On Tue, Jul 8, 2008 at 5:07 AM, Joel de Guzman <joel@boost-consulting.com> wrote:

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).

Thanks for your help. It works now. Is there any document that explains such things? I'm learning fusion and I'm really missing such a document that would go beyond the basic examples that appear in the documentation and would explain the connection between all the concepts used in fusion.

Best regards Istvan Buki