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Re: Storing !NULL in struct [message #83525]

Wed, 13 March 2013 07:46

Michael Galloy
Messages: 1114
Registered: April 2006

Senior Member

On 3/13/13 2:17 am, Tom Grydeland wrote:
> And it would be useful _to me_ if I could put a !NULL in a struct.
> I'm using the struct for UVALUEs in widgets, and it would be nice if
> they allowed _any_ well-defined IDL value.

But !null is undefined:

IDL> help, !null
<Expression> UNDEFINED = !NULL
IDL> print, size(!null, /type)
0

Mike
--
Michael Galloy
www.michaelgalloy.com
Modern IDL: A Guide to IDL Programming (http://modernidl.idldev.com)
Research Mathematician
Tech-X Corporation

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Re: Storing !NULL in struct [message #83529 is a reply to message #83525]

Wed, 13 March 2013 01:17

tom.grydeland
Messages: 51
Registered: September 2012

Member

On Tuesday, March 12, 2013 6:15:57 PM UTC, bobnn...@gmail.com wrote:

> Structure fields must be defined at the time of creation and the type (and rank and array sizes) cannot be changed after that. Thus, if you put a !null in the field you could never change it to anything else. Not very useful.

I know IDL won't let me change it, but I will happily rebuild structures to have the same field name and different contents to achieve the dynamism that IDL doesn't have. As long as my structures are anonymous, this works fine.

And it would be useful _to me_ if I could put a !NULL in a struct. I'm using the struct for UVALUEs in widgets, and it would be nice if they allowed _any_ well-defined IDL value.

> If you do not know the detail of whatever you are going to put in the structure you have several options. Traditionally, you would use a PTR. These days you could use a LIST or HASH instead.

Sure. Thanks!

> Bob

--T

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Re: Storing !NULL in struct [message #83530 is a reply to message #83529]

Tue, 12 March 2013 11:15

Bob[4]
Messages: 24
Registered: April 2009

Junior Member

On Tuesday, March 12, 2013 6:09:37 AM UTC-6, Tom Grydeland wrote:
> Any reason I shouldn't expect this to work?
>
>
>
> IDL> help, { foo: !null }
>
>
>
> --T

Structure fields must be defined at the time of creation and the type (and rank and array sizes) cannot be changed after that. Thus, if you put a !null in the field you could never change it to anything else. Not very useful. If you do not know the detail of whatever you are going to put in the structure you have several options. Traditionally, you would use a PTR. These days you could use a LIST or HASH instead.

Bob

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Re: Storing !NULL in struct [message #83640 is a reply to message #83525]

Fri, 15 March 2013 01:10

tom.grydeland
Messages: 51
Registered: September 2012

Member

> On 3/13/13 2:17 am, Tom Grydeland wrote:
>> And it would be useful _to me_ if I could put a !NULL in a struct.
>> I'm using the struct for UVALUEs in widgets, and it would be nice if
>> they allowed _any_ well-defined IDL value.

On Wednesday, March 13, 2013 2:46:17 PM UTC, Mike Galloy wrote:
> But !null is undefined:

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).

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.

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.

IDL> c = List({ t: 0})
IDL> print, c[0].t
0
IDL> c[0].t = 1
% Attempt to store into an expression: Structure reference.
% Execution halted at: $MAIN$

:-(

> Mike

--T

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