Re: Inter-object pointer problem

Re: Inter-object pointer problem

[Georg Bauhaus]

Alex Catlin wrote:
> Say I have Packages A and B, implementing data structure A and B
> respectively. I want to create a new B everytime I call a certain
> procedure, and insert a pointer to that data structure

Do you mean, a pointer to a package instance?

> The problem is that generic package A is expecting to be instantiated
> with an access type, but the Ptr_B doesnt exist until I have called
> procedure X.

Is there a reason that you are instantiating the package inside
the procedure?

> If this were C I would instantiate A_Structure as a static
> type

In this case your idea and the use of a generic _package_ do
not correspond so well, I'd say. But I might have misunderstood
your description of problem and solution.

Could you clarify a bit, e.g. "structure" is a rather ambigous
term, used in C, ML, and elsewhere, meaning different things.
But it's not clear (to me at least) what it is supposed to mean here.

Do you have a slightly more specified example, one that could
be compiled, if not successful?

Re: Inter-object pointer problem

[Jeffrey R. Carter]

Alex Catlin wrote:
> 
> The problem is that generic package A is expecting to be instantiated
> with an access type, but the Ptr_B doesnt exist until I have called
> procedure X. If this were C I would instantiate A_Structure as a static
> type during procedure X so it didn't keep making new A's every call,
> but I dont think I can do that in ADA. Is there a fix based on some
> concept i'm completely missing or am I going to have to completely
> change the organisation of the program?

This is so far from being correct Ada that it's impossible to tell what 
you're trying to do, but I'd suggest you pull the instantiation of B_Pkg 
out of the procedure.

If you can provide a better example, with information such as the 
specifications of the packages and the definition of B_Pointer_Type, we 
can probably help you better. It's likely you don't need access types or 
values at all outside the packages; pointers are needed far less often 
in Ada than in C (compiler writers excepted).

Finally, "Ada" is a woman's name, not the American Dental Association 
(or other acronym). People on c.l.a get more or less upset with others 
who don't capitalize it correctly.

-- 
Jeff Carter
"If you don't get the President of the United States on that
phone, ... you're going to have to answer to the Coca-Cola
Company."
Dr. Strangelove
32

Re: Inter-object pointer problem

[Martin Dowie]

Alex Catlin wrote:
> Say I have Packages A and B, implementing data structure A and B
> respectively. I want to create a new B everytime I call a certain
> procedure, and insert a pointer to that data structure into the main
> data structure A to keep track of all these B's flying around.
> 
> I'm trying something like:
> 
> with A_Pkg, B_Pkg;
> .....
> package A_Structure is new A_Pkg (B_Pointer_Type);
> B_List : A_Structure;
> 
> procedure X (Data : in Data_Type) is
>     package B_Structure is new B_Pkg;
>     New_B : B_Structure
>     Ptr_B is access New_B;
> begin
>     B_Structure.Insert(B_Structure,Data);
>     B_List.Insert(B_List,Ptr_B);
> end X;
> .....
> 
> The problem is that generic package A is expecting to be instantiated
> with an access type, but the Ptr_B doesnt exist until I have called
> procedure X. If this were C I would instantiate A_Structure as a static
> type during procedure X so it didn't keep making new A's every call,
> but I dont think I can do that in ADA. Is there a fix based on some
> concept i'm completely missing or am I going to have to completely
> change the organisation of the program?
> I'd be grateful if anyone could point me in the direction of something
> to try, Thanks.
> 

If you want to have a container holding lots of "B"'s and call that 
container an "A" then try the new Ada.Container.* hierarchy in Ada2005 
(there are 'backports' to Ada95 out there if you have even a gentle look).

Only you wouldn't be inserting a pointer to a "B" in the structure "A" 
but an "Iterator".

Cheers
-- Martin

p.s. As other have pointed out, your example highlights some serious 
misconception about Ada - but at least Ada is fun to learn! :-)

Re: Inter-object pointer problem

[Jeffrey R. Carter]

Alex Catlin wrote:
> 
> I have a pairing heap package, I'm going to store a load of integers in
> multiple pairing heaps.
> I have a binary tree package, I want to store pointers to these
> different heaps in the binary tree. The idea is that I can search
> through the binary tree (sorted by key), and find out which heap the
> Personal_Data is with that ID.

Why pointers? Why not the heaps themselves?

> with Ada.Text_IO, RB_Tree_Package, Pairing_Heap;
> 
> package body Main is
> 
>          --The tree package needs a pointer to a heap datatype to be
> instantiated
>          --One (obviously invalid) way would be to put
> Pairing_Heap.Heap_Ptr in here.

    package Personal_Data_Heap is new Pairing_Heap (Personal_Data);
    -- I assume Personal_Data is visible, since you use it that way
    -- later.

> 	package RB_Tree is new RB_Tree_Package(Key, *a pointer to a heap*);

"Key" is undefined here, and later you want to use Integer for your key.

    package RB_Tree is new RB_Tree_Package
       (Integer, Personal_Data_Heap.Pairing_Heap);

> 	Group_Tree : RB_Tree.Red_Black_Tree;
> 
> 	--Create new heap, put Personal_Data in it, then put key field and
> pointer to the
> 	--new heap in tree.
> 	procedure Process (Key : in Integer;
> 			Data : in Integer) is
>               New_Entry : Personal_Data := (Key, Data);
> 	    package Heap is new Pairing_Heap (Personal_Data);
> 	    type Heap_Ptr is access Heap;

Get rid of Heap and Heap_Ptr. Note you cannot have a pointer to a package.

    Heap : Personal_Data_Heap.Pairing_Heap;

> 	begin
>                    Ptr_To_Heap := Heap;
> 		Heap.Insert(Heap,Personal_Data);
> 		RB_Tree.Insert( T => Group_Tree,
> 				Key => Key,
> 				Content => Ptr_To_Heap);

Nor can you pass a type (Personal_Data) to a procedure.

    Personal_Data_Heap.Insert (Main_Heap => Heap, D => New_Entry);
    RB_Tree.Insert (T => Group_Tree, Key => Key, Content => Heap);

> 	end Process;
> end Main;

It would help to have the spec of Main. Note that you have exactly one 
pair in each heap, so why not simply store the pair in the tree and skip 
the heap altogether:

package RB_Tree is new RB_Tree_Package (Integer, Personal_Data);
...
RB_Tree.Insert (T => Group_Tree, Key => Key, Content => New_Entry);

?

-- 
Jeff Carter
"If you don't get the President of the United States on that
phone, ... you're going to have to answer to the Coca-Cola
Company."
Dr. Strangelove
32