Re: pointer questions

[David <kdgreen@fastmail.fm.au>]

Robert A Duff wrote:
> "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


I develop larg'ish mission-critical systems in Ada and cannot use 
pointers, dynamic memory allocation, tasks, variants, recursion, etc. 
etc.  You can always find a way to not use pointers as long as you stick 
to Ada (not interfacing C/C++).   Memory mapped hardware can be accessed 
using an expression like "for x'address use at <hardware address>".  You 
can declare one variable on top of another the same way.

Trying to get code safety-certified with pointers is very difficult to 
impossible.  Code that doesn't use pointers is invariably simpler and 
more reliable but it may require more thought depending upon mind-set.

regard ... David