Re: In a pickle.

["chris.danx" <chris.danx@ntlworld.com>]

Jim Rogers wrote:
> chris.danx wrote:
> 
>> Hi,
>>
>> Is there a resolution to the following problem that allows convienant
>> assignment of iterators?  An iterator points to a node in a list, like 
>> this
>>
>>    -- Bi-Directional Iterator definition for positions within
>>    -- objects of List_Type.
>>    --
>>    type Iterator_Type is new Ada.Finalization.Controlled
>>    with record
>>       List       :   List_Base_Access;
>>       Position   :   Node_Access;
>>    end record;
>>
>> While more than one iterator points to a node it cannot be 
>> deallocated, with
>> the number of iterators pointing to the node, recorded in the node 
>> itself.
>> The problem is assignment!  If we do
>>
>> Iterator := Next (Iterator);
>>
>> The iterator is assigned to the next position, but the count of iterators
>> isn't decremented so the node always thinks an iterator is on it.
>>
>> I've just noticed something in Cohen that may help but am not sure if my
>> interpretation is correct.
>>
>> At the top of page 573 he says "The target variable in an assignment
>> statement is finalized just before a new value is about to be copied 
>> to that
>> variable".
>>
>> Does this mean that in the above assignment Iterator is finalized 
>> after Next
>> (Iterator) is evaluated but before the assignment takes place?  That 
>> would
>> mean that a suitably coded finalize would do the job, correct?
> 
> 
> 
> That is my understanding of how assignment works.
> 
> Jim Rogers


ARGHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!!!

Managing all the finalisation calls is a nightmare!

raised PROGRAM_ERROR : aqua-lists-linear-unbounded-doubly.adb:56

caused by finalization of an iterator in a call in a subprogram.  I reckon I
know how to fix it, but it will require each iterator modifing subprogram
setting a flag, while basic query ops clear it.  Finalization will check the
flag and if it's cleared do nothing, otherwise adjust the iterator.

Think that's ugly?  Anyone think of a better way?