| Re: Structure Containing Structure: question about parentheses [message #55243] |
Thu, 09 August 2007 14:12  |
larkn10
Messages: 10
Registered: July 2006
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Junior Member |
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On Aug 9, 2:45 pm, JD Smith <jdsm...@as.arizona.edu> wrote:
> On Thu, 09 Aug 2007 08:36:34 -0700, larkn10 wrote:
>> Hi All,
>
>> I have a question about using parentheses with structures that are
>> members of a structure. I include a sample program below that
>> demonstrates the behaviors.
>
>> Basically I have no problem if I run the command
>
>> state.str.c = 50
>
>> But if I run the command
>
>> (state.str).c = 50
>
>> then I get the error:
>
>> Attempt to store into an expression: Structure reference.
>> Execution halted at: TESTSTRUCT 32 C:\teststruct.pro
>> $MAIN$
>
>> Can someone explain this behavior to me?
>
> This is the exact analog of being unable to pass a structure member by
> reference. (state.str) creates a temporary variable, a copy of the
> str structure inside of state. If you assign to that, IDL is smart
> enough to realize you don't want to change the temporary copy, but the
> real one, as if you hadn't written the parens. Once you add a further
> structure dereference, however, IDL stops saving you from yourself.
> Since you can't assign to temporary variables, you get the error.
>
> Not only is the above paren set superfluous, if it worked, it would be
> much slower, since it creates an unnecessary copy of the structure
> directly. Since it does work when used for anything but assignment,
> you should avoid it.
>
> Parentheses are useful and needed when you have nested pointers inside
> of structures; see the operator precedence tutorial.
>
> JD
Thanks David and JD! That explains the unexpected behavior!
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