Help on void *

[#define]

Hello,

Does any body know the advantage of writing function argument as void*
(rather than using int*, char* etc). what is the advantage or
disadvantage of this.
I've seen most of the library functions are implemented this was, as
they provide generic interface.

/*Function defination*/

int Msg_Edit(void *ptr)
{

return *((int*)ptr); //here i should know the type before hand as
int....


}


/* Function call */

Msg_Edit(&msg);


Now msg buffer can be of any type char, int, float....(Am i right)

Is there a way in way in C, by which i know the type of pointer passed
to the function on runtime.
otherwise i'll need to typecast ptr to a specific type (int*, char*
etc) which should be prior knownto me.

Then, what is the main advantage of this approach...

Best Regards,

#define.

[Eric Sosman]

Re: Help on void *



#define wrote On 12/19/05 10:33,:[color=blue]
> Hello,
>
> Does any body know the advantage of writing function argument as void*
> (rather than using int*, char* etc). what is the advantage or
> disadvantage of this.[/color]

Disadvantage: A `void*' carries no information about
the actual type of the data it points to, so the function
needs to obtain that information in some other way.

Advantage: A `void*' carries no information about the
actual type of the data it points to, so a function that
doesn't need to know the type isn't burdened with trying
to figure it out.
[color=blue]
> I've seen most of the library functions are implemented this was, as
> they provide generic interface.[/color]

Exactly. memcpy() doesn't need to know the type of
the data it's copying, so it can use `void*'. Without a
"generic" pointer of some kind, you'd need separate versions
of memcpy() for copying characters, shorts, ints, floats,
doubles, ... And, of course, for all the kinds of types a
programmer might invent: struct foo, union bar, ...

On the other hand, some functions *do* require specific
types, and they don't use `void*'. For example, strcpy()
is only for copying character strings -- not arrays of
`unsigned long', for example -- so it uses `char*' pointers.
[color=blue]
> /*Function defination*/
>
> int Msg_Edit(void *ptr)
> {
>
> return *((int*)ptr); //here i should know the type before hand as
> int....
>
>
> }
>
>
> /* Function call */
>
> Msg_Edit(&msg);
>
>
> Now msg buffer can be of any type char, int, float....(Am i right)[/color]

Yes.
[color=blue]
> Is there a way in way in C, by which i know the type of pointer passed
> to the function on runtime.[/color]

There is nothing built-in. If you need to know the type
of some piece of data, you need to keep track of it yourself.
[color=blue]
> otherwise i'll need to typecast ptr to a specific type (int*, char*
> etc) which should be prior knownto me.
>
> Then, what is the main advantage of this approach...
>
> Best Regards,
>
> #define.
>[/color]

--
Eric.Sosman@sun .com

[dukguru]

Re: Help on void *

in c++,
void FuncA(void *p)
{
if(typeid(*p) == typeid(int)) // RTTI check
{
// p is int pointer
}
}

// #include <typeinfo>

no way in c

==
dukguru@gmail.c om

[dukguru]

Re: Help on void *

in C++

you can check RTTI



no way in C

[Dave Thompson]

Re: Help on void *

On 20 Dec 2005 18:33:53 -0800, "dukguru" <dukguru@gmail. com> wrote:
[color=blue]
> in c++,
> void FuncA(void *p)
> {
> if(typeid(*p) == typeid(int)) // RTTI check
> {
> // p is int pointer
> }
> }
>
> // #include <typeinfo>
>[/color]
<OT> No, (even) in C++ the runtime/dynamic part of RTTI only works for
classes with virtual methods; 5.2.8[expr.typeid] 10.3[class.virtual].
typeid(*voidptr ) is statically 'void' and never == that of int.

IIRC Icon does support type inquiry on all types, and maybe Eiffel.
And LISP, of course. <G>

- David.Thompson1 at worldnet.att.ne t