Hi,
I know this is a bit OT, but talking about OSG design... What about dropping
the singleton pattern entirely?
The more I use it or the more I encounter it, the more I think the singleton
pattern is completely broken. I know this is quite a bold claim, but bare with
me and let me explain.
- The life time of the singleton is not well defined in most cases, leading to
typical problems in C++ such as the initialization order problem.
- The singleton pattern is an ambiguous design. It states that there is one
instance, but it does not state "per what". Is it per thread? Is it per
process? Is it per library/DLL? Is it per application? Is it per OS? Is it per
computer?
These two reasons alone make the typical singleton pattern broken in a lot of
cases. Most of the time without the programmer realizing it (myself included).
Anyone having to deal with singletons, static libraries and DLLs knows how
dangerous it becomes (e.g. you can often have several unwanted instance of the
singleton: one per DLL/EXE). Add thread-safety into the mix, and it all becomes
a nightmare.
We've had already several problems with OSG in our project due to that, and had
to resort to ugly work arounds (e.g. manually calling undocumented OSG release
functions).
May I suggest that a better design is investigated for OSG 3.0?
For example:
class Initializer : private noncopyable
{
public:
Initializer()
{
if (m_ref++ == 0)
initializeGlobalInstances();
}
~Initializer()
{
if (--m_ref == 0)
uninitializeGlobalInstances();
}
private:
initializeGlobalInstances();
uninitializeGlobalInstances();
atomic_counter m_ref;
};
Such as solution clearly defines the lifetime of the OSG library. It also puts
the responsibility of managing the lifetime of the library in the user hands,
who's more apt to know what is right for his application. It's also
exception-safe.
Note that that implementing
initializeGlobalInstances/uninitializeGlobalInstances as thread-safe needs
careful thinking (see
http://www.justsoftwaresolutions.co.uk/articles/implementing_mutexes.html
especially the part about initialization and uninitialization).
Also note that the Initializer has to be implemented in a DLL (i.e. shared
library) and not a static library, as it does not solve the multiple singleton
instances per process problem. However, I'm currently not aware of any robust
ways of solving that problem.
I hope I don't come as too arrogant on this. But I really think this part of
OSG doesn't do justice to the rest of the library.
Cheers,
Tanguy
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Jean-Sébastien
Guay
Sent: Wednesday 11 February 2009 16:21
To: OpenSceneGraph Users
Subject: Spam: Re: [osg-users] memory leak false positives on Windows
Hi Cory,
> I don't think anybody has questioned the design of OSG.
Yes, Neil has said that perhaps we should consider "restructuring" to
avoid the false positives. This comes down to changing the design.
> I also agree with you that making code concessions to accommodate tools
> is unfortunate, but it happens all the time.
Yes, the coding changes to accomodate tools. But the design will stay
the same. It's the implementation that changes. That's what I was
talking about.
> On the other hand, I'd like to know if code that explicitly unloads
> OSG would ever be accepted into the repository. I'm getting the sense
> that it would be if it were sufficiently transparent, simple and
> inexpensive.
Yes, Robert said in this thread that he would accept something that
unloads/unrefs all singletons / static objects, but it might be hard to
get to all singletons since some exist in the implementation files only.
>> (btw, has anyone compiled valgrind for Windows?)
> valgrind is Linux only.
I suspect it's not Linux-specific, but more gcc/g++ specific, and so
might be buildable on cygwin or mingw? If it is, then it might be usable
for Windows executables...
J-S
--
______________________________________________________
Jean-Sebastien Guay [email protected]
http://www.cm-labs.com/
http://whitestar02.webhop.org/
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