Re: [Boost-users] [boost.mpl] Implementing a numeric index access to fields of a composite class created with mpl::inherit_linearly

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-01-10 19:05:00

Khalil Shalish wrote:
> Hello,
> 
> I created the following composite class using mpl::inherit_linearly:
> 
> template <class T>
> struct DataField {
>     T  data;
> }
> 
> typedef mpl::inherit_linearly<
>    mpl::vector< int, int, int>
>    mpl::inherit< DataField<_2>, _1 >
>> Int3Composite;
> 
> Int3Composite  int3comp;
> 
> Since the type of each data field is an int, I cannot use the type of the 
> field to access the data in the field.  
> 
> Q: Is there a way to write a template (call it get_field) to access the data 
> using numeric position in the type sequence.    Something like this:
> 
> int field_0 = get_field<0>(in3comp); //This returns the first int data field.
> int field_1 = get_field<1>(int3comp);
> 
> I am very new to C++ template programming  and any help on this will be very 
> much appreciated.

Is the use of mpl::inherit_linearly merely to create a structure with
data from an MPL type sequence? If yes, you can simply use Fusion.
Example:

     typedef mpl::vector<int, int, int> types;
     fusion::result_of::as_vector<types>::type int3comp;

Then:

     fusion::get<0>(int3comp) = 1;
     fusion::get<1>(int3comp) = 2;
     fusion::get<2>(int3comp) = 3;

(Boost.Fusion has been recently accepted into boost and is now in the
Boost CVS)

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net