Pointers

[ByteSurfer]

I am realy confussed bout pointers.. Can you guys please tell me what
is the difference or is there any kinds of pointers ?

char** a;
char *a;
*char a;

etc if there is any that i don't know or i am wrong.

Thanks

Regards,
Billy

[Mark McIntyre]

On 8 Mar 2005 06:39:45 -0800, in comp.lang.c , "ByteSurfer "
<by********@gma il.com> wrote:

I am realy confussed bout pointers.. char** a;
pointer to a pointer to a char
char *a;
pointer to a char
*char a;

syntax errror.
If you need to understand this better, usenet is a REALLY bad place to do
distance learning. Sit down calmly with your C textbook, and work carefully
through the chapter on pointers.

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Christopher Benson-Manica]

<11************ *********@f14g2 000cwb.googlegr oups.com>
ByteSurfer <by********@gma il.com> wrote:

char** a;
a is a pointer to a pointer to a char.
char *a;
a is a pointer to a char.
*char a;

Syntax error.

--
Christopher Benson-Manica
ataru(at)cybers pace.org

[bjrnove]

*char a; IS NOT VALID AT ALL.

Look at this example:

char chr = 'a'; /* Just another char */
char* pchr = &chr; /* A pointer to the char above */
char** ppchr = &pchr; /* A pointer to the pointer to the char */
char*** pppchr = &ppchr; /* A pointer to a pointer to a pointer to a
char */

This can go on for a while (your code will probably crash after added
tons of *, but there doesn't have to be a limit. It's all compiler and
machine specific.)

Now you can use this pointers. Adding a * in front of a pointer (not
when declaring it, but ever after) will give you the stuff inside a
pointer. You could therfor do something like this:

printf("The char chr is: %c\n", chr);
printf("The char pointed to by pchr is: %c\n", *pchr);
printf("The char pointed to by ppchr is: %c\n", **ppchr);
printf("The char pointed to by pppchr is: %c\n", ***pppchr);

Now I know this may sound a bit confusing, but when you understand
pointers it wil be much clearer. I can recomend you to read this
chapter in a online book on c:
http://publications.gbdirect.co.uk/c_book/chapter5/
It will basicly try to teach you everything about pointers.

One last thing. Usaly you never need more than a pointer and in some
cases a pointer to a pointer. It's actualy pretty weird to see a
pointer to a pointer to a pointer.

--
bjrnove

[ByteSurfer]

some times is am kinda confused.
they like to code

char* a;
char *a;

is that both are of the same ??

and some more is it :

*const char a;
-> a pointer of a constant address to a character

const char* a; or const char *a;
-> a constant value of a char.

then how bout *const char* a;

will the spaces char* and char * the same ??

[bjrnove]

> some times is am kinda confused.

they like to code

char* a;
char *a;

is that both are of the same ?? Yes.

and some more is it :

*const char a;
-> a pointer of a constant address to a character No. Its an error. You want:
char const * a;

const char* a; or const char *a;
-> a constant value of a char. It's a pointer to a char. NOT a char.
char a; is a char.

then how bout *const char* a; Not valid.
char* const* a; is valid thow.

will the spaces char* and char * the same ??

Yes.
char * a;
is the same as
char* a;

Spaces doesn't mather as long as you have one space it's a space. The
rest is ignored by the compiler.

[Jens.Toerring]

ByteSurfer <by********@gma il.com> wrote:

some times is am kinda confused.
they like to code char* a;
char *a; is that both are of the same ??
Yes, it doesn't matter if you have white space before or after the '*'.
There's also the third and fourth possibility:

char * a;
char*a;

That shows it doesn't matter at all where and how much white space
you put before ot after the asterisk.

Some people prefer 'char*' to 'char *' because they feel that it
makes it more obvious that the variable to be defined is a pointer,
while others don't like it because they feel that it makes it easier
to overlook that in

char* a, b;

'b' isn't a pointer but a simple char variable despite the '*' after
the 'char'.
and some more is it : *const char a;
-> a pointer of a constant address to a character
No, that's a syntax error. You can't have a '*' at the start when
defining a variable.
const char* a; or const char *a;
-> a constant value of a char.
No, that's a pointer to an array of chars with the attached promise
that you won't change the contents of what the pointer points to.
This is quite useful when you use pointers that point to literal
strings, i.e.

const char *my_hello = "Hello, boys and girls!";

because the compiler may be able to gently remind you you're doing
something stupid when you don't keep your promise, or when you use
this for function arguments to indicate that the function is not
going to change the elements of the array it receives. An example is

char *strcpy( char *dest, const char *src );

This makes it immediately clear that strcpy() will only change the
destination string, but will leave the source string alone. And
again it doesn't matter if you have white space before or after
the '*' or before and after the asterisk or none at all.
then how bout *const char* a;
will the spaces char* and char * the same ??

That's also a syntax error.
Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@p hysik.fu-berlin.de
\______________ ____________ http://www.toerring.de

[Randy Yates]

Je***********@p hysik.fu-berlin.de writes:

while others don't like it because they feel that it makes it easier
to overlook that in

char* a, b;

'b' isn't a pointer but a simple char variable despite the '*' after
the 'char'.

This is a flaw in the C standard, in my opinion. Logically the "*" is
a modifier of the type. To associate it with the identifier rather
than the type is to confuse its function.
--
Randy Yates
Sony Ericsson Mobile Communications
Research Triangle Park, NC, USA
ra*********@son yericsson.com, 919-472-1124

[Randy Howard]

In article <xx************ *@usrts005.corp users.net>,
ra*********@son yericsson.com says...

Je***********@p hysik.fu-berlin.de writes:

while others don't like it because they feel that it makes it easier
to overlook that in

char* a, b;

'b' isn't a pointer but a simple char variable despite the '*' after
the 'char'.

This is a flaw in the C standard, in my opinion. Logically the "*" is
a modifier of the type. To associate it with the identifier rather
than the type is to confuse its function.

No, it's a flaw in the way people use it.

If they did it correctly, I.e.

char *a, b;

or even better

char *a;
char b;

then it would be obvious. Multiple variables on one line isn't
necessarily lazy, but it does lead to a lot of strange bugs,
particularly for those not already fluent in the language.
--
Randy Howard (2reply remove FOOBAR)
"Making it hard to do stupid things often makes it hard
to do smart ones too." -- Andrew Koenig

[Keith Thompson]

Randy Yates <ra*********@so nyericsson.com> writes:

Je***********@p hysik.fu-berlin.de writes:

while others don't like it because they feel that it makes it easier
to overlook that in

char* a, b;

'b' isn't a pointer but a simple char variable despite the '*' after
the 'char'.

This is a flaw in the C standard, in my opinion. Logically the "*" is
a modifier of the type. To associate it with the identifier rather
than the type is to confuse its function.

The principle is that declaration mimics use (more or less). Specifically,

char *a;

means that "*a" is of type char (from which you can infer that a is of
type pointer-to-char). Similarly,

char arr[10];

means that arr[whatever] is of type char (from which you can infer
that arr is of type array-of-char).

Yes, it can be confusing, and if you could only apply a single "*" or
"[...]" to a type name it would be silly, but you need to understand
the principle (or have a copy of the cdecl program) to be able to
decipher things like

void (*signal(int sig, void (*func)(int)))( int);

It's often helpful to avoid putting too many things on one line,
either by declaring only one variable per line or by using
intermediate typedefs.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.