generic pointer question

[Logan Lee]

int
main()
{
int i;
char c;
void *the_data;

i = 6;
c = 'a';

the_data = &i;
printf("the_dat a points to the integer value %d\n", *(int*) the_data);
^^^^^^^
the_data = &c;
printf("the_dat a now points to the character %c\n", *(char*) the_data);
^^^^^^^^
return 0;
}

On ^^^^ why can't them be typecasted with int and char respectively like
this:
(int) the_data
(char) the_data

[Richard Heathfield]

Re: generic pointer question

Logan Lee said:

<snip>

On ^^^^ why can't them be typecasted with int and char respectively like
this:
(int) the_data
(char) the_data

It's a pointer. Interpreting it as something else would be like trying to
interpret the Suez Crisis as a small currant bun topped with icing.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Logan Lee]

Re: generic pointer question

Fri, 28 Dec 2007 10:50:27 +0000에, Logan Lee 썼습니다:

int
main()
{
int i;
char c;
void *the_data;
>
i = 6;
c = 'a';
>
the_data = &i;
printf("the_dat a points to the integer value %d\n", *(int*) the_data);
^^^^^^^
the_data = &c;
printf("the_dat a now points to the character %c\n", *(char*) the_data);
^^^^^^^^
return 0;
}
>
On ^^^^ why can't them be typecasted with int and char respectively like
this:
(int) the_data
(char) the_data

i know why (int) the_data... are wrong. but how about (int)*the_data?

[Richard Heathfield]

Re: generic pointer question

Logan Lee said:

<snip>

i know why (int) the_data... are wrong. but how about (int)*the_data?

Because you can't dereference a void *, which in turn is because to do so
would be meaningless.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Logan Lee]

Re: generic pointer question

Fri, 28 Dec 2007 11:12:44 +0000에, Richard Heathfield 썼습니다:

Logan Lee said:
>
<snip>
>

>i know why (int) the_data... are wrong. but how about (int)*the_data?

>
Because you can't dereference a void *, which in turn is because to do so
would be meaningless.
>

Is (int*)the_data meaningful?

[Joachim Schmitz]

Re: generic pointer question

"Logan Lee" <logan.w.lee@st udent.uts.edu.a uschrieb im Newsbeitrag
news:4774dc03$0 $13262$afc38c87 @news.optusnet. com.au...

Fri, 28 Dec 2007 11:12:44 +0000?, Richard Heathfield ????:
>

>Logan Lee said:
>>
><snip>
>>

>>i know why (int) the_data... are wrong. but how about (int)*the_data?

>>
>Because you can't dereference a void *, which in turn is because to do so
>would be meaningless.
>>

>
Is (int*)the_data meaningful?

Depending on context yes. Here you force the (otherwise meaningless) void *
into an int *, which in this case is exactly what you want, as you assigned
it the address of an int before.

Bye, Jojo

[Richard Heathfield]

Re: generic pointer question

Logan Lee said:

<snip>

Is (int*)the_data meaningful?

It is meaningful if and only if the_data points to an int.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[James Kuyper]

Re: generic pointer question

Logan Lee wrote:

Fri, 28 Dec 2007 11:12:44 +0000에, Richard Heathfield 썼습니다:
>

>Logan Lee said:
>>
><snip>
>>

>>i know why (int) the_data... are wrong. but how about (int)*the_data?

>Because you can't dereference a void *, which in turn is because to do so
>would be meaningless.
>>

>
Is (int*)the_data meaningful?

"the_data" points at a memory location, but does not point at any
particular data type, and therefore cannot be dereferenced. If the_data
happens to be correctly aligned (as it is in your code), then
(int*)the_data converts it into a pointer which points at the same
memory location, but is now pointing at an 'int'. When you dereference
it, the memory at that location is interpreted as an int, and result is
the value of that int.

[Keith Thompson]

Re: generic pointer question

Richard Heathfield <rjh@see.sig.in validwrites:

Logan Lee said:
>
<snip>
>

>On ^^^^ why can't them be typecasted with int and char respectively like
>this:
>(int) the_data
>(char) the_data

>
It's a pointer. Interpreting it as something else would be like trying to
interpret the Suez Crisis as a small currant bun topped with icing.

(Missing context: ``void *the_data;'')

A cast does not interpret something as something else, it *converts*
something to something else. (In some cases, the conversion may just
reinterpret the bits; in others, such as an int-to-float conversion,
it re-expresses the value in the new type.)

<OT>C++ has something called a "misinterpret_c ast" -- or is it
"reinterpret_ca st"? C doesn't.</OT>

In this case, converting a void* expression to int or to char is
perfectly legal. The result is implementation-defined and is not
necessarily meaningful, though.

In effect, the language says you can convert the Suez Crisis to a
small currant bun topped with icing, but it doesn't guarantee that the
result will be at all edible.

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
[...]
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Richard Heathfield]

Re: generic pointer question

Keith Thompson said:

<snip>

A cast does not interpret something as something else, it *converts*
something to something else.

