[PATCH] Simplify MigrationImpl::subtract in desktop

[Stephan Bergmann]

On 12/28/2012 06:49 PM, julien2412 wrote:
> By reading the function MigrationImpl::subtract in
> desktop/source/migration/migration.cxx, see
> http://opengrok.libreoffice.org/xref/core/desktop/source/migration/migration.cxx#847
>
> I thought about this patch to use "set_difference"
> desktop_patch.txt
> <http://nabble.documentfoundation.org/file/n4026318/desktop_patch.txt>

The original code is broken to begin with, as it ignores the return 
values from the std::unique calls, so excess elements remain at the end 
of va and vb.

However, the patch is also broken:

* Why "vb.end()+5"?

* The output range of std::set_difference must not overlap with either 
input range.

* The return value of std::set_difference must be observed, to cut off 
excess elements at the end of the returned range.

* If va contains m identical elements and vb contains n < m elements 
identical to them, m - n of those elements will remain in the output of 
std::set_difference, so avoiding std::unique is unsound here.

Stephan