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Newsgroups: comp.lang.ada
Subject: Re: Ada Popularity: Comparison of Ada/Charles with C++ STL (and Perl)
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From: Matthew Heaney <matthewjheaney@earthlink.net>
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Date: Wed, 29 Sep 2004 22:44:52 GMT
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Date: 2004-09-29T22:44:52+00:00
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Mark Lorenzen <mark.lorenzen@ofir.dk> writes:

> After having thought about this a bit, I would say that an equivalent
> of C++s function classes is missing.
>
> If function classes were introduced, we could build pretty powerful
> algorithms traversing the different containers and f.ex. implement an
> infinite list (also called lazy list) container. Right now the
> containers only allow for a procedure to process the elements of a
> container.

Well, you can do that using the procedure, and depositing a result
somewhere prior to returning.  

For example, in the wordcount example, you need to compute a predicate
function on map cursors:

   Sort_Vector :
   declare
      function "<" (L, R : Wordcount_Maps.Cursor) return Boolean is
         Result : Boolean;

         procedure Query_L (LK : String; LN : Natural) is
            procedure Query_R (RK : String; RN : Natural) is
            begin
               if LN > RN then
                  Result := True;
               elsif LN < RN then
                  Result := False;
               else
                  Result := Ada.Strings.Less_Case_Insensitive (LK, RK);
               end if;
            end Query_R;
         begin
            Query_Element (R, Query_R'Access);
         end;
      begin
         Query_Element (L, Query_L'Access);
         return Result;
      end;

      procedure Sort is
         new Wordcount_Vectors.Generic_Sort;
   begin
      Sort (V);
   end Sort_Vector;

This does everything the a "function class" would do in C++.

The ARG is currently discussing whether to allow functions to return
anonymous access types, which would allow us to write the predicate as:

      function "<" (L, R : Wordcount_Maps.Cursor) return Boolean is
         Result : Boolean;

         LN : Natural renames Query_Element (L).all;
         LN : Natural renames Query_Element (R).all;
      begin
         if LN > RN then
            return True;
         end if;

         if LN < RN then
            return False;
         end if;

        declare
           LK : String renames Query_Key (L).all;
           RK : String renames Query_Key (R).all;
        begin
           return Ada.Strings.Less_Case_Insensitive (LK, RK);
        end;
      end "<";

Please give me a specific, concrete example of something you can do in
C++, that you believe cannot be done using the AI-302 container library.