void* question

[Janet]

What is the major reason for using void*?
When should we use void* for both input arguments and return value?
How can we cast the void* pointer to the type we need?
Thanx

[Malcolm McLean]

Re: void* question


"Janet" <no@mail.comwro te in message news

What is the major reason for using void*?
When should we use void* for both input arguments and return value?
How can we cast the void* pointer to the type we need?
Thanx
>

Take a proposed addition to the standard library.

void memswap(void *ptr1, void *ptr2, size_t len)

it simply exchanges two areas of memory.
So to swap two integers we would write

int i, j;
memswap(&i, &j, sizeof(int));

to exchange two doubles

double x, y;
memswap(&x, &y, sizeof(double)) ;

and so on.

The void * parameters mean we don't need ugly casts. Internally, however,
memswap will handle the pointers as unsigned char *s. It doesn't need to
understand what it swaps, it just sees two areas of memory.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Eric Sosman]

Re: void* question

Janet wrote:

What is the major reason for using void*?

To deal with a pointer when the type of the thing
it points to is unknown or unimportant. For example,
the malloc() function allocates memory but does not
know or need to know what kind of data you intend to
store in it.

When should we use void* for both input arguments and return value?

As above.

How can we cast the void* pointer to the type we need?

With a cast operator, like `(int*)ptr' or `(char*)ptr'.
Since the void* pointer carries no information about the
kind of data it points to, you need some other way to
figure out which cast operator is appropriate.

--
Eric Sosman
esosman@ieee-dot-org.invalid

[Ben Pfaff]

Re: void* question

Janet <no@mail.comwri tes:

What is the major reason for using void*?

A pointer to void is often used when the type of data that a
pointer points to is unknown at compile time.

When should we use void* for both input arguments and return value?

When the above is true about input arguments and return values.

How can we cast the void* pointer to the type we need?

There is rarely a need to cast a pointer to void, because most
conversions from a pointer to void to a pointer to another object
type can take place implicitly, but the syntax is no different
from that for any other cast.
--
"In My Egotistical Opinion, most people's C programs should be indented six
feet downward and covered with dirt." -- Blair P. Houghton

[=?iso-8859-1?q?Tom=E1s_=D3_h=C9ilidhe?=]

Re: void* question

Janet:

What is the major reason for using void*?
When should we use void* for both input arguments and return value?
How can we cast the void* pointer to the type we need?
Thanx

A void* is really just a char* that has few more "features" (...or even
"lack of features").

The differences between char* and void* are:
1) void* has implicit conversion to and from every other pointer type.
2) You can do pointer arithmetic on a char*, but NOT on a void*.
3) You can dereference a char*, but not a void*.

Basically the idea is that you use a void* when you aren't certain what
you're going to be pointing to. You might be pointing to an int one minute,
then a double the next.

At the end of the day tho, both void* and char* really just store the
address of a byte.

--
Tomás Ó hÉilidhe

[Malcolm McLean]

Re: void* question


"Janet" <no@mail.comwro te in message

On 11 Feb 2008 at 4:03, Jack Klein wrote:

>On Sun, 10 Feb 2008 22:41:58 +0000, Flash Gordon
>><spam@flash-gordon.me.ukwro te in comp.lang.c:
>>

>>Eric Sosman wrote, On 10/02/08 21:49:

The functions all take the same arguments, but they have different
return values, e.g.
int f1(int x, int y);
double f2(int x, int y);
etc.
>
As I'm never actually going to need the return value I was hoping to use
a void* instead of actual function pointers to let me ignore the
difference between the return types of the functions. Is that going to
work?
>

You need wrapper functions

void F1(int x, int y)
{
gf(x, y);
}

void F2(int x, int y)
{
f2(x, y);
}

Now you can have a list of

typedef void (*fptr)(int x, int y);

fptrs (function pointers) to select the appropriate function.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Peter 'Shaggy' Haywood]

Re: void* question

Groovy hepcat Tomás Ó hÉilidhe was jivin' in comp.lang.c on Mon, 11 Feb
2008 11:17 am. It's a cool scene! Dig it.

Janet:
>

>What is the major reason for using void*?
>When should we use void* for both input arguments and return value?
>How can we cast the void* pointer to the type we need?
>Thanx

>
A void* is really just a char* that has few more "features" (...or
even "lack of features").

Wrong, I'm afraid. A void * is not a char * at all. Although they do
have the same size and representation, this does not make them the same
thing, by any means.

The differences between char* and void* are:
1) void* has implicit conversion to and from every other pointer type.

Wrong again. void * has implicit conversion to and from every pointer
to object and incomplete type, but not pointer to function type.

--
Dig the sig!

----------- Peter 'Shaggy' Haywood ------------
Ain't I'm a dawg!!

[=?iso-8859-1?q?Tom=E1s_=D3_h=C9ilidhe?=]

Re: void* question

Peter 'Shaggy' Haywood:

Although they do have the same size and representation, this does not
make them the same thing, by any means.

Same size and representation is good enough for me -- they're the same
thing just with a different interface to the programmer.

void * has implicit conversion to and from every pointer
to object and incomplete type, but not pointer to function type.

Nice catch, thanks for the correction.

--
Tomás Ó hÉilidhe

[Ian Collins]

Re: void* question

Tomás Ó hÉilidhe wrote:

Peter 'Shaggy' Haywood:
>

>Although they do have the same size and representation, this does not
>make them the same thing, by any means.

>
>
Same size and representation is good enough for me -- they're the same
thing just with a different interface to the programmer.
>

Try dereferencing them and see what you get...

--
Ian Collins.

[=?iso-8859-1?q?Tom=E1s_=D3_h=C9ilidhe?=]

Re: void* question

Ian Collins:

>Same size and representation is good enough for me -- they're the same
>thing just with a different interface to the programmer.
>>

Try dereferencing them and see what you get...

I covered this in my list of differences between char* and void* upthread.

--
Tomás Ó hÉilidhe