Re: A little help please

From: Rich (Someone_at_somewhere.com)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 13:02:58 +0000 (UTC)

> You don't need the cast.

But according to the question if I read it correctly I do???
The question is exactly as written to newsgroup.

> Don't do that! Why would you want something like that when there are plenty
> of alternatives?

:) It is the question I have to answer
>

> You cannot do that since arr is of type void*. You need to cast it to
> another type. Since func1() has no way of knowing the actual type, this
> function is invalid.

Well I guess it would be if I didn't know I was passing an array of ints
disguised as a void* :)
>
>
>>arr += nBytes // increment by nBytes to get next element
>
>
> That shows us you don't know how pointers work and that means you shouldn't
> use them.

??? if arr points to first element of int array then I thought by
incrementing the pointer by nBytes I was pointing to next array element.?

> That's illegal.
>
> char ch = ' ';
>
> or
>
> char ch = 0;
>
> depending on what you want.

:) this was a typo on my part it was supposed to be char ch = ' ';
>
>
>>int* i = &arrX[0]; // set i as pointer to first element of array
>>void * vp = static_cast<void*> (i); // convert i to a void*
>
>
> You don't need that
>
> void *vp=i;

right now I am confused, the question states that I should cast the int
array to a void* using a static cast.

>
>
>> // assign to vp
>>func1( vp, sizeof( arrX ), 8 );
>>for( int i = 0; i < 10; i++ )
>
>
> Prefer ++i over i++.

if I did ++i would this not make the start of loop start at 1 ?? since
it increments before evaluating?

> That's because you cannot operate on void pointers, you need to cast them.

This is what I wondered about needing to cast back to int array in function?

> Your code is bound to be invalid. You'll have to assume a type (such as
> char, as does memset()), ask the user for one or forget that function.
> Templates are meant for this kind of problem.

We have not covered such things yet, we have played with vectors but not
templates yet.
>
>
>>My third question, is am I correct in thinking that the only time to use a
>>void* is when the variable being assigned could be of any type?
>
>
> I don't understand that.
>
I meant the only reason I could see for using a vcid* is for when you
don't know the type to be assigned to a pointer so by declaring the
pointer void* I can assign any type to it, but can't dereference a void*
   but instead have to cast it back to dereference it.

For example

void* vp;
vp = &i;

// *vp = 4 // can't do this can't dereference a void*
// have to do this instead

i = static_cast<int> (vp); // cast back to int*
*i += 4; // i += 4 would make pointer point 4 ints further in memory?

Thanks anyway

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