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Subject: Re: [boost] Formal Review Request: TypeErasure
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-06-26 15:18:58


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:
> compiles and runs OK.

That would be a bug. The code should
not compile with either setting.

> However, with:
> 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.

In Christ,
Steven Watanabe

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