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

On 09/18/13 22:46, Matt Calabrese wrote:

On Wed, Sep 18, 2013 at 7:14 PM, Daryle Walker wrote:

...

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.)

I agree. Use of that kind of stuff comes up all the time when dealing with variadics. Having sensible 0 argument and 1 argument forms is important.

My reply to Augustin mentioned a variadic and_ and or_ which takes any number of args, including 0. I looked closer at the code and found: http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/mpl/null_uni... which is used, indirectly, by the and_ and or_ metafunctions. The null_unit comments indicates it short-circuits the evaluation based on whether the current argument is the Null argument. In the case of and_, Null would be mpl::false_, In the case of or_, Null would be mpl::true_. At least that's my understanding after a brief review of code. HTH. -regards, Larry