| Re: Storing !NULL in struct [message #83638 is a reply to message #83531] |
Fri, 15 March 2013 06:31  |
lecacheux.alain
Messages: 325
Registered: January 2008
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Senior Member |
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Le vendredi 15 mars 2013 09:10:45 UTC+1, Tom Grydeland a écrit :
>> On 3/13/13 2:17 am, Tom Grydeland wrote:
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>>> And it would be useful _to me_ if I could put a !NULL in a struct.
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>>> I'm using the struct for UVALUEs in widgets, and it would be nice if
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>>> they allowed _any_ well-defined IDL value.
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> On Wednesday, March 13, 2013 2:46:17 PM UTC, Mike Galloy wrote:
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>> But !null is undefined:
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> Not so undefined that I cannot assign it to a variable, or return it from a function, or use it as a good 'missing value' indicator in most cases. It is also pretty damn useful if you want to build an array by parts (although, as Bob pointed out, I could use a LIST for that last case).
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> See, I was trying to stash away an arbitrary value (provided by the user for their later retrieval) under a given tag in a structure, and I didn't want to restrict what values they were allowed to use.
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> So instead of using a struct { internal: <whatever>, public: <your value here> }, I thought I could use a List(<whatever>, <your value here>), but that didn't work either, since I'm not allowed to assign to a struct that lives in a List.
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> IDL> c = List({ t: 0})
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> IDL> print, c[0].t
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> 0
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> IDL> c[0].t = 1
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> % Attempt to store into an expression: Structure reference.
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> % Execution halted at: $MAIN$
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> :-(
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>> Mike
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> --T
Are'nt you mixing three different concepts: undefined value, unassigned value and missing data ?
In IDL, you can use : !Null, New_Ptr() and !Values.F_NAN, respectively.
alx.
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