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From: europa!ark@RESEARCH.ATT.COM
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Message-ID: <8903210206.AA20438@prep.ai.mit.edu>
Date: 21 Mar 89 02:23:09 GMT
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One difficulty has not yet been mentioned in making sizeof
respect virtual functions.  Suppose D is derived from B and
bp is a B*.  Then bp can `really' be pointing either to a
B or to a D.  One might think that sizeof(*bp) should be
able to tell which.

However, the C language definition says that if the argument
of sizeof() is an expression, that expression is not evaluated!
It is obviously impossible to tell whether bp points at a B or
a D without evaluating it.

You might think it doesn't really matter -- after all, how can
it hurt to evaluate an expression like *bp to figure out the value
of sizeof(*bp)?  But what about sizeof(f()), where f is a function
returning a B*?  You can't call f, or it wouldn't be C -- and
it would be very sleazy to say that sizeof evaluates its argument
if virtual functions are involved but not otherwise.

No, it seems that this kind of facility must be named something
else.