char* vs void*

[Donald Maffly]

In article <1991Jan2.171429.11566 at glinj.gli.com> bobj at glinj.gli.com (Robert Jacobs) writes:
>
>>Pointers of types |char *| and |void *| must both be able to hold the
>>address of any object.  The reason why this is so differs for the two
>>types.  For |char *| it's existing practice, and for |void *| it's the
>>decree of ANSI.
>>
>> [ ... ]
>>
>
>What I can't understand is that ANSI decided that one cannot do pointer
>arithmitic on a void*, like a char*. Because of this, new work is forced
>to use char* as the generic pointer that can do pointer arithmitic.
>This continues the existing practice that char* is special. Void*
>should be allowed to do pointer arithmitic so that char* does not
>have to be used for new work.


Why did ANSI decide that one cannot do pointer arithmetic on a void* 
pointer, you ask.

Well, BY DEFINITION, void* means that 
we are dealing with a pointer to an object of unknown type and size.  
In pointer arithmetic, how could a C compiler possibly know how much 
to decrement or increment a pointer by if it didn't know the size of 
the object it was pointing to? 
(rhetorical).

For example, if we had a dynamically allocated array of objects of
unknown size, and "ptr" of type (void*) points to some object in the array,
what sense would "ptr++" make?  Surely, the intent is to reset the
pointer to point to the next object in the array.  But without the
contextual knowledge of the size of each object, the expression "ptr++" 
is not well defined.

I haven't been following this char* vs. void* discussion very closely,
so excuse me if I have missed something crucial and I am way off base.

Donald Maffly