How to use the Singleton pattern right

Hi everbody.

I recently did my first serious implementation of the Singleton
pattern and stumbled into mindboggling troubles immediately.

Imagine the following situation: A data management system shall be
equipped with a mechanism that tracks all kinds of errors, warnings
and status messages that occur during runtime. This mechanism - let's
call it the event logger - must be accessible by virtually every
object in the system but may as well never be called at all.

I chose to plan the event logger as a singleton which looked quite
plausible to me. Here comes the problem: Who is responsible for
cleaning up the event logger when it is no longer needed?

Any reference counting approach (like Java's garbage collection) would
fatally screw up things because there will be times in which the event
logger has been created but the objects that requested it have already
been destroyed. Garbage Collection steps in, deletes the event logger
with all the accumulated messages - just before a new error occurs and
the event logger gets referenced again.

The crucial distinction here is between "stateless" and "stateful"
singletons. Stateless singletons just do something and can then well
be destroyed again, whereas stateful singletons are required to retain
a certain internal state which can be queried later.

How would you implement stateful singletons?

Kind regards,
Malte

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On Tue, 07 Oct 2003 22:57:36 +0200, Malte Persike wrote:
[snip]
> The crucial distinction here is between "stateless" and "stateful"
> singletons. Stateless singletons just do something and can then well
> be destroyed again, whereas stateful singletons are required to retain
> a certain internal state which can be queried later.
>
> How would you implement stateful singletons?


In ModernC++Design there is a walkthrough though many different kinds of
singletons. The one you are talking about is a Phoenix singleton, which
is capable of respawning.
http://www.moderncppdesign.com/

You can download Loki, which is the code which comes with the book.
It contains a nice implementation of the phoenix singleton.
http://sourceforge.net/projects/loki-lib/

--
Simon Strandgaard

>You can download Loki, which is the code which comes with the book.
>It contains a nice implementation of the phoenix singleton.
>http://sourceforge.net/projects/loki-lib/


Dear Simon,

thanks a LOT (if formatting was allowed in Newsgroups, that would have
been king-size). This was helpful!

Regards,
Malte

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By the way, this newsgroup seems to be not the most active one. I
wonder why this is the case, especially when considering the relevance
of the topic for most programmers out there.
Do you have a clue?

Sorry for being somewhat off-topic,
Malte

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On Wed, 08 Oct 2003 02:51:15 +0200, Malte Persike wrote:

> By the way, this newsgroup seems to be not the most active one. I
> wonder why this is the case, especially when considering the relevance
> of the topic for most programmers out there.
> Do you have a clue?

There is also much talk about patterns on comp.object
and on my favorite newsgroup comp.lang.ruby

I guess people more often ask questions in the language specific
newsgroups, like how do I implement observer-pattern in 4004 assembler.

Agree, more activity in this forum would be interesting :-)

--
Simon Strandgaard

>Agree, more activity in this forum would be interesting :-)

So let's get it on ;-)

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> I'd suggest managing the state outside of the singleton itself. If you
> create the singleton as a stateless entity and then delegate stateful
> operations elsewhere, you'll have fewer problems with concurrent access,
> supporting higher load, and issues with distributed systems.

Thanks. Any examples of this?