Re: [Boost-users] [fusion] join result

From: Istvan Buki (buki.istvan_at_[hidden])
Date: 2008-07-08 09:02:07

On Tue, Jul 8, 2008 at 5:07 AM, Joel de Guzman <joel_at_[hidden]>
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