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5. An Ownership-based MOP for Expressing Concurrency Abstractions
of primitives that need to be covered. However, if the number of primitives
grows too large, this design can become cumbersome.
An alternative solution to list all primitives in the form of prim* is to use a
generic intercession handler similar to the one provided to handle all methods
in a uniform way. Such a handler needs to encode the primitive as a parameter,
for instance like #requestPrim: prim on: obj with: arguments.
Handling of Reified Execution State Since the presented implementations
are based on Smalltalk, they have direct access to runtime state for instance
to stack frames, i. e., context objects that are used for execution (cf. Sec. 4.2.2).
Context objects, and similar objects relevant for execution semantics, need
to be treated carefully. For each of them, it has to be determined whether
the notion of ownership should be provided or whether that would lead to
unexpected consequences.
In the case of some context object it is conceivable that it has an arbitrary
owner and that it is treated like any other object. As long as it is merely
used for introspection, it is feasible to allow arbitrary ownership changes and
the enforcement of arbitrary domain semantics. However, if a context object
is used by the VM for its purpose as a stack frame, neither the notion of
ownership nor the enforcement of arbitrary domain semantics are viable. The
VM needs direct access to them, for instance to perform the necessary stack
operations during execution.
Therefore, the implementations discussed in Chapter 7 regard context objects
as metaobjects that are not subject to the OMOP.
5.6. Related Work
This section contrasts the proposed OMOP with other MOPs that cover closely
related concerns or MOPs that have been used in the field of concurrent and
parallel programming. As in Sec. 5.1, this section relies on the categorization
of Tanter [2009] discussing metaclass-based, metaobject-based, as well
as group-based hybrid approaches.
Metaclass-based Approaches The C++ extensions Open C++ [Chiba and
Masuda, 1993] and PC++ [Stroud and Wu, 1995], which is built on top of
Open C++, both use metaclass-based MOPs. Open C++ is used in the context
of distributed programming and Chiba and Masuda [1993] give examples on
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