| 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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| Re: Structure Containing Structure: question about parentheses [message #55247 is a reply to message #55243] |
Thu, 09 August 2007 11:45  |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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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
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| Re: Structure Containing Structure: question about parentheses [message #55249 is a reply to message #55247] |
Thu, 09 August 2007 10:20 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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On Aug 9, 9:36 am, lark...@gmail.com wrote:
> 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?
Here is an article that explains what is going on:
http://www.dfanning.com/misc_tips/ptr_struct.html
Cheers,
David
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