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Re: ANSI-strict pointer aliasing rules

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Martijn van Oosterhout <kleptog(at)svana(dot)org>, mark(at)mark(dot)mielke(dot)cc, Andrew Dunstan <andrew(at)dunslane(dot)net>, Taral <taralx(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: ANSI-strict pointer aliasing rules
Date: 2006-04-27 16:50:10
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Lists: pgsql-hackers
Greg Stark <gsstark(at)mit(dot)edu> writes:
> Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
>> You're right, PostgreSQL uses a form of subclassing, so that a (for
>> example) struct ArrayRefExprState is occasionally referred to using a
>> (struct ExprState*) or even a (struct Node*). In C, the logical way to
>> get that to work it by casting, do you have a better way?

> There are other ways of achieving the same thing. Structs containing a union
> for the subclass fields for example.

Doesn't achieve the same thing, unless you mandate that every part of
the system use the identical massively-overloaded union struct to refer
to every node.  That would (a) defeat the purpose of extensibility, and
(b) make the code more error prone not less so (since it'd be
notationally easy to refer to a field that's not actually present in the
given node subtype).

			regards, tom lane

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