Re: [Boost-users] [mpl][fusion] Metafunction for extracting a type from a map

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-03-19 20:09:14

Christian Henning wrote:
> Hi there, next metaprogramming problem. Is it possible to extract a
> tagged type from a map?
> 
> namespace fields
> {
>    struct _id_;
>    struct _name_;
>    struct _birthday_;
>    struct _address_id_;
> }
> 
> namespace tags
> {
>    struct _primary_key_;
>    struct _foreign_key_;
>    struct _value_;
> }
> 
> typedef fusion::map< fusion::vector< fields::_id_ , std::string,
> tags::_primary_key_ >
>                    , fusion::vector< fields::_name_      ,
> std::string, tags::_value_       >
>                    , fusion::vector< fields::_birthday_  ,
> std::string, tags::_value_       >
>                    , fusion::vector< fields::_address_id_,
> std::string, tags::_foreign_key_ >
>                    > table_type;
> 
> int main()
> {
>  /// how to get the primary key type vector
> }

at_key<fields::_id_>(m)  ???

> I was looking at fusion's filter_view but that is only available for
> non-metaprogramming.

Not sure what you mean.

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