| Re: Storing !NULL in struct [message #83593 is a reply to message #83531] |
Mon, 18 March 2013 20:26   |
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Bob[4]
Messages: 24
Registered: April 2009
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Junior Member |
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On Monday, March 18, 2013 12:26:51 PM UTC-6, Yngvar Larsen wrote:
> On Monday, 18 March 2013 17:48:54 UTC+1, Mike Galloy wrote:
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>> On 3/18/13 6:40 AM, Tom Grydeland wrote:
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>> I think what you are showing here is that any variable passed by value
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>> does not end up modified at the calling level.
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> Right.
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>> It doesn't work for arrays of structures either:
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>> IDL> sarr = replicate({ foo: 0 }, 10)
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>> IDL> modify, sarr[0]
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>> IDL> print, sarr[0]
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>> { 0}
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> Yes, but you can still modify an element, or even a range of elements, of a structure array at your current calling level:
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> IDL> sarr[0].foo = 4
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> IDL> print, sarr[0].foo
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> 4
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> IDL> sarr[3:5].foo = 4
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> IDL> print, sarr[0:6].foo
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> 4 0 0 4 4 4 0
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> This is not the case for lists, as already discussed:
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> IDL> larr = list(length=10) & for n=0, 9 do larr[n] = {foo: 0}
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> IDL> print, larr[0].foo
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> 0
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> IDL> larr[0].foo = 4
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> % Attempt to store into an expression: Structure reference.
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> % Execution halted at: $MAIN$
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> I fail to see why the latter isn't allowed.
I agree. LISTs and HASHes should return a reference to their elements and not a temporary copy.
In addition, IDL needs a reference type that would be similar to a PTR but would not need to be de-referenced to get at what it is pointing at. It could use de-referencing (or a function call) to set the reference but then would allow syntax like a normal variable. This would greatly simplify IDL programing and could perhaps be used to fix the mess that IDL LISTs and HASHes are.
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