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From: oporat@yahoo.com (Ofer Porat)
Newsgroups: comp.std.c++
Subject: Defect Report: new sequencing rules
Date: Sat,  2 Jun 2007 23:42:12 GMT
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In "1.9.16 [intro.execution]" of N2284, the following
expression
is still listed as an example of undefined behavior:

i = ++i + 1;

However, it appears that the new sequencing rules make
this
expression well-defined:
a) The assignment side-effect is required to be
sequenced after
the value computations of both its LHS & RHS (5.17.1.)
b) The LHS (i) is an lvalue, so its value computation
involves
computing the address of i.
c) In order to value-compute the RHS (++i + 1), it is
necessary
to first value-compute the lvalue expression "++i" and
then
do an lvalue-to-rvalue conversion on the result.  This
guarantees
that the incrementation side-effect is sequenced
before the
computation of the addition operation, which in turn
is sequenced
before the assignment side effect.  In other words, it
yields
a well-defined order and final value for this
expression.

It should be noted that a similar expression

i = i++ + 1;

is still not well-defined, since the incrementation
side-effect
remains unsequenced with respect to the assignment
side-effect.

It's unclear whether making the expression in the
example well-defined
was intentional or just a coincidental byproduct of
the new
sequencing rules.  In either case either the example
should be fixed,
or the rules should be changed.

Ofer Porat
oporat@yahoo.com



 
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