On Sat, Apr 12, 2008 at 11:58 AM, Thorsten Ottosen < thorsten.ottosen@dezide.com> wrote:
Corrado Zoccolo skrev:
On Fri, Apr 11, 2008 at 9:38 AM, Sebastian Redl < sebastian.redl@getdesigned.at> wrote:
If this variant of swap() is really more efficient, perhaps metaprogramming could be used to support it *if* the type has a default constructor. But I don't know how to detect this. TypeTraits doesn't contain a has_default_constructor trait. It may be that this will only work with 09 concepts - DefaultConstructible.
We can use has_nothrow_default_constructor . This will also exclude the specialization when the default constructor performs some allocation, and could result in lower performance.
I can't imagine that it would result in lower performance in general ... have you seen many types for which default construction was more expensive than copy-construction?
For example shared-pimpl objects (mentioned by Andrey), will only increment a reference count on copy construction, but may need to allocate memory on default construction.
has_nothrow_default_constructor is certainly useable, albeit it requires compiler inrinsics. Do we know how many compilers that do support them?
I think boost could provide some specializations it for stl types that require it, and the user is allowed to specialize it for his types. I think variant has a similar optimization for exception safety purposes, that works under similar assumptions. Corrado
-Thorsten _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
-- __________________________________________________________________________ dott. Corrado Zoccolo mailto:zoccolo@di.unipi.it PhD - Department of Computer Science - University of Pisa, Italy -------------------------------------------------------------------------- The self-confidence of a warrior is not the self-confidence of the average man. The average man seeks certainty in the eyes of the onlooker and calls that self-confidence. The warrior seeks impeccability in his own eyes and calls that humbleness. Tales of Power - C. Castaneda