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Path: utzoo!attcan!uunet!mcvax!enea!sommar
From: sommar@enea.se (Erland Sommarskog)
Newsgroups: comp.lang.ada
Subject: Re: Garbage Collection
Message-ID: <4222@enea.se>
Date: 6 Jan 89 22:17:19 GMT
Organization: ENEA DATA AB, Sweden
List-Id: <comp.lang.ada>

For the third time I asked Bill Wolfe how his idea with deallocation
on block exit. With his last answer, I realized that due to unclarity
from my side, he may have misunderstood my example. My apologies to
you all, and Bill in particular. 

So I try again, and see if I can get it right:
We have:
     Generic
        Data_type is limited private;
        with Assign, "<", ">"    -- What you expect 
     Package Binary_trees is
        Tree_type is private;
        Procedure Insert(Tree in out : Tree_type; Data : in Data_type);
        ...
     End Binary_trees;
     
     Package Use_trees is  -- Could be a procedure. Could be generic.
        Type Any_type;     -- Could also be generic parameter, or imported.
        Type Some_type;
        Type Some_access_type is access Some_type;
        Procedure Assign(A : in out Some_access_type; 
                         B : in     Some_access_type) is 
        Begin A := B End Assign; 
        -- Also add "<" ">" to mean comparison on Any_type here.
        Package Tree_handler is new Binary_trees(Some_access_type, Assign);
        ...
        Procedure Put_something_in_tree(Tree : in out Tree_type;
                                        Anything : Any_type) is
        Reference : Some_access_type;                                
        Begin                                 
           Reference := new Some_type;
           Do_something(Reference.all, Anything);
           Tree_handler.Insert(Tree, Reference);
        End;
        ...
    End Use_trees;
    
Question: What happens with Reference.all when Put_something_in_tree
is exited? I read your proposal as that it should be deallocated, 
which would be disastrous. Or is this a misunderstanding from my 
side? If it is not: is this code erroneous with your scheme? 
  Side note: you said that the tree package should take a Destroy
procedure. For what benefit? You can't automatically call it when
your Destroy_tree is called. The caller might refer to the stored 
objects in some other place. Or does your Destroy_tree have a special
parameter to control this? Seems like adding two parameters (one
generic and one routine) that aren't necessary, but clutters up the
user's code. The user can always traverse the tree and destroy what
he has there.
-- 
Erland Sommarskog
ENEA Data, Stockholm              This signature is not to be quoted.
sommar@enea.se