You are, of course, quite correct.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[steve.pagliarulo@gmail.com]

Re: generic pointer question

On Dec 28, 2:50 am, Logan Lee <logan.w....@st udent.uts.edu.a uwrote:

int
main()
{
  int i;
  char c;
  void *the_data;
>
  i = 6;
  c = 'a';
>
  the_data = &i;
  printf("the_dat a points to the integer value %d\n", *(int*) the_data);
                                                      ^^^^^^^
  the_data = &c;
  printf("the_dat a now points to the character %c\n", *(char*) the_data);
                                                      ^^^^^^^^
  return 0;
>
}
>
On ^^^^ why can't them be typecasted with int and char respectively like
this:
(int) the_data
(char) the_data

It sounds like you are looking for a typeless language. C is not a
typeless language. C++ templates can help but not straight C.

[CBFalconer]

Re: generic pointer question

Logan Lee wrote:

>
int main() {
int i;
char c;
void *the_data;
>
i = 6;
c = 'a';
the_data = &i;
printf("the_dat a points to the integer value %d\n", *(int*) the_data);
the_data = &c;
printf("the_dat a now points to the character %c\n", *(char*) the_data);
return 0;
}
>
On why can't them be typecasted with int and char respectively
like this:
(int) the_data
(char) the_data

Works fine, with the appropriate #include:

[1] c:\c\junk>cat junk.c
#include <stdio.h>

int main(void) {
int i;
char c;
void *the_data;

i = 6;
c = 'a';

the_data = &i;
printf("*the_da ta is integer value %d\n", *(int*)the_data );
the_data = &c;
printf("*the_da ta now is the char %c\n", *(char*)the_dat a);
return 0;
}

[1] c:\c\junk>cc junk.c

[1] c:\c\junk>.\a
*the_data is integer value 6
*the_data now is the char a

--
Merry Christmas, Happy Hanukah, Happy New Year
Joyeux Noel, Bonne Annee, Frohe Weihnachten
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>



--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

Re: generic pointer question

Richard Heathfield wrote:

Logan Lee said:
>
<snip>
>

>On ^^^^ why can't them be typecasted with int and char
>respectively like this:
>(int) the_data
>(char) the_data

>
It's a pointer. Interpreting it as something else would be like
trying to interpret the Suez Crisis as a small currant bun topped
with icing.

I'm getting hungry.

--
Merry Christmas, Happy Hanukah, Happy New Year
Joyeux Noel, Bonne Annee, Frohe Weihnachten
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>



--
Posted via a free Usenet account from http://www.teranews.com

[Keith Thompson]

Re: generic pointer question

steve.pagliarul o@gmail.com writes:

On Dec 28, 2:50 am, Logan Lee <logan.w....@st udent.uts.edu.a uwrote:

>int
>main()
>{
>  int i;
>  char c;
>  void *the_data;
>>
>  i = 6;
>  c = 'a';
>>
>  the_data = &i;
>  printf("the_dat a points to the integer value %d\n", *(int*) the_data);
>                                                      ^^^^^^^
>  the_data = &c;
>  printf("the_dat a now points to the character %c\n", *(char*) the_data);
>                                                      ^^^^^^^^
>  return 0;
>>
>}
>>
>On ^^^^ why can't them be typecasted with int and char respectively like
>this:
>(int) the_data
>(char) the_data

>
It sounds like you are looking for a typeless language. C is not a
typeless language. C++ templates can help but not straight C.

What makes you think the OP is looking for a typeless language? I
have no idea *why* he wants to cast the_data to int or to char. (In a
typeless language, casting would be meaningless.)

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
[...]
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[spaglia]

Re: generic pointer question

On Dec 28, 8:38 pm, Keith Thompson <ks...@mib.orgw rote:

steve.pagliar.. .@gmail.com writes:

On Dec 28, 2:50 am, Logan Lee <logan.w....@st udent.uts.edu.a uwrote:

int
main()
{
  int i;
  char c;
  void *the_data;

>

  i = 6;
  c = 'a';

>

  the_data = &i;
  printf("the_dat a points to the integer value %d\n", *(int*) the_data);
                                                     ^^^^^^^
  the_data = &c;
  printf("the_dat a now points to the character %c\n", *(char*) the_data);
                                                     ^^^^^^^^
  return 0;

>

}

>

On ^^^^ why can't them be typecasted with int and char respectively like
this:
(int) the_data
(char) the_data

>

It sounds like you are looking for a typeless language. C is not a
typeless language. C++ templates can help but not straight C.

>
What makes you think the OP is looking for a typeless language?  I
have no idea *why* he wants to cast the_data to int or to char.  (In a
typeless language, casting would be meaningless.)
>
--
Keith Thompson (The_Other_Keit h) <ks...@mib.or g>
[...]
"We must do something.  This is something.  Therefore, we must do this.."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"- Hide quoted text -
>
- Show quoted text -

As you say I was focusing on the why and pointing out that C does not
intrinsically handle polymorphic types. In his example he wants to
treat the_data as a polymorphic type. Perhaps I'm reading too much
into his quetion and it is in fact about casts.