Re: [boost] operator implementation

From: Victor A. Wagner, Jr. (vawjr_at_[hidden])
Date: 2002-06-28 10:15:57

"missed"?  Unnamed return value optimization has been around longer.
Scott Meyers (I left his books at the office, so I can't quote which) 
talked at length about why naming the temporary was likely to generate 
WORSE code on most (at the time he wrote it) compilers.

Though, as I recall, he used something equivalent to:

friend T operator*(const T& x, const U& y) { return  T(x) *= y; }

which is an explicit call to the constructor leaving an UNnamed thing to 
return, and hence optimize

At Friday 2002/06/28 00:01, you wrote:
>Hi,
>
>I noticed that boost libraries sometimes implement some operators in a
>way that prevents optimizations. Example from boost/operators.hpp:
>
>template <class T, class U, class B = ::boost::detail::empty_base>
>struct multipliable2 : B
>{
>      friend T operator*(T x, const U& y) { return x *= y; }
>      friend T operator*(const U& y, T x) { return x *= y; }
>};
>
>This works, but it prevents the NRVO (named return value optimization).
>An implemenation that solves this is:
>
>template <class T, class U, class B = ::boost::detail::empty_base>
>struct multipliable2 : B
>{
>      friend T operator*(const T& x, const U& y)
>        { T nrv( x ); nrv *= y; return nrv; }
>
>      friend T operator*(const U& y, const T& x)
>        { T nrv( x ); nrv *= y; return nrv; }
>};
>
>For further details, see also the thread "Temporaries and optimization?"
>in csc++. (A message not yet approved explains why the name should be
>'nrv', it should show up on csc++ in the next hours).
>
>Or is there anything I missed?
>
>Regards, Daniel
>
>--
>Daniel Frey
>
>aixigo AG - financial training, research and technology
>Schloß-Rahe-Straße 15, 52072 Aachen, Germany
>fon: [phone], fax: [phone]
>eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de
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Victor A. Wagner Jr.      http://rudbek.com
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