Re: [boost] Formal Review Request: TypeErasure

Subject: Re: [boost] Formal Review Request: TypeErasure
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-06-26 17:07:13

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.