Re: [boost] [Boost.utility]

Subject: Re: [boost] [Boost.utility]
From: Andrew Chinkoff (achinkoff_at_[hidden])
Date: 2010-01-26 11:22:25

> I think you missed the point.  A typical spelling is
some_singleton_class::instance().  That member function
> must determine whether the Singleton has been instantiated and that, often
> in a thread safe way.  Thus, 
> there's a function call and some sort of synchronized access/manipulation
> of state.  That's costly.

> In my version, that cost is paid once per context by the smart pointer
> constructor.  All accesses via that 
> smart pointer *do not* incur the instance() overhead.

Below is the typical realization of A::Instance():

static A& Instance()
{
        if (instance_ == NULL)
        {
                boost::mutex::scoped_lock locker(mtx_); // this is the thread safe cost!
                instance_ = new A();
        }
        return *instance_;
}

You should note that:
1) Cost for thread safe synchronization is paid only once. After instance
had created this cost is no longer paid.
2) Cost for function call (A::Instance()) is replaced with smart_ptr::get()
one.

Did I miss the point?

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