Re: [boost] Proposed changes to operators.hpp

From: Daniel Frey (d.frey_at_[hidden])
Date: 2002-07-03 09:11:00

On Wed, 2002-07-03 at 14:37, David Abrahams wrote:
> From: "Daniel Frey" <d.frey_at_[hidden]>
> 
> > See also Scott Meyers "More Efficient C++", Item 4.7. If you read the
> > book, you might get the impression I misunderstood it completely, but
> > please read Scott's errata for this item, available at:
> >
> > http://www.aristeia.com/BookErrata/mec++-errata_frames.html
> 
> And there it says:
> 
>   7/19/96 sdm   109  In July 1996, the standardization committee
> 9/26/96
>                       decided that named objects may be treated
>                       essentially the same as unnamed objects for
>                       purposes of performing the return value
>                       optimization.  I added a footnote to this effect.
> 
> 
> Which leads me to believe that there is no "NRVO", just an "RVO".
> Furthermore, I believe that more compilers implement the RVO for unnamed
> objects than for named objects. So why aren't you using this implementation
> instead?
> 
> friend const T operator+( const T& lhs, const T& rhs )
> {
>   return T(lhs) += rhs;
> }
> 
> -Dave

There are several entries for Item 4.7, I refered to this one:

! 11/23/99 jm2   109  Both the first and second implementations on this
                      page suffer from the problem that they are
                      returning whatever operator=+ returns.  In
                      general, there is no way to know what this is
                      (yes, it's a reference to an object of type T,
                      but a reference to which T object?), hence no
                      way for compilers to optimize away the copy to
                      operator+'s return value.  The way to write
                      operator+ such that the return value
                      optimization can be performed is with this body:

                        T result(lhs);
                        result += rhs;
                        return result.

Regards, Daniel