Composition of before/after metaclasses

When there are two before/after metaclasses -"Barking" (as before) and "Fierce", which has a sommBeforeMethod and sommAfterMethod that both growl (that is, both methods make a "grrrr" sound when executed). The preceding discussion demonstrated how to create a "FierceDog " or a "BarkingDog ", but has not yet addressed the question of how to compose these properties of fierce and barking. Composability means having the ability to easily create either a "FierceBarkingDog" that goes "grrr woof woof grrr " when it responds to a method call or a "BarkingFierceDog " that goes "woof grrr grrr woof" when it responds to a method call.
Example for composition of before or after metaclasses.

There are several ways to express such compositions. Figure 1 depicts SOM IDL fragments for three techniques in which composition can be indicated by a programmer. These are denoted as Technique 1, Technique 2, and Technique 3, each of which creates a "FierceBarkingDog" class, named "FB-1", "FB-2", and "FB-3", respectively, as follows:

• In Technique 1, a new metaclass ("FierceBarking") is created with both the "Fierce" and "Barking" metaclasses as parents. An instance of this new metaclass (that is, "FB-1") is a "FierceBarkingDog" (assuming "Dog " is a parent).

• In Technique 2, a new class is created which has parents that are instances of "Fierce" and "Barking" respectively. That is, "FB-2" is a "FierceBarkingDog" also (assuming "FierceDog" and "BarkingDog" do not further specialize "Dog").

• In Technique 3, "FB-3", which also is a "FierceBarkingDog", is created by declaring that its parent is a "BarkingDog" and that its explicit (syntactically declared) metaclass is "Fierce".

┌─────────────────────────┬─────────────────────────┬─────────────────────────┐│TECHNIQUE 1              │TECHNIQUE 2              │TECHNIQUE 3              │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│interface                │interface                │interface                │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│FB-1:Dog                 │FB-2:FierceDog,          │FB-3:BarkingDog          │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│{                        │BarkingDog               │{                        │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│...                      │{                        │...                      │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│implementation           │...                      │implemetation            │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│{                        │Implementation           │{                        │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│metaclass=FierceBarking; │{                        │metaclass=Fierce;        │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│...                      │   ...                   │   ...                   │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│ };                      │ };                      │ };                      │
├─────────────────────────┼─────────────────────────┼─────────────────────────┤
│};                       │};                       │};                       │
└─────────────────────────┴─────────────────────────┴─────────────────────────┘

Figure 1.Three techniques for composing before/after metaclasses.

Note that the explicit metaclass in the SOM IDL of "FB-1" is its derived class, "FierceBarking". The derived metaclass of "FB-2" is also "FierceBarking". Lastly, the derived metaclass of "FB-3" is not the metaclass explicitly specified in the SOM IDL; rather, it too is "FierceBarking."