Re: [Boost-users] challange to MPL gurus

On 12/12/11 22:49, Robert Ramey wrote:

Jeremiah Willcock wrote:

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On Mon, 12 Dec 2011, Larry Evans wrote:

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On 12/12/11 17:04, Larry Evans wrote:

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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/c04...

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 struct is_end {};

template<typename I2> struct is_end { 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_iterboost::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