explanation needed on const pointers

[deepunayak@gmail.com]

I need complete explanation on constant pointers.

e.g

char *const ptr ;
const char *ptr ;
char *const* ptr;

please explain me the differences.

[Sreekanth]

Re: explanation needed on const pointers

Hi,
You can find out something regarding the const pointer Useage at
following URL:

http://www.geocities.com/spur4444/prog/const_and_pointers.html

deepunayak@gmai l.com wrote:[color=blue]
> I need complete explanation on constant pointers.
>
> e.g
>
> char *const ptr ;
> const char *ptr ;
> char *const* ptr;
>
> please explain me the differences.[/color]

[tatto0_2000@yahoo.com]

Re: explanation needed on const pointers


deepunayak@gmai l.com wrote:[color=blue]
> I need complete explanation on constant pointers.
>
> e.g
>
> char *const ptr ;
> const char *ptr ;
> char *const* ptr;
>
> please explain me the differences.[/color]

Is this ==== char *const* ptr; ====== declaration valid ??

[Pedro Graca]

Re: explanation needed on const pointers

deepunayak@gmai l.com wrote:[color=blue]
> I need complete explanation on constant pointers.
>
> e.g
>
> char *const ptr ;[/color]

ptr = NULL; /* not allowed */
ptr[0] = 42; /* allowed */
[color=blue]
> const char *ptr ;[/color]

ptr = NULL; /* allowed */
ptr[0] = 42; /* not allowed */
[color=blue]
> char *const* ptr;[/color]

ptr = NULL; /* allowed */
ptr[0] = NULL; /* not allowed */
ptr[0][0] = 42; /* allowed */

--
If you're posting through Google read <http://cfaj.freeshell. org/google>

[Ben Bacarisse]

Re: explanation needed on const pointers

On Wed, 15 Feb 2006 23:20:17 -0800, deepunayak@gmai l.com wrote:
[color=blue]
> I need complete explanation on constant pointers.[/color]

Ah, you'll need a good book for that.
[color=blue]
> char *const ptr ;
> const char *ptr ;
> char *const* ptr;
>
> please explain me the differences.[/color]

If you use the rule I outlined in:

http://groups.google.co.uk/group/comp.lang.c/browse_thread/thread/860be00649b49b4a/e44e3d249f1cd41f?lnk=st&rnum=1&hl=en#e44e3d249f1cd41f

you can read these as English words which will help to get you started.

<OT>What is the best way to reference a previous posting? The above
looks horrible and I can't from the style is the URI in long-lived or
not.</OT>

--
Ben.

[Seemanta Dutta]

Re: explanation needed on const pointers

tatto0_2000@yah oo.com wrote:[color=blue]
> deepunayak@gmai l.com wrote:[color=green]
>> I need complete explanation on constant pointers.
>>
>> e.g
>>
>> char *const ptr ;[/color][/color]
value pointed by ptr is constant.
ptr can be changed.
[color=blue][color=green]
>> const char *ptr ;[/color][/color]
value pointed by ptr can change.
ptr itself can't change.[color=blue][color=green]
>> char *const* ptr;
>>[/color][/color]
neither ptr, nor the value pointed by ptr can change.
[color=blue][color=green]
>> please explain me the differences.[/color]
>
> Is this ==== char *const* ptr; ====== declaration valid ??
>[/color]
Yes, see above.

By the way, did you try looking it up in some book before posting it
here?

~seemanta

[Antonio Contreras]

Re: explanation needed on const pointers

Seemanta Dutta wrote:[color=blue]
> tatto0_2000@yah oo.com wrote:[color=green]
> > deepunayak@gmai l.com wrote:[color=darkred]
> >> I need complete explanation on constant pointers.
> >>[/color][/color][/color]

I think you need to study some more C before giving advice.
[color=blue][color=green][color=darkred]
> >> e.g
> >>
> >> char *const ptr ;[/color][/color]
> value pointed by ptr is constant.
> ptr can be changed.[/color]

Just the other way arround. Here ptr is a constant pointer to a char,
which means that the char it points to can be changed, but ptr itself
cannot be changed.
[color=blue][color=green][color=darkred]
> >> const char *ptr ;[/color][/color]
> value pointed by ptr can change.
> ptr itself can't change.[/color]

Again, the other way arround. Here ptr is a pointer to a constant char,
which means that ptr can be changed but you cannot change the char it
points to through this variable.
[color=blue][color=green][color=darkred]
> >> char *const* ptr;
> >>[/color][/color]
> neither ptr, nor the value pointed by ptr can change.[/color]

Completely wrong. Here ptr is a pointer to a constant pointer to a
char. This means that ptr can be changed. The pointer it points to
cannot be changed, but the char pointed by the pointer pointed to by
ptr can be changed.
[color=blue][color=green][color=darkred]
> >> please explain me the differences.[/color]
> >
> > Is this ==== char *const* ptr; ====== declaration valid ??
> >[/color]
> Yes, see above.
>
> By the way, did you try looking it up in some book before posting it
> here?[/color]

Did you try to looking it up before posting advice that was incorrect?

[stathis gotsis]

Re: explanation needed on const pointers

<deepunayak@gma il.com> wrote in message
news:1140074417 .452662.66880@g 43g2000cwa.goog legroups.com...[color=blue]
> I need complete explanation on constant pointers.
>[/color]

My guesses would be the following.
[color=blue]
> char *const ptr ;[/color]

constant pointer to char
[color=blue]
> const char *ptr ;[/color]

pointer to constant char
[color=blue]
> char *const* ptr;[/color]

pointer to constant pointer to char

[John Bode]

Re: explanation needed on const pointers


deepunayak@gmai l.com wrote:[color=blue]
> I need complete explanation on constant pointers.
>
> e.g
>
> char *const ptr ;[/color]

ptr may not be modified; *ptr may be modified:

*ptr = 'b'; /* allowed */
ptr = NULL; /* not allowed */
[color=blue]
> const char *ptr ;[/color]

ptr may be modified; *ptr may not be modified:

*ptr = 'b'; /* not allowed */
ptr = NULL; /* allowed */
[color=blue]
> char *const* ptr;[/color]

ptr may be modified; *ptr may not be modified; **ptr may be modified:

ptr = NULL; /* allowed */
*ptr = NULL; /* not allowed */
**ptr = 'a'; /* allowed */[color=blue]
>
> please explain me the differences.[/color]

[Christian Bau]

Re: explanation needed on const pointers

In article <1140074417.452 662.66880@g43g2 000cwa.googlegr oups.com>,
"deepunayak@gma il.com" <deepunayak@gma il.com> wrote:
[color=blue]
> I need complete explanation on constant pointers.
>
> e.g
>
> char *const ptr ;
> const char *ptr ;
> char *const* ptr;
>
> please explain me the differences.
>[/color]

Any good C reference will do.