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Subject: Re: [boost] [Boost.utility]
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-01-26 11:30:04

Andrew Chinkoff wrote:
> 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.

That code has a race condition, so it appears more efficient than it should.

> 2) Cost for function call (A::Instance()) is replaced with
> smart_ptr::get() one.
> Did I miss the point?

Yes. The smart pointer's operators are inlined functions that merely access a pointer data member. Zero overhead. Your function cannot be inlined and, depending upon how you manage the synchronization, it can be fairly costly, but most certainly includes function call overhead.

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

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