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Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-03 12:55:35

Chad Nelson wrote:
> On 05/03/2010 07:34 AM, Stewart, Robert wrote:
> > PMFJI, but the point of move semantics is to avoid copying, not to
> > make it faster. If Using COW is faster than move semantics, that
> > means there are copies being made. If move semantics were applied
> > with the expectation of reducing copies where they weren't really
> > needed, from rvalues, then perhaps Boost.Move has been misapplied.
> I was trying to use it to reduce the copying that is
> otherwise necessary
> when returning values (and not using copy-on-write). I think
> that's its intended use.

You may be missing an important aspect: reusing temporaries. (N)RVO generally handles efficiently returning values. Where move semantics can really help, though, is in permitting you to use those rvalues for intermediate computations. Refer to the examples given earlier in the thread about moving intermediate results, in a temporary, to the full expression's result.

I recall that you did mention multiplication as a case where that couldn't be done because the product would be larger than the multiplicands, though obviously not if either is 1 or 0, so this aspect may not have eluded you. Nevertheless, I wanted to be sure you understood the difference.

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

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