Xref: utzoo comp.lang.c:37711 comp.std.c:4553
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.lang.c,comp.std.c
Subject: Re: increment casted void pointer  --  ANSI?
Message-ID: <15619@smoke.brl.mil>
Date: 28 Mar 91 22:43:40 GMT
References: <288@nazgul.UUCP> <15589@smoke.brl.mil> <3492@inews.intel.com>
Followup-To: comp.lang.c
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 24

In article <3492@inews.intel.com> bhoughto@pima.intel.com (Blair P. Houghton) writes:
-In article <15589@smoke.brl.mil> gwyn@smoke.brl.mil (Doug Gwyn) writes:
->In article <288@nazgul.UUCP> bright@nazgul.UUCP (Walter Bright) writes:
->-In article <1991Mar8.165300.11692@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
->-/There is no way to say "pretend this variable has a different type" in C.
->-The usual to do a 'type paint', which is to change the type of an object
->A strictly conforming method is to use a union type.
-Where in the std is it prohibited to fake unions by using a struct
-and externally prohibiting attempts to dereference a value
-that was superseded by one of a different type?

There are constraints on what is strictly conforming use of pointed-to
objects, etc.

-It seems to say only that a union has to be at least large enough
-to hold it's member with the most bits, and to align members
-suitably, but not that it has to overlap types.

Sure it "overlaps", or acts as if it did.  A conforming implementation
that, e.g. stores floating-point data in a different address space from
integer data may have to jump through hoops if it wishes to make the
aliasing of the representations actually work.  However, such a program
would have to be successfully translated, it just may not work right (in
which case the fake pointer business would probably malfunction too).