Re: Access to array slices?

["David Thompson" <david.thompson1@worldnet.att.net>]

sk <sk@noname.com> wrote :
> tmoran@acm.org
>  > it will usually have "chars **buf" or "Handle *h" where ...
>
> A handy rule of thumb which I hope to remember (I think that
> "Handle *h" is largely a MS/Window thing isn't it ?).
>
MSWindows certainly uses handles extensively, but they
are far from the only people to do so.  MacOS did; I believe
X did; I've used lots of other libraries/packages which each
have their own (collection of) handle types.  It's a generic
and widely useful concept, or if you like pattern.

> david.thompson1@worldnet.att.net
>  > Allocation has nothing to do with it; a C routine cannot
>  > allocate unless it *returns* a pointer or receives a pointer
>  > *to* a pointer (or receives a pointer to or returns a struct
>  > *containing* a pointer, but that's much rarer).
>
> I seem to recall that some of the Window C routines allocated
> some of the data structures internally and passed a (or updated
> a passed in) pointer back to the caller ?
>
You can't "update a pointer passed in" in C; more generally,
no called routine can ever update a caller's actual argument,
only the value pointed to by a pointer argument and/or provide
a return value (which may be stored, or discarded, by the caller).

Another thing you can do (in general) is *not* give out actual
pointers; instead a called library maintains its own private
storage, possibly with dynamic/heap allocation and pointers
and possibly not, and gives out only numbers or other (private)
values that identify things within that private storage; these go
by a variety of names often including "number" or "index" or
"capability" e.g. a color index in a palette.  I don't know of
MSWindows using these but I haven't looked hard.  In C of
course this means e.g. return an int, or take a pointer to int.

--
- David.Thompson 1 now at worldnet.att.net