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From: glenlow@pixelglow.com (Glen Low)
Newsgroups: comp.std.c++,comp.lang.c++.moderated
Subject: Unions up in arms!
Date: Fri, 10 Sep 2004 15:13:17 +0000 (UTC)
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Color me stupid, but after months of staring I still can't grok this
clause in the Standard 3.10/15:

----

If a program attempts to access the stored value of an object through
an lvalue of other than one of the following types the behavior is
undefined:

an aggregate or union type that includes one of the aforementioned
types among its members (including, recursively, a member of a
subaggregate or contained union),

----

Suppose

int j;
union u { int i; float f; };

Then which of the following is illegal, if any, and why? (Putting
aside, if possible, the legality of the popular type-punning practice
of putting x into a union and getting back y.)

A.

j = 12;
int k = ((u&) j).i;

B.

j = 12;
u v = (u&) j;

C.

u v = {12};
float g = v.f;

D.

j = 12;
float g = ((u&) j).f;

And finally, what does "including, recursively, a member of a
subaggregate or contained union" mean?

Cheers,
Glen Low, Pixelglow Software
www.pixelglow.com


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