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| Re: structure arguments sometimes behave like value types - why? [message #53604 is a reply to message #53603] |
Fri, 20 April 2007 14:26   |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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justspam03@yahoo.de writes:
> according to the IDL reference, structures (as a whole) are treated
> like reference types when supplied as an argument. A small test
> program of the kind
> confirms this - after the call to changeval, a.value has value '4'.
> However, when the structure is an object variable as in the example
> appended below, it seems that only a copy of structtest.val is
> exchanged, not a reference to it. The final call to printValue in
> 'main' prints a '1'.
> Could someone please explain why?
In one case you are passing an IDL variable, which is passed
by reference. In the other you are passing a structure reference,
which is NOT an IDL variable, and like everything else that is NOT
an IDL variable, is passed by value:
http://www.dfanning.com/tips/read_subscripted_array.html
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: structure arguments sometimes behave like value types - why? [message #53723 is a reply to message #53605] |
Mon, 23 April 2007 10:31 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Fri, 20 Apr 2007 13:26:42 -0700, justspam03 wrote:
>
> erg, addendum.
> Admittedly, in the second case the structure is not a function
> argument but a return value - I still assumed that it doesn't matter
> in the case (just as it doesn't for pointers or object references).
> Does it?
> Thanks again
In the second case, you are returning a copy of the structure from
inside the object, and then modifying that copy. The copy actually
occurred at the statement "self.val". Had you instead used a pointer
to a structure, ala:
pro structtest__define
obj = { STRUCTTEST , val: ptr_new({nullableString}) }
end
function structtest::getStruct
return, self.val
end
you could then return that *pointer*, and then modify directly the
object's internal copy. Note that you're still returning a copy of
*something* with:
b=x->getStruct()
but that something in this case is a (lightweight) copy of the pointer
to the internal structure, rather than a full copy of the structure.
However, just as it's dangerous to hand out too many sets of house
keys, it's often not a good idea to pass pointers to your important
internal data out to whomever may happen by.
Note that objects are similar to pointers in that they are (always,
unlike C++) lightweight references to variables on the global IDL
memory heap (it may help if you call them "object pointers"). They
are accessed differently, but otherwise serve a similar function: you
can make many copies, all of which refer to the same object.
JD
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