Re: SDL-News: Pure virtuality in SDL

Subject: Re: SDL-News: Pure virtuality in SDL
From: yves Lejeune ( )
Date: Thu Dec 10 1998 - 09:44:51 GMT

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-----From (yves Lejeune) to sdlnews -----

> Does the language provide a mechanism to declare a virtual object
> (structure type, transition, procedure, etc.) as PURILY virtual ?
> The aim is, just as in C++ for example, to FORCE the users of my
> specification to redefine the object.
> In addition, this would prevent users from instantiating a block-type
> which is not supposed to be instanciated.
> Regards,
> Fabrice Dubois.
You can not declare PURILY virtual objects, but there are several ways to FORCE
the users to redefine an object. It is not elegant, but it works :
- for a process type or a procedure, you can omit the start transition,
  so that is will not be possible to make instances of them
  (this is detected at compilation time). You will probably put a comment
  about what is supposed to be the start state, and an initialisation procedure
  to be called by the start transition of the subtype.
- for a transition you can use the "error" term to make it impossible
  to execute the transition. This is less satisfying as the error will only
  appear at execution time.
- for a block type it is less obvious. Mayby one can omit a signalroute
  or omit a signal in a signalroute, which makes the instanciation
Yves Lejeune
150, rue Nicolas Vauquelin - BP 1310
31106 Toulouse Cedex - France
Tel : +33 5 61 19 29 39
Fax : +33 5 61 40 84 52
e-mail :

-----End text from (yves Lejeune) to sdlnews -----
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