Re: [boost] Review Request: Singularity

Subject: Re: [boost] Review Request: Singularity
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-08-25 08:56:25

Mathias Gaunard wrote:
> On 24/08/2011 15:29, Alexey Tkachenko wrote:
> > On 24-Aug-2011, at 9:18 PM, Mathias Gaunard wrote:
> >> On 24/08/2011 04:28, Ben Robinson wrote:
>
> >>> 1) You have little control over when the Singleton is
> >>> destroyed.  Should you have multiple Singleton which
> >>> depend on each-other, very often you need a very
> >>> specific teardown order.
> >>
> >> It should be the inverse order of the construction order.
> >
> > Consider dependency between singletons
>
> Your point being?
>
> Construct your singletons in the order required by your
> dependency, then let destruction happen in the inverse order.
>
> function-level statics are constructed *on the first call of
> the function*, not in an unspecified order like class-level
> statics or global variables.

If an application has a logging Singleton and each of the other Singletons in that application used the logging Singleton in its destructor, but one or more of those other Singletons were created before the logging Singleton, you'd have a problem.

This is the destruction order problem which leads to so many different partial solutions, such as allowing the logging Singleton to be implemented as a function local static pointer and left to leak.

When you know that a particular Singleton is needed after all others, you might try to create it early, but if there are objects with static storage duration that access any of the Singletons, then you lose control over the initialization order.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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