese tell me why the value of i and *p is different??
See above, your program is broken.
Uli
Thomas Maeder
"vijay" <mt.v...@gmail.com> writes:
> #include "iostream.h"
This a non-Standard header. Consider #includeing <iostream> (and
writing std::cout etc.). For your code to reliably compile and work,
you also have to #include <ostream>, by the way.
> void main(void)
The return type of main() has to be int.
> {
> const int i=10;
> int *p;
> p=const_cast<int*>(&i);
> *p=111;
Modifying a constant object has undefined behavior.
> cout<<i<<" "<<*p;
> }
>
> output:
>
> 10 111
>
>
> Can you plaese tell me why the value of i and *p is different??
This is just an instance of undefined behavior.
We can speculate that the compiler "knows" that i, being const, will
never change, and substitutes usages of i with usages of the value 10.
kanze
> Code:
> #include "iostream.h"
Two problems right off the bat: it's more or less dangerous to
use the #include "..." form for headers -- you should use
#include <...> instead --; and with any modern compiler, it's
not iostream.h that you want, but iostream and ostream, .e.g:
#include <iostream>
#include <ostream>
> void main(void)
And this line shouldn't compile -- main returns an int. Also,
the use of void to specify that the function takes no parameters
is a C'ism, which pretty much marks one as unfamiliar with C++.
> {
> const int i=10;
> int *p;
> p=const_cast<int*>(&i);
> *p=111;
And this line has undefined behavior. You're attempting to
modify a const object. On some systems, for example, you might
get a core dump. If i were static, you'd probably get it on
most systems.
> cout<<i<<" "<<*p;
> }
> output:
> 10 111
> Can you plaese tell me why the value of i and *p is
> different??
Because the program has undefined behavior. Anything in
possible. (What's probably happening is that the compiler knows
the value of i in the output statement, and simply uses it
directly. Which is, after all, the intent of const objects.)
--
James Kanze GABI Software
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Seungbeom Kim
>
> Using const_cast to remove the const from a pointer/reference that refers
> to a const object (i.e. when the const in the pointer/ref was not added
> lateron) invokes undefined behaviour.
Isn't it the action of actually trying to modify a const object
(through a pointer that const was taken off from by const_cast)
that invokes an undefined behaviour?
AFAIK, just using const_cast won't invoke a UB.
--
Seungbeom Kim
Gerhard Menzl
> #include "iostream.h"
>
> void main(void)
> {
> const int i=10;
> int *p;
> p=const_cast<int*>(&i);
> *p=111;
> cout<<i<<" "<<*p;
> }
>
> output:
>
> 10 111
>
>
> Can you plaese tell me why the value of i and *p is different??
Because your code tries to modify a const object, which invokes
undefined behaviour. The program could do anything.
--
Gerhard Menzl
#dogma int main ()
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Francis Glassborow
<musi...@bawi.org> writes
>Ulrich Eckhardt wrote:
>>
>> Using const_cast to remove the const from a pointer/reference that refers
>> to a const object (i.e. when the const in the pointer/ref was not added
>> lateron) invokes undefined behaviour.
>
>Isn't it the action of actually trying to modify a const object
>(through a pointer that const was taken off from by const_cast)
>that invokes an undefined behaviour?
>
>AFAIK, just using const_cast won't invoke a UB.
That depends. If your code attempts to generate a pointer to read only
memory which can be written through there is nothing in the Standard
that denies the system from aborting the program.
--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
James Kanze
> In article <e5mt5d$e2f$1...@news.Stanford.EDU>, Seungbeom Kim
> <musi...@bawi.org> writes
>> Ulrich Eckhardt wrote:
>>> Using const_cast to remove the const from a pointer/reference that
refers
>>> to a const object (i.e. when the const in the pointer/ref was not added
>>> lateron) invokes undefined behaviour.
>> Isn't it the action of actually trying to modify a const object
>> (through a pointer that const was taken off from by const_cast)
>> that invokes an undefined behaviour?
>> AFAIK, just using const_cast won't invoke a UB.
> That depends. If your code attempts to generate a pointer to
> read only memory which can be written through there is nothing
> in the Standard that denies the system from aborting the
> program.
Where do you read that in the standard. I don't think the
const_cast is a problem in itself; only what you do with the
results. And I'm pretty sure that a char* to const data is
legal.
--
James Kanze kanze...@neuf.fr
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34