Re: [Boost-users] [fusion]push_back<list<>, int> > fails compilation

Subject: Re: [Boost-users] [fusion]push_back<list<>, int> > fails compilation
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-07-16 10:58:42

Larry Evans schrieb:
> The code in the 1st attachment fails to compile as shown in
> 2nd attachment.  If that's intended, the what's the justification
> for not allowing appending to an empty sequence? If that's
> not intended, then should I file a bug report?
> 
> Based on:
> 
> <fusion.doc>/html/fusion/algorithm/transformation/functions/push_back.html
> 
> it seems push_back should work on a 'Forward Sequence' and
> based on:
> 
> <fusion.doc>/html/fusion/container/list.html
> 
> list is a 'Forward Sequence'.  And based on the following quote from
> the Synopsis of the list.html:
> 
>> The variadic class interface accepts 0  to FUSION_MAX_LIST_SIZE  elements
> 
> an empty list type, i.e. list<>, is a list, the attached code should
> compile.

The problem boils down to fusion::push_back having a const-qualified
sequence argument, whereas fusion::result_of::push_back passes its
sequence argument right through to the fusion::joint_view .
The documentation is misleading in this regard as the declaration of
fusion::push_back in algorithm.qbk does not pass the const to
fusion::result_of::push_back either. The actual code does so of course.

typedef result_of::push_back<list_t0,int>::type
denotes to
fusion::joint_view<list_t0, fusion::single_view<int> const>
whereas
decltype(fusion::push_back(list_v0,1))
denotes to
fusion::joint_view<list_t0 const, fusion::single_view<int> const>

See

https://svn.boost.org/trac/boost/ticket/3358

for more information.

To fix your code, const-qualify the sequence argument of the
fusion::result_of::push_back instantiation.

BTW. the upcoming C++11 port of fusion fixes this by overloading all
functions with both, const- and non-const qualified arguments.

-Christopher