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From: glenlow@pixelglow.com (Glen Low)
Newsgroups: comp.std.c++,comp.lang.c++.moderated
Subject: Re: Unions up in arms!
Date: Mon, 13 Sep 2004 15:37:26 +0000 (UTC)
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 > Aren't you making a fairly big assumption?
 > What if sizeof(int) != sizeof(float)?

Let's assume that it is so.

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

Well, interpretation A was that:

an object of type int can be accessed by a lvalue of type union { int
i; float f }; and also an lvalue of type union { union { int i; float
f}; float g; };

But that doesn't make sense, since if the lvalue is constrained to be
of struct type instead, you could possibly access an undefined value
e.g. the float value.

Interpretation B is that

an object of type int can be accessed by a lvalue of type union { int
i; float f; }.i or union {int i; float f; }.f etc.

This hinges on the phrase "through an lvalue", since the actual lvalue
is no longer of union (or struct) type. I see a lot of gcc users
advocating this, e.g. Mark Mitchell of Codesourcery who contributed
the type-based alias analysis code to gcc.

Interpretation C is that

an object of type union { int i; float f; }; or union { union { int i;
float f; } float g; }; can be accessed by a lvalue of type int or
float

This seems the safest, but relies on twisting the order of the
language in the Standard.

So which is it?

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


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