Re: pointer questions

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jeffrey R. Carter" <spam@spam.com> writes:

> Szymon Guz wrote:
> 
> > I've got some maybe stupid questions but I don't understand some things:
> > 1. Is there a pointer like (void *) from C that points to anything ?
> > 2. Is there a universal (like above) pointer for procedure|function
> > that  can point to any kind of procedure|funcion ?
> 
> The short answer, as you've already heard, is No.

Well, type System.Address can be used in that fashion.
Or you can say "type Void_Star is access all Void;"
and use unchecked conversions.  So I'd say the answer is
"Yes, but you don't usually want to do things this way."

> The long answer is that you'll never need these to use Ada. 

Never?  Well, these kinds of low-level hacks are _rarely_ a good idea.
But you might need them when interfacing to some other language
that uses such trickery.

>...In fact,
> you'll rarely need pointers at all.

I've heard that claim many times in this newsgroup.  It doesn't match my
experience at all.  I use pointers all the time.

I just checked one project.  About 500 named access types in about a
quarter million lines of code.  That's about one named access type for
every 500 lines of code.  Do you call that "rare"?  (I didn't count
anonymous access types -- there are probably hundreds of those
in this project, too.)

There are also about 500 packages in this project.

You need pointers in Ada whenever you have complex (linked) data
structures.  Also, whenever you don't know the size of an object
at the point it is created.  Also, whenever the size of an object
changes wildly over time.

And whenever you have recursive types.  (E.g. in a compiler, an
expression contains subexpressions; in Ada you will need pointers
to accomplish that, since you can't do it directly.)

And whenever you want to pass a composite type as a discriminant -- you
have to create a bogus pointer:

    type T(Some_Task: access Some_Task_Type) is ...

(because "Some_Task: Some_Task_Type" is annoyngly illegal.

Maybe the kinds of programs I write are different.  Question for those
who rarely use access types: what sort of application areas are you
talking about?  Mine are usually software development tools -- such as
compilers.

It is true that C requires or encourages pointers in cases where Ada
does not.  Two examples: (1) a common idiom in C is to loop through an
array using a pointer.  In Ada, you would use an index instead
(which you can also do in C).  (2) In C, you need to use heap
allocation if a struct contains a field whose size is not known
at compiler time.  In Ada, you could use a discriminated record
instead.

If I designed a language, I would go even further in that direction.
I would allow mutually recursive types without intervening pointers.
The only need for pointers in my language would be where
reference semantics is desired -- e.g. when you want two pointers
to the same object, such that updates to that object can be seen
through both.

>... If you think you do, especially for
> beginning Ada, then you're probably misunderstanding something.

I don't really agree.  A beginner to Ada who knows how to program
(in some other language, such as C) might well want to make a
linked list, for example, and that requires pointers of some
sort -- usually access types.

> About the only time you'd need anything like these is when interfacing
> to C. Judging from your questions, I suspect your understanding of Ada
> is not yet at a level where you should be doing that.

Well, a programmer coming from C might well have some C code lying
around, and might well want to call it from Ada!

- Bob