Re: [Boost-users] challange to MPL gurus

Subject: Re: [Boost-users] challange to MPL gurus
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-12-13 00:30:02

On 12/12/11 22:49, Robert Ramey wrote:
> Jeremiah Willcock wrote:
>> On Mon, 12 Dec 2011, Larry Evans wrote:
>>
>>> On 12/12/11 17:04, Larry Evans wrote:
>>>> On 12/12/11 16:39, Jeremiah Willcock wrote:
>>> [snip]
>>>> Looking at your code, it seems you solved the problem by removing
>>>> the nested specialization of is_end, with a specialization in file
>>>> scope of check_unless_end.  I've a vague memory of hearing some
>>>> similar solution elsewhere.  Is there some reason why
>>>> specializations cannot occur within the class?
>>>>
>>>> -regards,
>>>> Larry
>>>
>>> The reason is explained in this thread:
>>>
>>> http://groups.google.com/group/comp.lang.c++.moderated/browse_frm/thread/c040d6d750bf5c08/26bc1ce0b89b5979?#26bc1ce0b89b5979
>>
>> I don't see anything in that thread that talks about why member
>> templates can't be specialized in-class.  I don't see a good reason
>> for that limitation; how is a class member specialization different
>> from one in a namespace other than that there can be template
>> parameters on the class itself?
>>
>> -- Jeremiah Willcock
> 
> First of all I want to thank everyone who contributed to this thread. I 
> carefully read
> every single message and the links.  I was really lost.
> 
> I was really bothered with the idea that the template specialization would 
> work
> outside the class but not in it - as you were.  The links didn't seem on 
> point on
> this subject.  I took my original solution and moved the specializations 
> outside
> the class.  This failed because the I wasn't defined so I added it do the 
> template
> arguments.  Then a few syntax corrections and it started to work - OK,
> problem solved - but now I was left with this really annoying feature whose
> behavior changed when the specialization was moved out side the class.  So
> I moved it back in - and voila - worked as expected.  Here is my solution:
> 
> 
> template<typename I>
> struct ForwardIterator {
>     template<typename I, typename B>
>     struct is_end {};
> 
>     template<typename I2>
>     struct is_end<I2, false_> {
>         typedef typename deref<I2>::type t1;
>         typedef typename next<I2>::type t2;
>         BOOST_CONCEPT_ASSERT(( ForwardIterator<t2> ));
>     };
> 
>     typedef typename is_end<
>         I,
>         typename boost::is_same<
>             boost::mpl::l_iter<boost::mpl::l_end>,
>             I
>         >
>     > type;
>     typedef typename boost::mpl::print<
>         I
>     >::type t1;
> 
>     BOOST_MPL_ASSERT((
>         is_convertible<
>             typename I::category,
>             forward_iterator_tag
>         >
>     ));
> };
> 
> Basically, I just had to avoid depending on the "outter" template parameter. 
> This had bothered
> me before but I wasn't aware of the right syntax to avoid it.   I had to use
> the specialization syntax above rather than the template<> syntax I was 
> familiar with.  So
> I guess that resolves the problem.
> 
> Again, thanks for everyone who contributed - really would never have found 
> it by myself.
> 
> Robert Ramey
Hi Robert,

I couldn't get this code to compile with gcc-4.7-20111008.  Before any
revision, got:

ramey_iter_concept_check.cpp:22:14: error: declaration of 'class I'
ramey_iter_concept_check.cpp:20:10: error:  shadows template parm 'class I'
ramey_iter_concept_check.cpp:32:22: error: expected nested-name-specifier
ramey_iter_concept_check.cpp:38:7: error: invalid declarator before 'type'

After eliminating the showing I, as shown in the attached, I got:

/home/evansl/download/gcc/4.7-20111008/install/bin/g++ -c -Wall
-Wstrict-overflow -ftemplate-depth-300 -O0 -g3 -fno-inline -std=gnu++0x
-I/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox/ro/numeric_bindings
-I/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox/rw/non_variadic_templates
-I/home/evansl/prog_dev/boost-svn/ro/trunk   -DTEMPLATE_DEPTH=300
ramey_iter_concept_check.cpp -MMD -o
build/gcc4_7n/boost-svn/ro/trunk/sandbox/rw/variadic_templates/libs/mpl/example/ramey_iter_concept_check.o

ramey_iter_concept_check.cpp:31:22: error: expected nested-name-specifier
ramey_iter_concept_check.cpp:37:7: error: invalid declarator before 'type'
make: ***
[build/gcc4_7n/boost-svn/ro/trunk/sandbox/rw/variadic_templates/libs/mpl/example/ramey_iter_concept_check.o]
Error 1

I guess you're using MS or an earlier gcc.

-regards,
Larry