On 07/27/12 14:27, Steven Watanabe wrote:
AMDG
On 07/27/2012 11:54 AM, Larry Evans wrote:
On 07/27/12 13:08, Steven Watanabe wrote:
You're passing an argument of type ph_value<_a>. How would constructible<_a(const _a&)> (which is the copy constructor) be viable?
Because, when _a in constructible<_a(const _a&)> is replaced by the binding, it becomes, IIUC,
constructible<ph_value<_a>(const ph_value<_a>&)>
This substitution happens in the dispatching layer. In the interface of any, constructible<_a(const _a&)> defines the constructor:
any<C, _a>::any(any<C, const _a&>);
It can't possibly define any<C, _a>::any(const ph_value<_a>&); because it doesn't know about ph_value<_a>.
Why doesn't it know about ph_value<_a>? It's the contained type corresponding to _a, and that must be available somewhere in the binding<Concept> arg ; otherwise, it couldn't create the a_dup even when the constructible is: constructible<_a(const a_type&)> instead of: constructible<_a(const _a&)>
and this is the same as the compiling version of binding_of.cpp. I thought this replacement is what would happen based on the example:
constructible<_a(const _b&, const _c&)>
from:
http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/...
Why is the replacement done in the construction.html example and not in bindings_of.cpp? Hmmm... Ok, the example has any args, from which the bindings<ConceptA> can be gotten. But those same bindings are available from the binding<ConceptA> arg in bindings_of.cpp creation of a_dup.
Obviously I'm still missing something, but I've no clue what :(
The library deals with either any's or with the contained types, but not both, depending on what layer you're looking at. You're trying to mix the two.
Initially, that made no sense to me. Then, after further experimentation with the attached, where I used every combination of the defined or undefined for macros: HARDCODE_CTOR_ARG A_DUP_VALUE I finally realized that a placeholder in a concept does not stand for the contained type, instead it stands for a type_erasure::any of the contained type. If that's right, I can't remember seeing that anywhere in the docs. However, looking again at the examples: multi.html construction.html that seems to fit. Now the doc here: conceptdef.html does say: When a concept is instantiated with a specific set of type bindings, each placeholder is bound to a cv-unqualified non-reference type but it doesn't say the non-reference type must be an any. Also, the doc here: placeholder.html says: Placeholders act as a substitute for template parameters in concepts. The library automatically replaces all the placeholders used in a concept with the actual types involved when it stores an object in an any. but it doesn't say the actual type must be a type_erasure::any of some sort. I thought (quit understandably, IMO) the actual type could be the one specified by mpl::at<Map,Tag>::type where Map is the one used in static_binding<Map>. To help avoid future confusion, I suggest the docs make very clear that the placeholder's in the concepts stand for a type_erasure::any of some sort. Also, the doc: any.html#id2445089-bb contains: Requires: Concept must contain a matching instance of constructible. but no definition of "matching". So, again, to help avoid future confusion, please provide a definition of "matching" (maybe in a matching.html file, with a link to that file) which emphasizes that a placeholder does stand for a type_erasure::any and *not* for the contained type corresponding to the placeholder. I'm apologize to take up so much of your time and I appreciate your patience in helping me understand type_erasure. -regards, Larry