Rozental, Gennadiy wrote: > > I hope you mean "what it does/how to use it" as opposite to > > "how it does what it does" ;). > > In fact it is second ;-)
See http://aspn.activestate.com/ASPN/Mail/Message/boost/1156353 for the sketch. The full implementation is just way too complicated to describe in words. The code is always there, of course :). > > > // numbered placeholders semantics > > plus<_1, int_c<5> > == bind<meta_fun2<plus>,_1,int_c<5> > > > plus< int_c<5>,_1 > == bind<meta_fun2<plus>,int_c<5>,_1 > > > plus< int_c<5>,_2 > == bind<meta_fun2<plus>,int_c<5>,_2 > > > plus<_1,_2> == bind<meta_fun2<plus>,_1,_2> > > plus<_2,_1> == bind<meta_fun2<plus>,_2,_1> > > > > logical_or< is_same<_1,int>, is_same<_1,long> > > > = bind< meta_fun2<logical_or> > > , bind< meta_fun2<is_same>,_1,int > > > , bind< meta_fun2<is_same>,_1,long > > > > > > > > is_same< _1, add_pointer<_2> > > > == bind< meta_fun2<is_same> > > , _1 > > , bind< meta_fun1<add_pointer>, _2 > > > > > > It would help if you start with what meta_fun[n] are, cause > they are not covered in docs. 'meta_fun[n]' (to be renamed to 'quote[n]') converts a metafunction (http://www.mywikinet.com/mpl/paper/mpl_paper.html#metafunctions.simple) of arity 'n' into the corresponding metafunction class (http://www.mywikinet.com/mpl/paper/mpl_paper.html#metafunctions.classes). > Also if I understand correctly it require > template template parameters. Yes. > How then it works for MSVC6.5? It ('meta_fun') doesn't. Lambda facility does, because on deficient compilers it employs intrusive introspection: template< typename T > struct identity { typedef T type; BOOST_MPL_AUX_LAMBDA_SUPPORT(1,identity,(T)) // here }; The marked line is required if you want to use 'identity' in lambda expressions on compilers that don't support partial specialization or template template parameters. [...] > So the _ placeholder is substituted with appropriated numbered > one based on position in expression. How did you managed to achieve > this? Well, I just do that - when resolving each bind's argument I analyze if it is the unnamed placeholder, and if it is, then I substitute it for the current numbered equivalent and "increment" the count. See "boost/mpl/aux_/preprocessed/plain/bind.hpp" for the details. For comparison purposes, "boost/mpl/aux_/preprocessed/plain/basic_bind.hpp" contains the preprocessed version _without_ the unnamed placeholder support. HTH, Aleksey _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
