Larry Evans wrote:
On 04/19/2007 12:35 PM, Eric Niebler wrote:
Larry Evans wrote:
On 04/19/2007 11:46 AM, Eric Niebler wrote: [snip]
Oh! You're chaining or_'s together. Sure, that works. So for your initial state, all you need is a pattern that never matches anything. For that, you can use proto::not<proto::_>. I couldn't get that to compile; however, proto::logical_not<proto::_> does. Thanks.
proto::not_<> is a fairly recent addition. Be sure you're sync'ed up. logical_not<_> is certainly not what you want here. It will successfully match proto expressions such as !as_expr('a'). You want something that will never match anything. You could also use proto::if_<mpl::always<mpl::false_> >.
Yeah, I thought about it some more and the name, logical_not, suggests that if the argument, X, doesn't match, then logical_not<X>, would match, which, as you've noted, is certainly not what I want.
No, you're mixing two things up. logical_not<> doesn't logically negate its argument. It matches the ! operator. The pattern "logical_not< terminal< char > >" would match the expression template generated by !as_expr('a'). not_<> is different. It *does* logically negate its argument. The pattern "not_< terminal< char > >" will match any expression *except* those that match the pattern terminal<char>. The pattern _ is a wildcard that matches anything. So not_<_> is a degenerate pattern that matches nothing.
However, I think proto::not_ also suffers the same name confusion. Like logical_not<X>, I'd think not_<X> would mean "if X fails then not_<X> succeeds".
That's precisely what it does. -- Eric Niebler Boost Consulting www.boost-consulting.com