Re: Charles: Generic_Find

[mheaney@on2.com (Matthew Heaney)]

"Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net> wrote in message news:<Hx8A9.7947$6Z.7289@nwrddc01.gnilink.net>...
> "Hyman Rosen" <hyrosen@mail.com> wrote in message
> news:1037108893.573381@master.nyc.kbcfp.com...
> > It also introduces potentially significant optimization
> > opportunities, since for predicates, function call overhead can
> > easily dwarf the amount of computation done by the predicate
> > itself.
> 
> In other words, the possibility of inlining the function is possible with
> the template / generic approach,whereas the pointer-to-function approach
> would force a function call.

Yes, but not just that.  In Ada downward chains must be static. 
Consider my previous employee example, and assume we want to count how
many employees are male:

Employees : Container_Subtype;

function Count (Sex : Sex_Type) return Natural is

    function Predicate (E : Employee_Type) return Boolean is
    begin
       return E.Sex = Sex;
    end;

    function Find is
       new Generic_Find (Predicate);

    Result : Natural := 0;

    Back : constant Iterator_Type := 
       Employees_Types.Back (Employees);

    Iter : Iterator_Type = First (Employees);

begin

   loop

      Iter := Find (Iter, Back);

      if Iter = Back then
         return Result;
      end if;

      Result := Result + 1;

      Iter := Succ (Iter);

   end loop;

end Count;

This implementation (a locally declared subprogram that uses the
parameters of the enclosing subprogram) is very natural in Ada, but
doesn't have a direct translation in C++.