In the implementation of the Dog, BigDog, and LittleDog classes, a metaclass is not specified in the Metaclass section of the class definition files. This means that the metaclass of each of these classes is the metaclass of their parent class. The metaclass of the BigDog and LittleDog classes is the metaclass of its parent class, Dog. The metaclass of the Dog class is the metaclass of its parent class, SOMObject. The metaclass of the SOMObject class is SOMClass. SOMClass, then, supplies the constructors for the Dog, BigDog, and LittleDog classes, as well as for SOMObject.

If a metaclass is specified in the Metaclass section of the IDL files, a new metaclass is defined for the classes. The new metaclass provides new constructors for the classes. As an example, a new metaclass M_Dog is defined for the Dog class and added to DOG.IDL. The following sample code shows the new DOG.IDL file, including the changes made to the original DOG.IDL file.

/*
 *  Define a metaclass of the Dog class.
 *  By convention, metaclasses are the class name prefixed with M_
 */

#ifndef dogmeta_idl
#define dogmeta_idl

#include <somcls.idl>
#include <somobj.idl>

interface Dog;   /* Declared later in this file */
interface M_Dog : SOMClass
{
  Dog   CreateADog();

  // Create an instance of the Dog class and return a pointer to it
  #ifdef __SOMIDL__
  implementation

  {
    releaseorder:  CreateADog;

    //# Class modifiers

    callstyle = oidl;    // For compatibility with SOM1
    majorversion = 1;
    minorversion = 2;
  };
  #endif   /* __SOMIDL__ */
};

/*
 *   A class defining a set of objects (dogs) with metaclass M_Dog
 */

interface Dog : SOMObject
{
  attribute string breed;

  // The breed for the dog
  void display();

  // Display characteristics for this dog.
  // Show the breed and make the dog speak.
  void bark();

  // Have the dog bark
  #ifdef __SOMIDL__
  implementation

  {
    releaseorder:  _get_breed, _set_breed, display, bark;

    //# Class modifiers

    callstyle = oidl;    // For compatibility with SOM1
    majorversion = 1;
    minorversion = 2;
    metaclass = M_Dog;   // Explicitly specify a metaclass
  };
  #endif   /* __SOMIDL__ */
};
#endif   /* dogmeta.idl */