Istvan Buki wrote:

Dear boost experts,

I have difficulties using the fusion join algorithm result.
Here is a small sample demonstrating the problem.

namespace fields
{
 struct one ;
 struct two ;
}


typedef fusion::map <
 fusion::pair< fields::one, std::string >,
 fusion::pair< fields::two, std::string >
 > rec_t ;


int
main()
{
 rec_t r1( std::string( "1" ), std::string( "2" ) ) ;
 rec_t r2( std::string( "10" ), std::string( "20" ) ) ;

 typedef fusion::result_of::join< const rec_t, const rec_t >::type res_type ;
 res_type res = fusion::join( r1, r2 ) ;

 // Compile error!
 fusion::at_key< fields::two >( res )  = std::string( "200" ) ;    std::cout << res << std::endl ;
}

I can not modify the map fields because they are const.
I tried changing the result type from

   typedef fusion::result_of::join< const rec_t, const rec_t >::type res_type ;
to
 typedef fusion::result_of::join< rec_t, rec_t >::type res_type ;

but that doesn't work neither.

Can somebody explain how to assign the result of a join to a variable that I can modify later
using functions like at_key, erase_key,... ?

When you join a sequence, A, and another sequence, B, the result is
a joint_sequence -- a view. IOTW, when you join a map and another
map, the result is not a map. What you can do is to convert the
view to a map using as_map(res).