[Interest] Strange behaviour of QByteArray reserve/reallocation mechansim

[Bernhard]

Ok, thanks a lot for the answer! Thanks also to Bo!

I summarized the results and added them as an answer to the original SO
thread where hopefully a lot of people can find them without going through
the pain I had:

http://stackoverflow.com/a/26886634/1421332

I also will have a look into the contribution guidelines.

-- 
Kind Regards
Bernhard Lindner

> Von: interest-bounces+private=bernhard-lindner.de at qt-project.org
> [mailto:interest-bounces+private=bernhard-lindner.de at qt-project.org] Im
> Auftrag von Alejandro Exojo
> Gesendet: Mittwoch, 12. November 2014 09:22
> An: interest at qt-project.org
> Betreff: Re: [Interest] Strange behaviour of QByteArray
reserve/reallocation
> mechansim
> 
> El Monday 10 November 2014, Bernhard escribió:
> > So after all I am not supposed to use clear() or operator=() after
> > calling reserve()? That should be documented. Without that knowledge
> > it obviously is impossible to use reserve() in a sensible way.
> 
> There was a similar question on the development mailing list about
clearing
> QVector while keeping the capacity. The solution seems to use erase(), and
> that is not available on QByteArray, but see below.
> 
> http://lists.qt-project.org/pipermail/development/2014-
> November/019015.html
> 
> > If you don't have an idea how that could be improved on the code side
> > I would file a suggestion about extending the documentation a bit.
> 
> I think that this will work for you (if I understood your use case):
> 
>     QByteArray buffer;
>     buffer.reserve(1000);
>     buffer.append("foo");
>     qDebug() << "buffer" << buffer.capacity() << buffer;
> 
>     buffer.truncate(0);
>     buffer.append("bar");
>     qDebug() << "buffer" << buffer.capacity() << buffer;
> 
> This prints:
> 
> buffer 1000 "foo"
> buffer 1000 "bar"
> 
> As Bo explained, operator= doesn't work because of the implicit sharing.
It
> can be tricky to document which member functions will hold the reserved
> memory without adding lots of noise to each member description. Maybe
> the Qt developers don't even want to commit to keeping the same
> behaviour during all the Qt5 timespan.
> 
> Sending a patch to improve the documentation is easy once you do the
initial
> procedure:
> 
> http://qt-project.org/wiki/Qt-Contribution-Guidelines
> 
> --
> Alex (a.k.a. suy) | GPG ID 0x0B8B0BC2
> http://barnacity.net/ | http://disperso.net
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