On Sun, Jul 17, 2011 at 2:48 PM, Nathan Ridge <zeratul976@hotmail.com>wrote:
On 7/17/2011 4:10 PM, Edward Diener wrote:
I agree with Jeff's suggestion that the matching metafunction should take a metafunction that defaults to is_same rather than always testing for type equality (and now I see why he was also suggesting that in the Type Traits Extension review :).
I will look further into adding this but I strongly feel that I should not drop the simple functionality currently provided, where the end-user can just pass an actual type.
As a casual Boost user, I would welcome this. I would find TTI very useful, but I do not know MPL beyond the very basics, and I'd rather not have to learn it just so that I can use TTI. I think others may be in the same position.
My initial response is, grab a beer and go learn about Boost.MPL lambda expressions, you'll be happy you did (at worst, you're +1 drunk beer). Okay, my secondary response: I don't think it's really that bad to write has_type_xxx< T, boost::is_same< boost::mpl::_1, U > > // my proposed query rather than has_type_xxx< T, U > // current query (a) How often do you *really* want to check that T::xxx is precisely U? I think, much more frequently, you'll just want has_type_xxx<T>. And I'm *not* proposing to get rid of *that*. (b) To me, the former states more explicitly how U ties into the query. (c) With the current has_type_xxx semantics, it's going to get onerous one metaprogramming day when boost::is_same< boost::mpl::_1, U > just isn't want what you want. You want to check if T::xxx is an object type. Or whether it's const-qualified. Or any of a number of other properties in Boost.TypeTraits. Or :: gasp :: whether T::xxx itself has a nested type yyy :) Given that TTI is a "metaprogramming library proposed for Boost", I don't really see any issues (in theory) with TTI making good use of a library called "Boost Metaprogramming Library" (aka, Boost.MPL). Honestly, though, it did take me a few reads to dissect the nuances of the Boost.MPL "Metafunctions" section, specifically the subsections "Invocation" and "Composition and Argument Binding". Fortunately, I believe only a very small part of that is necessary to effectively use my proposed change to HAS_TYPE. You can get a lot of mileage out of just knowing about and using Boost.MPL placeholder expressions like boost::is_same< boost::mpl::_1, U >. I know *I* did for a long time and I just accepted the "magic" that went on behind the scenes that made them work until I mustered up the courage to really delve into the documentation. I think if TTI just added a couple of examples showing Boost.MPL placeholder expressions in action, everything would be fine. - Jeff