The notions of class, subclass and virtual procedure are fairly well understood and recognized as some of the key concepts in object-oriented programming. The possibility of modifying a virtual procedure in a subclass is a powerful technique for specializing the general properties of the superclass.In most object-oriented languages, the attributes of an object may be references to objects and (virtual) procedures. In Simula and BETA it is also possible to have class attributes.The power of class attributes has not yet been widely recognized. In BETA a class may also have virtual class attributes. This makes it possible to defer part of the specification of a class attribute to a subclass. In this sense virtual classes are analogous to virtual procedures. Virtual classes are mainly interesting within strongly typed languages where they provide a mechanism for defining general parameterized classes such as set, vector and list. In this sense they provide an alternative to generics. Although the notion of virtual class originates from BETA, it is presented as a general language mechanism.

The notions of class, subclass and virtual procedure are fairly well understood and recognized as some of the key concepts in object-oriented programming. The possibility of modifying a virtual procedure in a subclass is a powerful technique for specializing the general properties of the superclass.In most object-oriented languages, the attributes of an object may be references to objects and (virtual) procedures. In Simula and BETA it is also possible to have class attributes.The power of class attributes has not yet been widely recognized. In BETA a class may also have virtual class attributes. This makes it possible to defer part of the specification of a class attribute to a subclass. In this sense virtual classes are analogous to virtual procedures. Virtual classes are mainly interesting within strongly typed languages where they provide a mechanism for defining general parameterized classes such as set, vector and list. In this sense they provide an alternative to generics. Although the notion of virtual class originates from BETA, it is presented as a general language mechanism.

<p>The BETA programming language is a modern language in the SIMULA 67 tradition. It supports the object-oriented perspective on programming and contains comprehensive facilities for procedural and functional programming. BETA replaces classes, procedures, functions and types by a single abstraction mechanism called the <em>pattern</em>. Patterns may be organized in a classification hierarchy by means of <em>sub-patterns</em>. The notion of virtual procedure is generalized to virtual pattern. Virtual patterns combined with sub-patterns make it possible to delay the specification of an attribute in a pattern. Attributes may then have different bindings in different sub-patterns. BETA also provides a unified framework for sequential, coroutine and concurrent execution.</p><p>This paper is a tutorial introduction to BETA.</p>

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The notations of class, subclass and virtual procedure are fairly well understood and recognized as some of the key concepts in object-oriented programming. The possibility of modifying a virtual procedure in a subclass is a powerful technique for specializing the general properties of the superclass.<p>In most object-oriented languages, the attributes of an object may be references to objects and (virtual) procedures. In Simula and BETA it is also possible to have class attributes. The power of class attributes has not yet been widely recognized. In BETA a class may also have <em>virtual class attributes</em>. This makes it possible to defer part of the specification of a class attribute to a subclass. In this sense virtual classes are analogous to virtual procedures. Virtual classes are mainly interesting within strongly typed languages where they provide a mechanism for defining general parameterized classes such as set, vector and list. In this sense they provide an alternative to generics. Although the notion of virtual class originates from BETA, it is presented as a general language mechanism.</p>