Re: [Boost-users] Please help with metaprogramming question : termination of recursion

Subject: Re: [Boost-users] Please help with metaprogramming question : termination of recursion
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2010-07-08 20:40:37

> I'm using Fusion to create a list of Range objects.  Here is a simple
> function:
> 
> 		using namespace boost::fusion;
> 
> 		template <typename STLCollection, typename FusB, typename
> FusE>
> 		void process_append (STLCollection& collection, const FusB&
> begin, const FusE& end)
> 		   {
> 		   std::cout << "Yip! ";
> 		   typename result_of::deref<FusB>::type r= deref(begin);
> /*23*/	   collection.insert (collection.end(), boost::begin(r),
> boost::end(r));
> 		   typename result_of::next<FusB>::type  successor=
> next(begin);
> /*25*/	   if (successor != end)  process_append (collection,
> successor, end);
> 		   std::cout << "Yap! ";
> 		   }
> 
> I expect it to print Yip! n times and then Yap! n times.  What I get is a
> compiler error concerning the boost::begin(r) or boost::end(r), that I'm
> calling it on a fusion::void_ rather than a Range.  As you see, I have
> moved the end-of-recusion test from its normal position at the top of the
> function (test when called, and return immediately rather than doing the
> normal body) to the point of the recursive call (don't call with the end
> sentinel).  However, this appears to still trigger the function expansion,
> since the trail of errors seems to go from the point marked 23 as the
> original (inner-most) error, to several stops at point 25, to the caller,
> and so on back out.  So it expands several times and then errors, so I
> think it is objecting to the "end" case.
> 
> Now if simply mentioning the function, not calling it, is enough to make
> it expand the function, then more levels of indirection isn't going to
> help.  What is the proper way to do this?  Is there, somewhere, a proper
> example of using Fusion to iterate from begin to end of a Fusion
> collection?  Presumably this must be done using recursion because the type
> of "next(I)" is not necessarily the same as the type of "I".
> 
> If some other kind of overloading or partial specialization is the trick,
> I would expect that the "end" type, or the dereference of the "end" type,
> would be documented as something I could easily use for that.

You need to decide what function to call based on the _type_ of the fusion
iterators:

template <typename STLCollection, typename FusB, typename FusE>
void process_append (STLCollection& collection, const FusB& begin, const
FusE& end,
    mpl::true_)
{
    // do nothing
}

template <typename STLCollection, typename FusB, typename FusE>
void process_append (STLCollection& collection, const FusB& begin, const
FusE& end,
    mpl::false_)
   {
   std::cout << "Yip! ";
   typename result_of::deref<FusB>::type r= deref(begin);
   collection.insert (collection.end(), boost::begin(r), boost::end(r));
   typename result_of::next<FusB>::type  successor= next(begin);
   process_append (collection, successor, end, 
       is_same< typename result_of::next<FusB>::type, FusE>());
   std::cout << "Yap! ";
   }

template <typename STLCollection, typename FusB, typename FusE>
void process_append (STLCollection& collection, const FusB& begin, const
FusE& end)
{
    process_append(collection, begin, end, is_same<FusB, FusE>());
}

