[boost] Re: Singleton

From: Dave Handley (dave_at_[hidden])
Date: 2005-01-07 12:59:02

Mikhail Glushenkov wrote:

> What about adapting Loki's Singleton? It's the best Singleton
> I know about and (probably) the most widely-known one.

I agree that it is probably one of the best ones around.  My only issues 
are:

1)    Wouldn't it be better to use CRTP than the rather strange usage syntax 
that Loki uses with a typedef and a requirement to make the 
constructors/destructors private?  It is remarkably easy if you are not 
familiar with the Singleton implementation in Loki to make your 
constructors/destructors public by mistake and then use them, whereas a CRTP 
solution could make the base constructors/destructors protected.

2)    There are probably some more options that need to be included, for 
example making a Singleton be capable of destroying/recreating during the 
run of a program.  Consider a singleton that is managing a print queue.  For 
some reason all the printers go off-line, there is a potential for memory 
optimisation by closing down the print queue.  If printers come back on line 
the print queue could come back on line - but that isn't possible (as far as 
I know) with Loki.

3)    What issues are there with the licensing of Loki?

David Greene wrote:

> I think that's a good place to start.  What other patterns should
> we be looking at.  We ought to start a wiki page.
>
> I would prefer to put the files in the sandbox so they are
> versioned, have history, etc.  The yahoo files section is
> inconvenient due to the registration requirement.

I would be happy to submit what I have to the sandbox - who do I contact to 
get on the sandbox as a developer, and is there a set of submission 
guidelines?  Where would the wiki page be?  I also notice from a quick 
browse of the sandbox that there is a set of classes for visitors, and that 
they haven't really been touched for 2 years.  Did these ever make it into 
boost?  Is there any documentation for them?  And could we perhaps look 
there for a starting point for the visitor side of things?

In my view, the best start for a list of patterns to look at is probably the 
GoF book!  I have written generic code in the past for:

1)    Flyweights
2)    Composites
3)    Visitors
4)    Singletons
5)    Bridge (just to link the 2 classes easily)

There is certainly scope to include:

1)    Factory Method
2)    Abstract Factory
3)    Decorator
4)    Chain of responsibility

However, if we are to have a go at a project like this, I would suggest 
keeping the initial submission list quite short, and then if there is some 
interest further down track, follow on with more patterns.

Dave