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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2008-01-16 03:07:56

Giovanni Piero Deretta wrote:
> On Jan 15, 2008 12:21 PM, Anthony Williams <anthony_w.geo_at_[hidden]> wrote:
>> John Torjo <john.groups <at> torjo.com> writes:
>>
>>> Today starts the formal Fast-Track review of the Boost.Utility/Singleton
>>> library. This is a very useful utility, so I'm expecting lots of reviews ;)
>>> * What is your evaluation of the potential usefulness of the library?
>> 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.
> 
> I consider singletons just another form of globals and thus bad style.

Let's see:

     "I consider if, else, for, do, while and the ternary operator just
      another form of goto and therefore bad style."

;-)

> But sometimes they get handy,
> if you do not want to pass around a context to every function in a module.
> Anyways, I've regretted too every single singleton that I have used.
> 
> What about a form of thread local singleton which can be replaced with
> a scoped construct?
> 
> //global scope
> singleton<my_object> my_singleton;
> 
> void foo() {
>    my_singleton.instance().use_singleton(); // the globaly scoped one
> or a local replacement.
> }
> 
> void bar() {
>   scoped_replacer _ (my_singleton); // replace my_object of baz with a copy
>   my_singleton.instance().perform_local_modification();
>   foo();
>   // at this point my_object of baz is reinstantiated
> }
> 
> void baz() {
>   scoped_replacer _ (my_singleton, my_object()); // replace the
> initial my_object
> 
>         // with a copy of the second parameter
>   foo();
>   // at this point the original my_object is reinstantiated
> }
> 
> int main() {
>   baz();
> }
> 
> In practice it is a form of dynamic scoping, which might be more
> useful and flexible than a static singleton.
> You definitely want this dynamic scoped object to be per thread.
> 
> I haven't read Boost.Singleton documentation, maybe it already allows
> such a functionality.

I'm not sure I understand the purpose of the code provided, but it seems
to be sufficiently trivial to implement the facility you describe on top 
of what's provided.

Regards,
Tobias