Arnd Poetzsch-Heffter

            Dynamic Components as Semantic Entities:
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                  Concept and Static Support
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            Extended Abstract of Work in Progress

The notion of a software component is widely used. In most definitions, 
a component is considered as a static entity (e.g. Booch: ``A reusable 
software component is a logically cohesive, loosely coupled module that 
denotes a single abstraction''; Nierstrasz, Tsichritzis: ``A software 
component is a static abstraction with plugs''). In this talk, we will
look at components from a dynamic point of view. The idea is to 
conceptually structure the storage and code of a running program into 
a hierarchically organized set of encapsulated components with well-defined 
interfaces. Based on this concept, we investigate typing techniques and
syntactic constructs for access restriction to statically enforce the 
component structure.

The talk is organized into two parts. In the first part, we illustrate 
the concept of dynamic components by examples within an object-oriented 
setting. Then, we introduce the invariant expressing the component
hierarchy and describe what is necessary to maintain the invariant. 
We discuss the benefits of a component structure for program specification, 
verification, and reuse. The second part of the talk investigates 
language mechanisms that help to make the component structure explicit
and to enforce the structure invariant. In particular, we present
a typing mechanism for distinguishing objects inside and outside a
component.