On 07/26/12 15:32, Steven Watanabe wrote:
AMDG
On 07/26/2012 09:21 AM, Larry Evans wrote:
However, in my defense, I couldn't see any reason why, if:
any<Concept,_a> a_any
can be created with static_map<Concept>, it couldn't be created with a binding<Concept>; thus, I guess I just glossed over the any.html#id2445089-bb doc.
The reason I didn't provide it, is that I assumed that if the type you're constructing is known statically, the entire set of type bindings would be known statically.
BTW what *is* the reason for the restriction on the Concept in the binding<Concept> CTOR. It's obviously not needed for creating an instance of AnyA with a value<_a>::type even when ConceptAB does not "contain a matching instance of constructible".
Yes it is.
Then why can an AnyA be created with the static_binding<bindings_map_t> CTOR *without* the need for any constructible concept? The fact that AnyA can't be created with the binding<ConceptAB> CTOR is a an unnecessary restriction since, obviously, it can be created with the static_binding CTOR
You're not understanding what this constructor does properly.
template<class... T> explicit any(binding<Concept>, T&& ...);
calls a type erased constructor. The stored type is whatever the binding says, /not/ T. constructible<...> is required, because it is used to construct the contained object.
But this: constructible<...> is required, because it is used to construct the contained object. constradicts what happens with the static_binding CTOR because an AnyA *is* constructed with that CTOR when ConceptAB does *not* contain constructible<a_(...)>. Now it's obviously required by the binding<ConceptAB> CTOR, but that is, AFAICT, an unneeded restriction. Now if you made the 2 constructors consistent (i.e. both requiring constructible<a_(...)>, I could see your point; however, as long as the static_binding allows creation with constructible<a_(...)>, then the other one should also. One way to achieve consistency is shown in my other post: http://article.gmane.org/gmane.comp.lib.boost.devel/233032 That post contains a revised detail/construct.hpp. It's the same as the one in svn except, in case there's just one U, it behaves the same as the static_binding CTOR. IOW, with just one U arg, no construct<a_(...)> is needed either with the static_binding CTOR(obviously because there is only one CTOR arg besides the static_binding one) or with the binding<ConceptAB> CTOR. -regards, Larry