[xsde-users] variable length member : suggestion for "_present (bool x)" methods

Ivan Le Lann ivan.lelann at free.fr
Fri Sep 10 10:22:45 EDT 2010

Hi Boris,

I hope I do not bother you with all my comments!

----- "Boris Kolpackov" <boris at codesynthesis.com> a écrit :

> Two problems with this suggestion:
> 1. When exceptions are disabled 

I definitely overlooked the exception point.
"_present (bool)" could be switch to void/error, but that's quite ugly.

> 2. The more serious problem is the fact that the newly created object
>    (bar in the code above) is itself uninitialized. Since it is var-
>    length, it will most likely contain members that need
> initialization
>    (e.g., a string member when STL is disabled). So the patch only
>    postpones the problem for a little while:
>    f.bar ().baz ();       // ok
>    f.bar ().baz ().str () // segfault

Please let me stress that my point is not to avoid all segfaults.
It is to be able to write the same code for fix/var elements,
as much as possible.

I am focusing on "_present" but as you suggest, 
there might be other harder cases to harmonize, like simple accessors.

While "_prevent(false)" is ok, since it deallocates on non-null pointer,
"_present(true)" is not since it does not allocate on null pointer .

Two solutions to adapt my suggestion :
1) change the return type of _present(bool) to void/error
2) isolate new behavior in a "void/error _check_present()"

>Maybe not generate _present(bool) for var-length members at all?

You don't end with var/fix independant code, but at least,
this is compile time error. I consider it a better way.
This is the exact opposite of what I suggested, though! :-D

As for elements accessors without "_present" flag,
I indeed see no obvious solution that would fit with "no exception" support.


More information about the xsde-users mailing list