What's New in Component Pascal?

types are design tools, they denote interfaces of objects. They are the primary means of Component Pascal to model component interfaces. Denoting records as abstract allows to indicate more precisely the use of a record: as an interfacing construct rather than an implementation construct. Nevertheless, an abstract type may have all types of methods (see below), i.e., it is not forced to be fully abstract. LIMITED types are a special case of final types. They are special in that they can be instantiated only within the defining module, and they cannot be copied. For example, a client may not perform a NEW on variables of limited types. Since allocation is under complete control of the defining module, the programmer of this module can guarantee that all newly allocated variables are correctly initialized before they are made accessible to client modules. This means that clients can only see variables that respect the type's invariants (which are established during initialization). An implementor is free to change the type's internal representation with less risk of breaking client code; there is no need for lazy initialization schemes; there cannot be delayed run-time errors due to missing initializations; and invariants (e.g., invariants over hidden record fields) cannot be violated through copying. Typically, factory functions or factory objects are provided to create new instances of dynamic LIMITED types. Example: Semaphore = POINTER TO LIMITED RECORD END; PROCEDURE New (level: INTEGER): Semaphore; In the BlackBox Component Framework, most abstractions are represented as abstract types, which are implemented by (non-exported) final types. This is another approach that allows to guarantee correct initialization, but it is too inconvenient for simple non-extensible abstractions. Moreover, LIMITED types cannot be substituted by client-side extensions. This is important, because it allows to protect non-extensible services, such as a real-time kernel, from being used with illegal types. Record syntax The record syntax looks as follows: RecordType = [EXTENSIBLE | ABSTRACT | LIMITED] RECORD ["(" QualIdent ")"] FieldList {";" FieldList} END.