On functions and access types.

On functions and access types.

[Caffeine Junky]

In a related post where I asked for help with Generics I posted this spec
file...

generic
   Max : Positive;
   type Item is private;

package genstack is

   type Stack is limited private;

   procedure Push(X : in Item; S : in out Stack);
   function Pop(S: in Stack) return Item;
   function Is_Empty(S : in Stack) return Boolean;

private

   type Cell;
   type Stack is access Cell;
   type Cell is
      record
         Value: Item;
         Next: Stack;
      end record;

   -- This package will be expanded to include other forms of stacks --
   -- as time permits.                                               --

end genstack;


Now, function Pop was written thusley...


function Pop(S : in Stack) return Item is

	X : Item;

begin

	X := S.Value
	S := S.Next;
	return X;

end Pop;


Now the compiler tells me that I cannot assign to an 'in' parameter.
However, as I understand it, a function can only take an 'in' parameter.
I can copy S to a stack which is local to the function, but results in
the function only returning the last item in the stack.
If I use a procedure, it works fine. But I run into problems whenever I
try to use similiar acess types into a function.

I'm definitely overlooking something here. Probably something obvious.
I'll continue to search through my book for an answer, but any pointers
or references would be appreciated.


Now, eventually I changed this to a procedure for the sake of expediancy.
But I'm continuing to work on the problem.


Thanks for your patience.

Staple

Re: On functions and access types.

[Robert A Duff]

Caffeine Junky <nospam@hotmail.com> writes:

> Now the compiler tells me that I cannot assign to an 'in' parameter.
> However, as I understand it, a function can only take an 'in' parameter.

That's right.  If you want a function to modify its argument,
you can use an access parameter, like "S: access Stack;",
and call it with Some_Stack'Access.

But in this case, it's better to use a procedure.

> Now, eventually I changed this to a procedure for the sake of expediancy.

That's the right solution.

- Bob

Re: On functions and access types.

[SteveD]

"Caffeine Junky" <nospam@hotmail.com> wrote in message
news:eFrS8.4786$Uu2.750@sccrnsc03...
[snip]
> Now the compiler tells me that I cannot assign to an 'in' parameter.
> However, as I understand it, a function can only take an 'in' parameter.
> I can copy S to a stack which is local to the function, but results in
> the function only returning the last item in the stack.
...
> If I use a procedure, it works fine. But I run into problems whenever I
> try to use similiar acess types into a function.
>
> I'm definitely overlooking something here. Probably something obvious.
> I'll continue to search through my book for an answer, but any pointers
> or references would be appreciated.

The only thing you're overlooking is the philosophy that functions should
not change the state of the system.  By modifying their parameters for
example.

The concept of not permitting "out" or "in out" parameters to functions has
been debated in this group many times.

>
> Now, eventually I changed this to a procedure for the sake of expediancy.
> But I'm continuing to work on the problem.
>
That's the right answer anyway.

SteveD

>
> Thanks for your patience.
>
> Staple

Re: On functions and access types.

[Ted Dennison]

Robert A Duff wrote:
> Caffeine Junky <nospam@hotmail.com> writes:
> 
> 
>>Now the compiler tells me that I cannot assign to an 'in' parameter.
>>However, as I understand it, a function can only take an 'in' parameter.
> 
> That's right.  If you want a function to modify its argument,
> you can use an access parameter, like "S: access Stack;",
> and call it with Some_Stack'Access.

..or use a pointer. C also allows only "in" parameters to functions, and 
passing pointers is the standard work-around there. (C++ allows 
reference parameters though).

> But in this case, it's better to use a procedure.

It almost always is.

Re: On functions and access types.

[Grein, Christoph]

> function Pop(S : in Stack) return Item is
> 	X : Item;
> begin
> 	X := S.Value
> 	S := S.Next;
> 	return X;
> end Pop;
> 
> Now the compiler tells me that I cannot assign to an 'in' parameter.

Others have commented on 'in'. So if you change this into a procedure, watch out 
for garbage you're going to produce:

  S := S.Next;

Hint: Unchecked_Deallocation