Thursday, 16 April 2009

Protel II

In the first post I described the basic Protel language. In the mid 1990s it was extended with object oriented capabilities. This was done in a number of phases and was tied into the development of a project at Nortel called Generic Services Framework (GSF). This was an object oriented reimplementation of 'call processing' on the DMS with wide scope and a huge development team. Nortel even created an 'Object Center' somewhere in Canada with a helpline that confused designers could call to get OO Protel advice. I suspect that today it would be a 'Center of Object Excellence'. I heard that the GSF project was not an unqualified success, but it did drive the evolution of Protel-2 which was used later in other products. In retrospect it seems that GSF and Protel-2 were more motivated by the Objects-with-everything zeitgeist than any particularly compelling benefits.

Protel-2 supports single inheritance with a common root class called $OBJECT. Methods can be explicitly declared to be overridable in base classes, similarly to C++'s virtual keyword. It does not support operator overloading, or overloading method names with different signatures. It supports fully abstract classes.
Like Eiffel, Protel-2 allows parameterless methods which return a value to be called without parentheses. This allows data members to be refactored to be read directly or via an accessor function method without changing the callers. I'm not sure how valuable this is in practice, especially as it does not affect assignment. Perhaps some Eiffel practitioners have experience of finding this useful? Protel-2 methods can be declared to be read-only with respect to the object instances they operate on, in a similar way to specifying const-ness in C++.

The GENERIC keyword in Protel-2 allows class definitions to be parameterised by type. This allows the creation of type-safe generic collection classes and datastructures. The type parameterisation is similar to the Generics mechanism in Java, in that it is effectively a compile-time-only mechanism. The compiler generates a single underlying class implementation and checks type-correctness at compile time. A side-effect is that all access to the parameterised class must be made via a pointer. This fits well with the requirement for online load-replacing implementation etc, but it offers much reduced power compared to C++'s code-generation style templating mechanisms.

As with most C++ implementations, Protel-2 objects contain a vtbl pointer in the first word of their data followed by data members of superclasses and the class instance. On XA-Core, this sometimes presented a problem in that the normal transactional memory ownership mechanism could create a bottleneck when used for the vtbl ptr, but often data in the rest of the class should use the transactional memory mechanism. To deal with this, special libraries were created to allow the object header to be stored in WRITEBLOCKING memory and the rest of the object to be stored in BLOCKING memory.

That's scraping the bottom of the barrel on Protel-2 information in my head. I thought there was more there (or maybe just something more interesting). At least I've written it down.

1 comment:

vlad said...

It was interesting to see your posts about Protel.