| Re: Copying (Duplicating) Objects [message #31022] |
Thu, 06 June 2002 14:38  |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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"Randall Skelton" <rhskelto@atm.ox.ac.uk> wrote in message
news:Pine.LNX.4.33.0206061044560.10439-100000@mulligan.atm.o x.ac.uk...
> I agree with Ben... I too use Martin's copy method and it works very
> well. It does have problems when you try and clone a class that has
> requried positional parameters though as internally it relies on
> 'clone = obj_new( obj_class(self) )'. My 'I do not have time to
> think about this now' solution was to make classes with no required
> positional parameters and write an 'isValid' method that ensures the
> object is correctly populated with 'quasi-required' parameters
> before it is used. Of course, failing to use positional arguments
> for required parameters goes against some well established IDL
> programming conventions and therefore tends to upset some of the IDL
> Expert Programmer's Association Executive Committee members ;)
Well, I can't comment on that since the IEPAEC has unaccountably
failed to invite me to join. But just to spin this thread out a little
longer, I would like to make a couple of observations:
- Few, if any, standard IDL classes require parameters during
initialisation
- Where standard IDL classes have positional parameters, they tend
to "echo" them with keyword parameters.
The first means that you can create a blank object then set its
properties later. Allowing for this is good OO practice IMHO. The
second lets you take advantage of the power of IDL's keyword handling
(inheritance, keyword-structure equivalence).
--
Mark Hadfield "Ka puwaha et tai nei, Hoea tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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| Re: Copying (Duplicating) Objects [message #31025 is a reply to message #31022] |
Thu, 06 June 2002 06:44  |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Mark Hadfield wrote:
>
> "M. Katz" <MKatz843@onebox.com> wrote in message
> news:4a097d6a.0206050708.7dc1b091@posting.google.com...
>
>> After 4 months away from IDL, I can't remember how to duplicate an
>> object variable.
>
> That's OK. After 10 years with it, I can't remember it either.
>
>> I know that the simple command, b = a, only makes b and a have the
>> same reference--they point to the same object.
>
> Indeed.
>
>> What's the command to make b an entirely new object but identical to
>> a.
>
> There isn't one, at least not built-in. The simplest & most robust
> (though not foolproof) method of doing this is to SAVE the object to
> disk then RESTORE it.
Really? Wouldn't writing your own copy/assign function be more, uh, OO-y? When I create
definitions for structures with pointer components in Fortran the very next thing I do is
write the Initialize() [nullifies the pointers], Destroy() [disassociates the pointers],
Allocate() [associates the pointers], and Copy() [COPIES the entire structure, including
the allocation of memory for the pointer components] functions (or methods, to use the
obfuscatory OO terminology).
paulv
p.s. And in Fortran, if you wanted to, you could overload an Copy() subroutine with the
"=" so that
b = a
does the same thing as
CALL Copy( b, a )
Some people have expressed a wish that IDL could do something similar to be truly OO-ey.
The fact that fortran can do this (to some extent has *always* done it wrt intrinsic
functions) has always made me wonder why this capability was thought of as an OO
characteristic.
Anyway...
--
Paul van Delst Religious and cultural
CIMSS @ NOAA/NCEP/EMC purity is a fundamentalist
Ph: (301)763-8000 x7274 fantasy
Fax:(301)763-8545 V.S.Naipaul
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| Re: Copying (Duplicating) Objects [message #31154 is a reply to message #31025] |
Sat, 08 June 2002 11:49 |
MKatz843
Messages: 98
Registered: March 2002
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Member |
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Thanks for all the great suggestions. Here's the solution I've cobbled
together. It seems to work nicely.
I now define objects with the following specifications
(1) The GetProperty method has an all=all keyword that
returns a structure containing all of the paramters and
values of that object. (I know this works best when the
values are simple and not pointers and containers...)
(2) The Init method has an all=all keyword. If the user provides
an all structure when the object is first defined, then the
contents are used to set the initial values of the object.
I defined the following obj_copy() function
function obj_copy, a
a -> GetProperty, all=all ;Read a structure with all prameters
b = obj_new(obj_class(a), all=all) ;Create new identical object
return, b
end
While not a completely generalizable function, this will work for me
for now.
I also like the suggestion to make a Copy() method part of all object
definitions.
M. Katz
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