Re: [boost] [xint] Third release is ready, requesting preliminary review

Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-04 11:36:17

DE wrote:
>  Stewart, Robert wrote :
> > DE wrote:
> >>  Giovanni Piero Deretta wrote :
> >>
> >> > No, but swap is your friend:
> >>
> >> > X operator- (X x) {
> >> >  modify-in-place x;
> >>
> >> >  X ret; // this should be cheap
> >> >  ret.swap(x);
> >> >  return ret; // NRVO should kick in
> >> > }
> >>
> >> i don't think it is a good idea to write such curious code
> >> in chase of
> >> a _possibility_ (there are no guarantees) of rvo
>
> > That's perfectly reasonable code.  Consider
> > <http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/>.
>
> i don't understand how is that different to
>
>   X operator- (const X &x)
>   {
>     X ret(x);
>     //modify 'ret'
>     return ret;
>   }

As the article states, the compiler can elide the copy in various cases.  With your version, the copy is unconditional.

> what about idiomatic
>
>   return X().swap(x);  //x is modified

That incurs the default constructor overhead, whatever that may be, before the swap.  That also requires that the function argument be a reference to non-const X and the caller would be justifiably surprised at the effect.

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

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