Re: [Boost-users] [fusion] "modify" value in map

Subject: Re: [Boost-users] [fusion] "modify" value in map
From: Jean-Louis Leroy (jl_at_[hidden])
Date: 2009-09-27 03:12:22

Jean-Louis Leroy wrote:
> How can I create a map that has the same content as another rmap, except 
> for a given key which has a new associated value ? I'm looking for 
> something like replace_key<Key>(Sequence, Value) in the doc but I cannot 
> find it. I think I can build it with push_back(erase_key<Key>(seq), 
> make_pair<Key>(newval)) but if it already exists...

I'm encountering difficulties while trying to implement this. Consider :

   template<class K>
   struct replace_at_key {
     template<typename Any> bool operator()(Any) const { return false; }
     template<class V> bool operator()(boost::fusion::pair<K, V>) const
     { return true; }
   };

   void test() {
     using namespace boost::fusion;

     typedef map<
       pair<int, std::string>
     , pair<double, std::string> >
     map_type;

     map_type m(
         make_pair<int>("X")
       , make_pair<double>("Men"));

     BOOST_AUTO(v, boost::fusion::as_vector(
       replace_if(m, replace_at_key<int>(),
         make_pair<int, std::string>("Z"))));

     as_map(v); // error : reference to reference
   }

I get compiler errors (msvc9 and g++ 4.4.1) because it's trying to form 
a reference to a reference. Indeed, when I look inside v with the 
debugger I see that the type of v.vec.m1 is
`const pair<double, string>&' (note : reference). And m1 is passed by 
reference in the internals of fusion. Hence the error.

Am I doing something wrong here ?

Thanks,
J-L