Issue 52:  Omission in union C++ mapping (cxx_revision)
Source:  (, )
Nature:  Uncategorized
Severity: 
Summary: Summary: Several problems exist with the definition of union constructors.

Resolution:  fixed by adding clarifying text
Revised Text: 
Actions taken:
July 11, 1996: Received issue
February 19, 1999: closed issue
Discussion: 

End of Annotations:=====

Subject: Omission in union C++ mapping
To: cxx_revision@omg.org
Date: Thu, 11 Jul 1996 11:46:50 +1000 (EST)
Cc: corbanet@dstc.edu.au, trader@dstc.edu.au
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Hi,

the latest C++ mapping makes it clear that a default-constructed union
is in an undefined state, and may not be accessed by the application
before setting it.

It also states that the copy constructor and assignment operator
make a deep copy, but does not make it clear whether it is legal
to copy a default-constructed union or assign from a
default-constructed
union.

Further, a union with an implicit default member can be put back into
an uninitialized state with the _default() member function. It is not
clear whether it is legal to call the copy constructor or assignment
operator after calling _default().

Finally, it is not clear whether it is safe to destroy a union that is
not initialized, or destroy a union after calling _default().

Suggestion:

	Calling the copy constructor or assignment operator on
	an uninitialized union should be required to be safe.
	The copy will be uninitialized as well.

	Similarly, copy construction and assignment should be safe
	after calling _default(), and the copy should be
uninitialized.

	Finally, destroying a default-constructed union should be
safe,
	as should be destruction of a union after calling _default().


						Cheers,

							Michi.
-- 
Michi Henning              +61 7 33654310
DSTC Pty Ltd               +61 7 33654311 (fax)
University of Qld 4072     michi@dstc.edu.au
AUSTRALIA
http://www.dstc.edu.au/BDU/staff/michi-henning.html

>From geoff  Thu Jul 11 15:46:23 1996
Received: by amethyst.omg.org (5.4R2.01/1.34)
	id AA20909; Thu, 11 Jul 1996 15:46:23 -0400
Date: Thu, 11 Jul 1996 15:46:23 -0400
From: geoff (Geoffrey Speare)
Message-Id: <9607111946.AA20909@amethyst.omg.org>
To: michi@dstc.edu.au
Cc: issues, cxx_revision@omg.org, corbanet@dstc.edu.au,
trader@dstc.edu.au
In-Reply-To: Michi Henning's message of Thu, 11 Jul 1996 11:46:50
+1000 (EST) <199607110146.LAA11927@azure.dstc.edu.au>
Subject: Omission in union C++ mapping [Issue 52]


This issue has been entered into the OMG revision database as issue
#52.

>   the latest C++ mapping makes it clear that a default-constructed
union
>   is in an undefined state, and may not be accessed by the
application
>   before setting it.

>   It also states that the copy constructor and assignment operator
>   make a deep copy, but does not make it clear whether it is legal
>   to copy a default-constructed union or assign from a
>   default-constructed
>   union.

>   Further, a union with an implicit default member can be put back
into
>   an uninitialized state with the _default() member function. It is
not
>   clear whether it is legal to call the copy constructor or
assignment
>   operator after calling _default().

>   Finally, it is not clear whether it is safe to destroy a union
that is
>   not initialized, or destroy a union after calling _default().

>   Suggestion:

>	Calling the copy constructor or assignment operator on
>	an uninitialized union should be required to be safe.
>	The copy will be uninitialized as well.

>	Similarly, copy construction and assignment should be safe
>	after calling _default(), and the copy should be
uninitialized.

>	Finally, destroying a default-constructed union should be
safe,
>	as should be destruction of a union after calling _default().


>						Cheers,

>							Michi.
>   -- 
>   Michi Henning              +61 7 33654310
>   DSTC Pty Ltd               +61 7 33654311 (fax)
>   University of Qld 4072     michi@dstc.edu.au
>   AUSTRALIA
>   http://www.dstc.edu.au/BDU/staff/michi-henning.html


Geoff Speare
OMG
geoff@omg.org