Re: [Boost-users] [fusion] making an inner struct boost fusion compatible?

Subject: Re: [Boost-users] [fusion] making an inner struct boost fusion compatible?
From: Oswin Krause (Oswin.Krause_at_[hidden])
Date: 2012-09-21 05:16:32

Hi,

Again thanks a lot! I allready got my first small success after 
scanning the file. I figured that what i needed was
BOOST_FUSION_DEFINE_STRUCT_INLINE_MEMBERS

Now i can write:

struct MyType{
struct type  : boost::fusion::sequence_facade<
        type,
        boost::fusion::random_access_traversal_tag
> {
         #define attributes (vector<T>,a)(vector<U>,b)(vector<V>,c)
         BOOST_FUSION_DEFINE_STRUCT_INLINE_MEMBERS(type, attributes)
        //add own methods here...

         //however, does this work?
         struct reference{
             #define refAttributes (T&,a)(U&,b)(V&,c)
             BOOST_FUSION_DEFINE_STRUCT_INLINE_MEMBERS(reference, 
refAttributes)
         };
};
};

I am not sure whether i can put reference types into the structure. 
because in the macro there is a ctor created as:

#define BOOST_FUSION_MAKE_COPY_CONSTRUCTOR(NAME, ATTRIBUTES_SEQ)        
        \
     NAME(BOOST_PP_SEQ_FOR_EACH_I(                                       
        \
             BOOST_FUSION_MAKE_CONST_REF_PARAM,                          
        \
             ~,                                                          
        \
             ATTRIBUTES_SEQ))                                            
        \
...                                                                     
     \

#define BOOST_FUSION_MAKE_CONST_REF_PARAM(R, DATA, N, ATTRIBUTE)        
        \
     BOOST_PP_COMMA_IF(N)                                                
        \
     BOOST_PP_TUPLE_ELEM(2, 0, ATTRIBUTE) const&                         
        \
     BOOST_PP_TUPLE_ELEM(2, 1, ATTRIBUTE)

so there is an assumption that ATTRIBUTE is not a reference type.

Is there a reason why it is implemented that way? If not, i would like 
to change the argument type to something like(not compiled):
(with ELEM = BOOST_PP_TUPLE_ELEM(2, 0, ATTRIBUTE))
typename mpl::if_<is_reference<ELEM>, ELEM, typename add_const<typename 
remove_reference<ELEM>::type>::type& >::type

(if ELEM is a reference use ELEM as argument type, else try to get a 
const reference in a safe way)

Anyways, Keep up the good work! Fusion is great :)

Gretings,
Oswin Krause

On 2012-09-21 09:52, Nathan Ridge wrote:
>> thanks for the reply.
>> Is it possible to just copy and rename the macro, or does it depend 
>> on
>> newer features of fusion? I need to be compatible with boost 1.45 so 
>> i
>> can't use it directly (also i need to add a few more functions to 
>> the
>> structure later on)
>
> BOOST_FUSION_DEFINE_STRUCT_INLINE is implemented using the 
> sequence_facade
> and iterator_facade extension mechanisms, which were present in Boost 
> 1.45
> [1] [2]. So, it should be straightforward to back-port the macro to 
> 1.45.
>
> Regards,
> Nate
>
> [1]
> 
> http://www.boost.org/doc/libs/1_45_0/libs/fusion/doc/html/fusion/extension/sequence_facade.html
> [2]
> 
> http://www.boost.org/doc/libs/1_45_0/libs/fusion/doc/html/fusion/extension/iterator_facade.html
>
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