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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?

>> 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:
>> 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.


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