[boost] Re: composition, with expression templates!

From: jsiek_at_[hidden]
Date: 2000-01-29 16:33:36

Ok, I see what you mean about the need for typeof.  I just settled for
something like plus_trait, though at least the traits are already
defined for all builtin-types through the promote_trait class
(borrowed from Todd Veldhuisen).

For user-defined types they'll have to specialize the traits.

Valentin Bonnard writes:
 > Dave Abrahams wrote:
 > 
 > > on 1/29/00 3:15 PM, Valentin Bonnard at Bonnard.V_at_[hidden] wrote:
 > > 
 > > >> Basically, the idea is to overload all the typical operators
 > > >> (+, -, <, ==) for functor objects, and use Nico's composition
 > > >> library as the nodes in the expression tree. It's really not
 > > >> all that hard to do...
 > > >
 > > > How it is possible at all w/o typeof, which no compiler (not even
 > > > gcc) supports ?
 > > >
 > > > (and even then (w/ typeof) it would be ugly)
 > > 
 > > I did some thinking, and I think you could do it with traits. Preparing
 > > types for use in operations isn't pretty, but the resulting usage would be
 > > ;)
 > 
 > template <typename T>
 > T value ();
 > 
 > typeof (value<T> () + value<U> ())
 > 
 > can of course be rewritten
 > 
 > template <>
 > struct plus_trait<T, U>
 > {
 >   typedef whatever result_type; 
 > };
 > 
 > typename plus_trait<T, U>::result_type
 > 
 > but if the user is required to define too much traits, then 
 > he will simply define a named function to do the work 
 > directly instead of using all the compose library.
 > 
 > (In complex cases, the named function will be clearer than 
 > a bunch of combinators.)
 > 
 > -- 
 > 
 > Valentin Bonnard
 > 
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 > 
 >