CORBA Programming/Echo/Object

The echo object is a simple object with only one method. This method takes a single string parameter and returns that string unchanged.

CORBA IDL definition

Any CORBA object needs to be defined in CORBA's Interface Definition Language (IDL). The IDL syntax is based on C/C++ including the use of the C preprocessor. Hence we need the usual protection against a double include:

File: test-echo.idl (view, plain text, download page, browse all)

#ifndef  __TEST_ECHO_DEFINED
#define  __TEST_ECHO_DEFINED

Next we define a module name. While not strictly needed for such a small example it is a good practice never to pollute the global namespace with classes and to use modules instead.

 module Test
    {

A class is called an "interface" in CORBA IDL. This should not be mixed up with interfaces in Ada or Java. A CORBA interface is the public part of the class — the part of the class you can interface with.

    interface Echo
       {

As said, we define just one method. It takes a string called Message as input parameter and returns a string as result.

       string
       Echo_String (
          in string Message	
          );
 
       };
    // end interface Echo
 
    };
 // end module Test
 
 #endif

Ada Specification

Before you download the specification below you should try to call the following PolyORB command. It will generate an implementation template for you.

idlac -i test-echo.idl

Most CORBA implementations will generate specification and implementation template files for the chosen implementation language. Which implementation languages you can choose from depends on your ORB. In our case it is Ada.

File: test-echo-impl.ads (view, plain text, download page, browse all)

 with CORBA;
 with PortableServer;

The CORBA module (Test) and class (Echo) is mapped to an Ada package hierarchy — this is the normal way to do object oriented programming in Ada. The CORBA language mapping always tries to map close the implementation language. The Echo hierarchy contains several child packages which are fully generated and need no attention. Only the child package "Impl" contains the actual implementation of our Echo class and needs editing.

 package Test.Echo.Impl
 is

The type Object contains any data our class might want to store. However, all data stored is private, in fact all data in CORBA objects are private.

    type Object
    is new
       PortableServer.Servant_Base
    with private;
 
    type Object_Ptr is access Object'Class;

The Echo function is specified as a normal primitive operation to a normal Ada class. If you are interested to know how Ada classes are declared you can read Ada Programming/Object Orientation.

    function Echo_String (
       --  The Object itself
       Self : access Object;
       --  The string which should be echoed.
       Message : in CORBA.String)
    return
       --  The returned string - which is the same as Message
       CORBA.String;
 
 private

Here, in the private section of the package, we could now declare data members for our class. However our simple echo class needs no data, so we just declare a "null record". If you are interested to know what it would look like with data members you can read Ada Programming/Types/record.

    type Object
    is new
        PortableServer.Servant_Base
    with null record;
 
 end Test.Echo.Impl;

Ada Implementation

Now we know what the CORBA object looks like in Ada we have to actually implement the object. The implementation is a little more extensive than needed — using PolyORB logging facility to produce diagnostic output.

File: test-echo-impl.adb (view, plain text, download page, browse all)

 with PolyORB.Log;
 
 with Test.Echo.Skel;
 
 --
 --  The following packages are only initialized but not used otherwise.
 --
 pragma Warnings (Off, Test.Echo.Skel);
 
 --
 --  Test.Echo.Skel is only initialized but not used.
 --
 pragma Elaborate (Test.Echo.Skel);
 
 package body Test.Echo.Impl
 is
    --
    --  a Test Module to test the basic PolyORB functions
    --
 
 --------------------------------------------------------------------------------
 
    --
    --  Initialize logging from configuration file.
    --
    package Log is new  PolyORB.Log.Facility_Log ("test.echo");
 
 --------------------------------------------------------------------------------
 
    --
    --  Log Message when Level is at least equal to the user-requested
    --  level for Facility.
    --
    procedure Put_Line (
       Message : in Standard.String;
       Level   : in PolyORB.Log.Log_Level := PolyORB.Log.Info)
    renames
       Log.Output;
 
 --------------------------------------------------------------------------------
 
    --
    --  Echo the given string back to the client.
    --
    function Echo_String (
       --  The Object itself
       Self : access Object;
       --  The string which should be echoed.
       Message : in CORBA.String)
    return
       --  The returned string - which is the same as Message
       CORBA.String
    is
       pragma Unreferenced (Self);
    begin
       Put_Line (
          "Echoing string: « " &
          CORBA.To_Standard_String (Message) &
          " »");
       return Message;
    end Echo_String;
 
 --------------------------------------------------------------------------------
 
 end Test.Echo.Impl;