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From: ron@spamcop.net (Ron Natalie)
Newsgroups: comp.std.c++
Subject: Re: Is this really unspecified behavior?
Date: Thu, 24 Nov 2005 19:03:49 GMT
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pongba@gmail.com wrote:
> Hyman Rosen wrote:

> yeah, that's where my doubt lies exactly. The standard says that "i =3D
> v[i++]" has unspecified behavior, but according to what you have said,
> it should just be undefined behavior.
> Is there anything wrong with the corresponding standard wording?
>=20
The order of evaluation is unspecified, but changing the value twice
between sequence points is undefined behavior.   With built-in
operators there are no sequence points other than the end of the
full expression.


Fourth paragraph of Chapter 5 -- Expressions from the standard.

Except where noted, the order of evaluation of operands of individual=20
operators and subexpressions of individual
expressions, and the order in which side effects take place, is=20
unspecified.53) Between the previous
and next sequence point a scalar object shall have its stored value=20
modified at most once by the evaluation
of an expression. Furthermore, the prior value shall be accessed only to=20
determine the value to be stored.
The requirements of this paragraph shall be met for each allowable=20
ordering of the subexpressions of a full
expression; otherwise the behavior is undefined. [Example:
i =3D v[i++]; // the behavior is unspecified
i =3D 7, i++, i++; // i becomes 9
i =3D ++i + 1; // the behavior is unspecified
i =3D i + 1; // the value of i is incremented
=97end example]

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