No abstract

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>

Center-to-limb variations indicate that a type III burst is likely to be emitted in a radial direction while a type IIIb emission is more perpendicular to the stream.Present theories of type III bursts by stream-plasma interaction do not emphasize the center to limb variations of the characteristics of these bursts. Actually the occurrence probability of type III bursts does not reveal any strong center to limb effect, and these variations are assumed to be small. This is certainly true for the type II! bursts associated with -flares. But we shall show below that, on low frequencies, in particular at decameter wavelengths where the type III bursts are mostly observed in storms, several characteristics of these storms change with the longitude of the source. One of these effects has been found by Fainberg and Stone (1970) who showed that the average frequency drift rate of storm type III bursts is larger when the source is near the center of the disc than when it is close to the limb, but this effect can be explained by purely geometrical considerations. On the contrary, the effects that we shall describe refer to the physics of the radio emission by stream-plasma interaction.We shall first recall that: (a) the decameter storms are observed at the same time as type I storms at higher frequencies.(b) they are formed by a great variety of fast bursts, with a majority of type III and type IIIb bursts.(c) inside the decameter range, the type III bursts observed during a storm do not differ significantly from the flare associated type III bursts, except in their average intensity. Then the theory of the two emissions is likely the same, the difference being only in the origin of the streams of electrons.(d) the type IIIb bursts (de la NoE and Boischot, 1972) drift in frequency at a rate similar to that of type III bursts, and they are certainly due to a streamplasma interaction, too.(e) it is not yet clear whether type IIIb and type III bursts are emitted by the same stream of electrons or by two distinct streams of similar electrons.Then any difference in the directivity of type IIIb and type III bursts will give information on the different mechanisms by which a stream-plasma interaction can give an emission of radio waves.