Re: [boost] [mpl] and_<true_> returns false

Subject: Re: [boost] [mpl] and_<true_> returns false
From: Daryle Walker (darylew_at_[hidden])
Date: 2013-09-18 22:14:23

> Date: Wed, 18 Sep 2013 20:33:31 -0300
> From: kaballo86_at_[hidden]
>
> The following snippet fails to compile (tested with 1.54.0 and trunk,
> several compilers),
>
> #include <boost/mpl/and.hpp>
> #include <boost/mpl/bool.hpp>
> #include <boost/mpl/or.hpp>
>
> int main() {
>     using namespace boost::mpl;
>     static_assert(and_<true_>::value, "U KID?"); //this one fires
>     static_assert(or_<true_>::value, "U KID?"); //this one is ok
> }
>
> This issue actually showed up in a variadic template scenario. Is this a
> bug? I feel like I'm doing something really wrong...
>
> I guess one may not call `and_` with less than two arguments. If that's
> the case, it would be helpful if it would be impossible to instantiate
> it with a single argument.

I would go the other way and start support from zero arguments!  The "and" operation would default to TRUE while "or" defaults to "FALSE."  (Hint: the operations default to their "identity" operand value, and definitely not their "short-circuit" operand value.)

Daryle W.