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Subject: Re: [boost] [Boost.utility]
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-01-26 11:03:51

Jeff Flinn wrote:
> Stewart, Robert wrote:
> > Jeff Flinn wrote:
> >
> >> That said, in my experience singletons are
> >> generally used as a crutch to access global state.
> >
> > They do access global state, but there often is global
> > state in an application. If Singletons provide the only
> > means to access state, they formalize that access.
> > Singletons can also, using policies, control lifetime,
> > post-destruction behavior, etc. Consequently, Singletons are
> > more than a crutch, though often overused, abused, or misused.
> Well, I've yet to come across a case where global state couldn't have
> been avoided by passing the appropriate objects by value or reference
> along the function call chain.

That is certainly possible. It is also possible to use return codes rather than exceptions for all error reporting, though that's hardly a good idea. Throwing exceptions and catching them in higher level code means the code in the middle is ignorant of the error state. Singletons work similarly. Code buried well down the call stack needs access to some state. Without a Singleton for that state, all functions up the call stack must be changed to pass along that state. Using Singletons eliminates that burden.

> Doing so avoids the mess that ensues when trying to unit test classes
> and functions that use supposed singletons/global state.

I can't argue with that. It is also easier to unit test functions returning error codes, but that shouldn't be the only factor in design.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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