Re: [boost] [review] Fast track review of Boost.Utility/Singleton begins today

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2008-01-16 03:04:59

Anthony,

thanks for your review.

Anthony Williams wrote:
> I don't think this is at all useful. People should not be encouraged to use
> singletons, as there is already a tendency towards overuse. In most
> circumstances, singletons are a poor design choice, so there is little
> requirement for their use.

Any design that's picked carelessly is poor or just at best a fluke.

Having a framework internally use some Singletons can greatly simplify
its use. Exposing a singleton to a user provides more flexibility than
exposing a static interface (and can also improve performance).

A "tendency towards overuse" is not a good reason to reject a library,
as it won't stop overuse and encourages more half-baked solutions that
are written in a hurry.

The only thing you can do against overuse is to add your experience to
the documentation in order to educate users about applicability.

>> * What is your evaluation of the design?
>> * What is your evaluation of the implementation?
> 
> The design mixes several independent issues --- ensuring there is only one
> instance of a class and avoiding initialization order problems with on-demand
> initialization for starters.
> 
> A simple wrapper class that allows for on-demand initialization, would be
> useful. Conflating that with "there shall be at most one instance" is not.
> 
> Again, allowing for a preferred destruction sequence of objects with such
> on-demand initialization might also be useful, but does not belong with the
> one-instance constraint.

What is the point in managing construction order without static context?

What is the point of having more than one instance of a class that lives
in static context -- and how would it be captured syntacticly?

> thread_specific_singleton is overly complex for what should just amount to a
> simple use of thread_specific_ptr.

It uses 'thread_specific_ptr' and has additional responsibilities.

Could you be more specific, please?

> I've written code that used singletons (and regretted it later), and used
> code that other people have written containing singletons.

So you should like this library: The design chosen allows you to easily
substitute Singletons with Smart Pointers.

Regards,
Tobias