Re: IDL 5 Syntax ?

[davidf(at)dfanning.com (David Fanning)]

Eric Kihn (eak@ngdc.noaa.gov) writes:

>     I have a syntax question regarding IDL objects. I have a object
> which has as one of it's data elements
>     a pointer to a structure (data).  In a method (draw) if I try to
> access part of the structure like:
> 
>    plot, *self.data.X
> 
>    I get the dread "Must be a structure in this context" message. (Which
> 
> I think it is)
>    Where as if I do a :
>     ds  = *self.data
> 
>      Plot,ds.X
> 
>     Everypne is happy.  Why?

Because the pointer dereference has the lowest order
of precedence. Lower, even, than a structure dereference.
So in the code that doesn't work, the data structure is 
dereferenced first, and then the pointer is dereferenced.
But the pointer dereference is not dereferencing a pointer
at that point, so this syntax fails. 

What you can do (in all of IDL) is put parentheses
around something that you want to have a higher order
of precedence, since parentheses have the highest order
precedence. This will work:

   Plot, (*self.data).x

Cheers,

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
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