Regards Hartmut

---------------
Meet me at BoostCon
www.boostcon.com

> 
> Please help.
> Thanks in advance,
> 
> --John
> 
> 
> 2>D:\boost_1_43_0\boost/range/iterator.hpp(63) : error C2039: 'type' : is
> not a member of 'boost::mpl::eval_if_c<C,F1,F2>'
> 2>        with
> 2>        [
> 2>            C=true,
> 2>            F1=boost::range_const_iterator<boost::fusion::void_>,
> 2>            F2=boost::range_mutable_iterator<const boost::fusion::void_>
> 2>        ]
> 2>        blahblahblah.h(23) : see reference to class template
> instantiation 'boost::range_iterator<C>' being compiled
> 2>        with
> 2>        [
> 2>            C=const boost::fusion::void_
> 2>        ]
> 2>        blahblahblah.h(25) : see reference to function template
> instantiation 'void
> Aa::catranges_help::process_append<STLCollection,boost::fusion::cons_itera
> tor<const boost::fusion::nil>,FusE>(STLCollection &,const FusB &,const
> FusE &)' being compiled
> 2>        with
> 2>        [
> 2>            STLCollection=std::vector<RTPRICEREC>,
> 2>            FusE=boost::fusion::cons_iterator<const boost::fusion::nil>,
> 2>            FusB=boost::fusion::cons_iterator<const boost::fusion::nil>
> 2>        ]
> 2>        blahblahblah.h(25) : see reference to function template
> instantiation 'void
> Aa::catranges_help::process_append<STLCollection,boost::fusion::cons_itera
> tor<Cons>,FusE>(STLCollection &,const FusB &,const FusE &)' being compiled
> 2>        with
> 2>        [
> 2>            STLCollection=std::vector<RTPRICEREC>,
> 2>            Cons=const
> boost::fusion::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::
> allocator<RTPRICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocat
> or<RTPRICEREC>>>,boost::fusion::nil>,
> 2>            FusE=boost::fusion::cons_iterator<const boost::fusion::nil>,
> 2>            FusB=boost::fusion::cons_iterator<const
> boost::fusion::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::
> allocator<RTPRICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocat
> or<RTPRICEREC>>>,boost::fusion::nil>>
> 2>        ]
> 2>        blahblahblah.h(25) : see reference to function template
> instantiation 'void
> Aa::catranges_help::process_append<STLCollection,boost::fusion::cons_itera
> tor<Cons>,FusE>(STLCollection &,const FusB &,const FusE &)' being compiled
> 2>        with
> 2>        [
> 2>            STLCollection=std::vector<RTPRICEREC>,
> 2>            Cons=const
> boost::fusion::cons<std::vector<RTPRICEREC>,boost::fusion::cons<std::pair<
> std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC>>,std::_V
> ector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC>>>,boost::fusion
> ::nil>>,
> 
> 
> 2>            FusE=boost::fusion::cons_iterator<const boost::fusion::nil>,
> 2>            FusB=boost::fusion::cons_iterator<const
> boost::fusion::cons<std::vector<RTPRICEREC>,boost::fusion::cons<std::pair<
> std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC>>,std::_V
> ector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC>>>,boost::fusion
> ::nil>>>
> 2>        ]
> 2>        blahblahblah.h(55) : see reference to function template
> instantiation 'void
> Aa::catranges_help::process_append<std::vector<_Ty>,boost::fusion::cons_it
> erator<Cons>,boost::fusion::cons_iterator<const
> boost::fusion::nil>>(STLCollection &,const FusB &,const FusE &)' being
> compiled
> 2>        with
> 2>        [
> 2>            _Ty=RTPRICEREC,
> 2>            Cons=const
> boost::fusion::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::
> allocator<RTPRICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocat
> or<RTPRICEREC>>>,boost::fusion::cons<std::vector<RTPRICEREC>,boost::fusion
> ::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTP
> RICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC
> >>>,boost::fusion::nil>>>,
> 2>            STLCollection=std::vector<RTPRICEREC>,
> 2>            FusB=boost::fusion::cons_iterator<const
> boost::fusion::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::
> allocator<RTPRICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocat
> or<RTPRICEREC>>>,boost::fusion::cons<std::vector<RTPRICEREC>,boost::fusion
> ::cons<std::pair<std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTP
> RICEREC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC
> >>>,boost::fusion::nil>>>>,
> 2>            FusE=boost::fusion::cons_iterator<const boost::fusion::nil>
> 2>        ]
> 2>        blahblahblah.h(53) : while compiling class template member
> function 'Aa::catranges_help::object<R,Tail>::operator
> std::vector<_Ty>(void) const'
> 2>        with
> 2>        [
> 2>
> R=std::pair<std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICER
> EC>>,std::_Vector_const_iterator<RTPRICEREC,std::allocator<RTPRICEREC>>>,
> 2>
> 
> 
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