| Re: _Ref_Extra : BUG? (in Win2K 55b) corrected test file [message #27716 is a reply to message #27697] |
Fri, 02 November 2001 01:59   |
Martin Downing
Messages: 136
Registered: September 1998
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Senior Member |
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----- Original Message -----
From: "Mark Hadfield" <m.hadfield@niwa.cri.nz>
>
> So the routine that's crashing your machine (and mine) is as follows
> (shortened)...
>
> pro test_ref_extra1,_REF_EXTRA=e
> test_ref_extra2, _REF_EXTRA=e ; Here be dragons!
> end
>
> This isn't merely bad practice, it's wrong. The _REF_EXTRA keyword is only
> used in routine definitions. When *calling* a routine, the acceptable
forms
> are "_EXTRA" and (as of 5.5) "_STRICT_EXTRA".
>
> Still, it shouldn't crash IDL. It should be detected as a syntax error.
>
Hi Mark
It seemed logical to me that if arguments have to be passed through many
preprocessing routines to a later routine, that the best way would be by
reference to avoid copying (as would be the case if the Keyword was
specified), if the final routine insists on a local copy (by value) why
should that matter?
Well I guess I just didnt understand the manual (no change there!). Are you
saying that this is determined purely by the definition, and that calling
with _REF_EXTRA as below
> test_ref_extra2, _REF_EXTRA=e ; Here be dragons! (indeed)
is supposed to be a syntax error? That was just not clear to me from the
help (and for that matter the programmer who wrote the syntax parser!!!)
I am concerned as to which causes passing by reference/value, the definition
or the call?
This is almost as horrible as C++ inheritance simple in principle but boy
can some strange things happen when things get complicated - for now I guess
will have to stick to _extra which at least is easy to understand :(
roll on coffee time!
cheers
Martin
Help on Keyword Inheritance from 5.4 follows:
"Choosing a Keyword Inheritance Mechanism
The "pass by reference" (_REF_EXTRA) keyword inheritance mechanism was
introduced in IDL version 5.1, and in many cases is a good choice even if
values are not being passed back to the calling routine. Because the
_REF_EXTRA mechanism does not create an IDL structure to hold the
keyword/value pairs, overhead is slightly reduced. Two situations lend
themselves to use of the _REF_EXTRA mechanism:
1. You need to pass the values of keyword variables back from a called
routine to the calling routine.
2. Your routine is an "inner loop" routine that may be called many times. If
the routine is called repeatedly, the savings resulting from not creating a
new IDL structure with each call may be significant.
It is important to remember that if the routine that is passing the keyword
values through also needs access to the values of the keywords for some
reason, you must use the "pass by value" (_EXTRA) mechanism.
Note - Updating existing routines that use _EXTRA to use _REF_EXTRA is
relatively easy. Since the called routine uses _EXTRA to receive the extra
keywords in either case, you need only change the _EXTRA to _REF_EXTRA in
the definition of the calling routine.
By contrast, the "pass by value" (_EXTRA) keyword inheritance mechanism is
useful in the following situations:
1. Your routine needs access to the values of the extra keywords for some
reason.
2. You want to ensure that variables specified as keyword parameters are not
changed by a called routine.
Example: Keywords Passed by Value
One of the most common uses for the "pass by value" keyword inheritance
mechanism is to create "wrapper" routines that extend the functionality of
existing routines. In most "wrapper" routines, there is no need to return
values to the calling routine � the aim is simply to implement the complete
set of keywords available to the existing routine in the wrapper routine.
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