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

Subject: Re: [Boost-users] [fusion] "modify" value in map
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2009-09-27 06:00:02

Jean-Louis Leroy schrieb:
> 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

Yeah, that's definitely a bug of fusion. Please file a bug report.
For now you can use my fusion port, it has this bug fixed ;)

-Christopher