Tips on memory management with C++ and Qt

[Maximilian Kossick]

On Mon, Feb 16, 2009 at 8:40 AM, Mark Kretschmann <kretschmann at kde.org> wrote:
> Hey all,
>
> as you may have noticed, I've lately been focusing a lot on fixing
> critical bugs in Amarok. During this process it occurred to me that
> the vast majority of crashes and major stability issues are comprised
> of memory management errors.
>
> Why is that? Well, there are a few obvious reasons, e.g. memory
> management in C++ is not fully automatic (like in most "modern"
> programming languages), but rather manual or semi-automatic (with the
> help of libraries). Many of our new contributors come from a Java (or
> similar) background, and are used to automatic garbage collection. So
> it's a hurdle for them to get memory management right (it can be a
> little complex in the beginning). So I decided to start a HOWTO like
> mail, collecting a few tips from my experiences, and maybe also
> starting a discussion to see what other ideas you guys have for making
> Amarok more robust in this regard.
>
> One warning first: I'm in no way an expert authority on this field.
> I'm simply a somewhat experienced C++ programmer. That means that this
> document may contain mistakes, and if you find them, please let us
> know!
>
> So let's start with a few tips:
>
> 1)
> Use of "Smart Pointers":
> A smart pointer in C++ means (in its most simple incarnation) a
> pointer that automatically sets itself to 0, when the object it points
> to is being destroyed (deleted).
>
> Advantages:
>
> No risk of "dangling pointers". A dangling pointer is a pointer that
> has been deleted, but still contains the memory address of the object
> that has been destroyed. What happens if you delete this again is
> called a "double-free", and almost always leads to a crash. With a
> smart pointer, you can delete the object safely again, because
> deleting a 0-pointer is defined as a safe (doing nothing) operation in
> the C++ standard.
>
> Example:
>
> WRONG:
>
> QWidget* foo = new QWidget();
> delete foo;
> delete foo;
> <BOOOOM>
>
> RIGHT:
>
> QPointer<QWidget> foo = new QWidget();
> delete foo;
> delete foo;
> <not nice, but no crash>
>
>
> 2)
> Always make sure not to dereference a 0-pointer:
>
> This is _the_ single most common crash cause in Amarok 2 currently.
> It's easy to prevent, but unfortunately also easy to miss:
>
> Example:
>
> WRONG:
>
> Meta::TrackPtr foo;
> debug() << foo->prettyUrl();
> <BOOOOM>
>
> RIGHT:
>
> Meta::TrackPtr foo;
> if( foo )
>    debug() << foo->prettyUrl();
> <no output, and no crash>
>
>
> 3)
> Never, ever, use private d-pointer classes in QObject derived subclasses:
>
> What can happen is that you do a "delete d;" in your destructor, and
> then Qt goes ahead and auto-deletes other QObject pointers contained
> in the private class again, through means of its automatic deleting of
> QObjects with a parent Object. -> <BOOOOM>
>
> Read more about this topic in Michael Pyne's interesting blog article:
>
> http://www.purinchu.net/wp/2009/02/04/another-programming-tidbit/

This is not correct. Using d-pointers in QObjects is perfectly fine as
long as the destructors are written correctly. The referenced article
only says that one has to be careful about deleting a private
d-pointer when other classes keep pointers to members of the private
class around.

>
> 4)
> Use Valgrind:
>
> This is one of the most advanced memory debugging tools available,
> it's free, and we even have found volunteers that run regular Valgrind
> checks (both for memory access bugs and memory leaks) on Amarok trunk.
> Reading the Valgrind logs correctly is a bit of an art in itself, but
> I'm willing to explain this in another posting, if there is a demand.
>
>
> Recommended reading on the topic of memory management is this page of
> the excellent "C++ FAQ Lite":
>
> http://www.parashift.com/c++-faq-lite/freestore-mgmt.html
>
>
> So, this wasn't very many tips, but I think I've covered a few
> important topics that could already help us a lot. Please feel free to
> add your own ideas, and discuss :)
>
> --
> Mark Kretschmann
> Amarok Developer
> www.kde.org - amarok.kde.org
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