[saga-rg] context problem

[Andre Merzky]

Hi Pascal, 

Quoting [Pascal Kleijer] (Jul 20 2006):
>>
>>> In the case of non-OO languages it might be preferable to use by copy 
>>> then by reference, this will however have an impact on the runtime but 
>>> will avoid you a lot of troubles.
>>
>> I would assume that a non-OO implementations uses pointers
>> to structures as object representations, or handles or such.
>> Hence a copy would in fact represent a reference-copy, and
>> the same semantics can easily be achieved.
> 
> Well no. The problem with non-OO languages and C++ is that you must 
> explicitly garbage collect your objects (call a destroy). So if you just 
> copy the pointer you cannot test if it has been destroyed unless you 
> handle the pointer to the pointer and can test for a NULL (agrl in 
> Fortran). So the idea is to make a full copy of the object state and use 
> a locale copy within the thread. This might cause a big overhead and be 
> rather tricky to implement, especially if object contains other objects. 
> The recursion in copy might be either impossible, longish or very complex.

I disagree, and for two reasons even (that must be worth
something :-)

For one, as I said in the thread earlier, I think that deep
copy raises huge semantic problems:

  task t_1 = file.seek <saga::task::task> (100, saga::file::SeekSet);
  task t_2 = file.read <saga::task::task> (1, buf, &out);

  t_1.run ();  
  t_1.wait ();

  t_2.run ();  
  t_2.wait ();

I, as a user, would assume that the read reads byte number
100.  But if we do a deep copy on task creation, the object
state (e.g. the file pointer) gets _copied_.  So the seek
operates on a different file pointer than the read, and the
read in fact returns the first byte in the file.

That breaks a large number of use cases.



Secondly, you are right, C++ and C must handle destruction
explicit - but that does not mean that you cannot copy
references.  It just means that your implementaion needs to
keep track of them!

Now, that might be difficult, but (a) there are many libs
and tools helping with that and there also exist known
practices on how to do that, and (b) SAGA is about
ease-of-use, not ease-of-implementation.


Hehe, now you need three counterarguments :-)  Lets see what
you come up with :-D

Cheers, Andre.


-- 
"So much time, so little to do..."  -- Garfield