Re: [boost] [mpl] is there a or_seq like logical metafunction?

Subject: Re: [boost] [mpl] is there a or_seq like logical metafunction?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-02-10 14:21:45

On 02/10/09 12:45, vicente.botet wrote:
 > ----- Original Message -----
 > From: "Larry Evans" <cppljevans_at_[hidden]>
 > To: <boost_at_[hidden]>
 > Sent: Sunday, February 08, 2009 2:50 PM
 > Subject: Re: [boost] [mpl] is there a or_seq like logical metafunction?
[snip]

 >> Aren't true_ and false_ nullary logical metafunctions.  If
 >> so then I think what you're requiring is someway to
 >> "unpack" the args from Seq<T1,T2,...Tn> into and_<T1,T2,...,Tn>
 >> or or_<T1,T2,...,Tn>. Could:
 >>
 >> 
http://www.boost.org/doc/libs/1_37_0/libs/mpl/doc/refmanual/unpack-args.html
 >>
 >> be used to do that?
 >
 > Oh! this is quite interesting. I will try it to see how it
 > works. WHich implementation should perdorms better if any
 > difference?

I don't know.  I assume you mean the two methods:

   1) find_if a true_ or false_
   2) unpack then use and_ or or_

If you're using the variadic compiler, then the 2nd would work faster
because the unpacking is done by the compiler instead of by template
metaprogramming.  If not, then I really don't know because I've not
looked at the unpack code.

 >
 >> If not, then it's pretty easy to do that
 >> with the variadic version of mpl since all Sequences are
 >> derived from package<T1,T2,...,Tn> so:
 >>
 >>   template<typename... T>
 >>   or_seq<package<T...> >
 >>   : or_<T...>
 >>   {};
 >>
 >> A prototype variadic version of mpl is in the boost vault.
 >
 > Thanks for the pointer,
 >

You're most welcome.

BTW, what about the while_ template mentioned in my other recent
post. It seems that would be most general, and it just uses template
recursion and eval_if.  If simplified implementation *usually* means
faster execution, then maybe while_ would be fastest.  I know the
non-variadic and_ uses recursion (as I mentioned in my reply to
David), so; I don't think that would be a disadvantage.

-Regards,
Larry