On 06/26/12 14:18, Steven Watanabe wrote:
AMDG
On 06/26/2012 11:41 AM, Larry Evans wrote:
*/libs/type_erasure/doc/html/boost_typeerasure/conceptdef.html
states:
The template parameters of the concept may involve placeholders.
* Each template argument may be a cv and/or reference qualified placeholder type.
* If a template argument is a function type, its arguments and return type may be cv/reference qualified placeholders.
but, because of the 'may's it doesn't require a placeholder.
That's correct. The errors that you're getting stem from a different cause. The library doesn't actually require any placeholders, although it isn't terribly useful without them.
Now, obviously, the placeholder is the only way to retrieve the value of the data stored in the any, and you could reason that a placeholder is obviously needed; however, I think that needs to be explicitly stated so there's no confusion (I confess, I was actually confused by this and it took several trials before I finally concluded that a placeholder was required).
The attached file, when compiled with:
#define MODEL_DEFAULT_SELF
compiles and runs OK.
That would be a bug. The code should not compile with either setting.
However, with:
//#define MODEL_DEFAULT_SELF
gcc4.8 fails to compile it and gives a very obscure error message containing:
../../../boost/type_erasure/call.hpp:530:89: required from 'typename boost::type_erasure::detail::call_result<Op, void(U0&)>::type boost::type_erasure::call(const Op&, U0&) [with Op = concept<>; U0 = int; typename boost::type_erasure::detail::call_result<Op, void(U0&)>::type = void]' concept_ph.cpp:84:5: required from here ../../../boost/type_erasure/detail/get_signature.hpp:23:5: error: incomplete type 'void' used in nested name specifier BOOST_TYPEOF_NESTED_TYPEDEF_TPL(nested, &Concept::apply) ^
which seems to support the position that some placeholder is required in the concept's template arguments, AFAICT.
For some reason, gcc tends to make errors with call point to BOOST_TYPEOF.
Anyway, I can compile the following just fine with MSVC 10.
int i = 0; call(binding<incrementable<int> >(mpl::map0<>()), incrementable<int>(), i); // i = 1
The problems with your code are:
a) call(concept<int>(), y) - The library cannot deduce the the binding because none of the arguments is an any. b) call(concept<_self>(), a_any) - concept<_self> is not part of the requirements of a_any.
I tried the b) case again and it does not compile. I must of accidentally rm'ed the concept<> from the requirements while posting the code. I remember making a bunch of "minor" edits to make the code more presentable, and I must have failed to test compile it after the last edit :( Sorry for that typo. When corrected, and when: #define MODEL_DEFAULT_SELF the run is: ./concept_ph.exe :e=empty :apply(empty ) Thanks for the explanation.