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From: rb@cc.ic.ac.uk (Robin Becker)
Newsgroups: comp.std.c++
Subject: Re: case()
Message-ID: <1990Aug23.182857.107@cc.ic.ac.uk>
Date: 23 Aug 90 18:28:57 GMT
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Reply-To: rb@cc.ic.ac.uk (Robin Becker)
Organization: Imperial College Computer Center, London, UK
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There seems to be a fundamental reason why case/switch statements
with variable/expression tags are to be avoided and that is the
impossibility of guaranteeing that a unique tag will be selected.
This means that either some ordering of tag interrogation has to
be imposed which will kill optimisations (unless all tags are constant)
or we abandon determinism and let the compiler decide.  Of course
this construct has been discussed before by Hoare,  I seem to
remember something about programs shouldn't depend on the order
of guard evaluation.  Even worse in parallel processing two guards
may get evaluated to true so parallel languages like Occam must
do something about this, but I don't know what.