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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Generic in out object parameters
Date: 1997/04/17
Message-ID: <33561FCC.C38@gsfc.nasa.gov>#1/1
X-Deja-AN: 235461617
References: <Forum.861207636.14373.petera@woodlands>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Reply-To: Stephen.Leake@gsfc.nasa.gov
Newsgroups: comp.lang.ada
Date: 1997-04-17T00:00:00+00:00
List-Id: <comp.lang.ada>


petera@ece.uc.edu wrote:
> 
> A generic unit may have an generic object parameter of mode in out.  I understand that this acts as a renaming of the actual parameter supplied when the unit is instantiated.
> 
> Can someone give me a good motivating example of why this is useful?
> 

Here's a layer I use over Ada.Command_Lines:

--
--  Purpose :
--      Provide generic routines for reading parameters from the command
--      line.
--
--  Design :
--      Each Get_* procedure reads a particular parameter type from
--      Command_Line.Argument(Next_Arg) and then increments Next_Arg by
--      one.  PARAMETER_ERROR is raised for *any* error, after putting
an
--      appropriate message to Standard_Error.
--
with Ada.Strings.Unbounded;
package Command_Line_IO is

   PARAMETER_ERROR : exception;

   generic
      Default_Expecting : in STRING;
      Next_Arg : in out NATURAL; -- index of next argument to read
   procedure Get_String 
	(Item : out Ada.Strings.Unbounded.Unbounded_String;
         Expecting : in String := Default_Expecting);

   generic
      type Discrete_Type is (<>);
      Default_Expecting : in STRING;
      Next_Arg : in out NATURAL; -- index of next argument to read
   procedure Get_Discrete 
	(Item : out Discrete_Type;
         Expecting : in STRING := Default_Expecting);

   generic
      type Float_Type is digits <>;
      Default_Expecting : in STRING;
      Next_Arg : in out NATURAL; -- index of next argument to read
   procedure Get_Float (Item : out Float_Type;
                        Expecting : in STRING := Default_Expecting);

end Command_Line_IO;

Next_Arg is a global for each Get; this makes the final code read
eaiser:


procedure Get_Options is

   Next_Arg : POSITIVE := 1;  -- next argument to process

   procedure Get is new Command_Line_IO.Get_Discrete 
	(Com_Port_ID_Type, "Com port id", Next_Arg);

   procedure Get is new Command_Line_IO.Get_Discrete 
	(Interfaces.C.Unsigned_Long, "address", Next_Arg);

   procedure Get is new Command_Line_IO.Get_String ("String", Next_Arg);

   use type Ada.Strings.Unbounded.Unbounded_String;
begin
   --
   -- Process options.
   --
   loop
      exit when (Ada.Command_Line.Argument_Count < Next_Arg);

      if Ada.Command_Line.Argument (Next_Arg) = "--help" or
         Ada.Command_Line.Argument (Next_Arg) = "-?"
      then
         Put_Usage;
         raise Fatal_Error;

      elsif Ada.Command_Line.Argument (Next_Arg) = "--com_port" or
         Ada.Command_Line.Argument (Next_Arg) = "-p"
      then
         Next_Arg := Next_Arg + 1;
         Get (Com_Port_ID);

      elsif Ada.Command_Line.Argument (Next_Arg) = "--verbose" or
         Ada.Command_Line.Argument (Next_Arg) = "-v"
      then
         Next_Arg := Next_Arg + 1;
         Verbose := True;
      else
         -- should be root file name

         Get (Root_File_Name, "root file name");

      end if;
   end loop;
end Get_Options;


Clearly, I could have passed Next_Arg as a parameter to each procedure
call, rather than to each instantiation. However, it is somewhat clearer
this way that there must be exactly one Next_Arg, shared by all the Get
procedures. Since I did not overload Ada.Command_Line.Argument, I admit
the abstraction is muddy.

I don't think I've used a generic "in out" parameter any where else.
-- 
- Stephe