Re: How to avoid unreferenced objects (mutexes etc)

[dmitry@elros.cbb-automation.de (Dmitry A. Kazakov)]

On Thu, 17 Jan 2002 10:55:57 +0000 (UTC), lutz@iks-jena.de (Lutz
Donnerhacke) wrote:

>* Dmitry A. Kazakov wrote:
>>* lutz@iks-jena.de (Lutz Donnerhacke) wrote:
>>>* Dmitry A. Kazakov wrote:
>>>>I believe it is worth to allow that for all types, i.e. to allow
>>>>overriding a non-abstract subroutine with an abstract one. A
>>>>dispatching call to a disallowed subprogram will then raise an
>>>>exception [like when tags of arguments are different].
>>>
>>>I strongly discourage from such a change. Class wide programming using
>>>abstract tagged types are used for the guarantee of a working interface.
>>
>>There is no way to enforce LSP, if you aim at that. Just consider an
>>overriding like:
>>
>>procedure Do_Something (...) is
>>begin
>>   raise Program_Error;
>>end Do_Something;
>
>Then the reason is on client site. Which is easy to detect.

How it differentiates from:

procedure Do_Something (..) is abstract;

The later has an additional advantage that some calls to it can be
detected at compile time.

Consider disallowing methods as an additional constraint. Any
constraint imposed by a derived type is incompatible with LSP. Should
we then dismiss the idiom of constrained subtypes?

>>And who said that derived types shall be LSP subtypes?
>
>The generic preamble requiring the correct instantiation.

You must define the word correct. If correct = absolutely
substitutable then of course LSP is required.

Regards,
Dmitry Kazakov