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From: kevin.cline@gmail.com (Kevin Cline)
Newsgroups: comp.lang.ada
Subject: Re: Ada Popularity: Comparison of Ada/Charles with C++ STL (and Perl)
Date: 2 Oct 2004 13:39:56 -0700
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Date: 2004-10-02T13:39:56-07:00
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Brian May <bam@snoopy.apana.org.au> wrote in message news:<sa4wty9a196.fsf@snoopy.apana.org.au>...
> >>>>> "K" == K  <slatyb@yahoo.com> writes:
> 
>     K> dbmap<Order> order_map;
> 
>     K> order_map.insert("ORDER.NUMBER", &Order::number)
>     K> .insert("ORDER.CUSTOMER", &Order::customer_number)
>     K> .insert("ORDER.ACCEPTED", &Order::acceptance_date)
>     K> .insert("ORDER.PROMISED", &Order::promised_ship_date)
>     K> ...
> 
>     K> and then be able to write:
> 
>     K> Order order;
>     K> order_map.read(dbreader, order);
>     K> order.promised_ship_date = ship_date(order.line_items);
>     K> order_map.update(dbconnection, order);
> 
> Hmmmm... Thinking aloud: One approach that might work, at least for
> some applications:
> 
> declare
>         type Index_Type is (Number, Customer_Number, Acceptance_Date,
>                    Promised_Ship_Date, ...);
> begin
>         insert(Order_Map, Number);
>         insert(Order_Map, Customer_Number);
>         insert(Order_Map, Acceptance_Date);
>         insert(Order_Map, Promised_Ship_Date);

Sorry, I didn't make it clear that not all thes data members are
integers.  The dates have Date type, and other members will be strings
or other user-defined types.

>         ...
> end;
> 
> (not tested; syntax may not be exactly correct)
> 
> The obvious problem with this is that all values need be the same
> type. I am not sure if your code has this limitation or not.

No, it doesn't.  That's the point.  The insert function is a template
member function with a declaration like this:

   template <class RecordType>
   class DatabaseMap
   {
     vector<ColumnMap> entries;
     ...
     template <class MemberType>
     DatabaseMap& insert(Column c, MemberType
RecordType::*member_pointer)
     {
        entries.push_back(new ColumnMemberMap<RecordType, MemberType>
                              (c, member_pointer));
        return this;
     }
   }

Matt has suggested ways this can be done in Ada, but it requires to
create a function for every data member to be mapped, and additionally
you will have to instantiate the insert function for each different
member type.  That is so much work that it destroys the value of the
technique.