From 6920146996448486389 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,8fdf6b501ff64059 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2002-01-07 07:26:05 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!logbridge.uoregon.edu!dispose.news.demon.net!news.demon.co.uk!demon!mail2news.demon.co.uk!not-for-mail From: "Anthony Williams" Newsgroups: comp.std.c++ Subject: Re: Possible DRs Date: Mon, 7 Jan 2002 15:25:10 GMT Organization: Nortel Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: References: <3C36019C.32739F68@sensor.com> X-Trace: mail2news.demon.co.uk 1010417149 mail2news:23952 mail2news mail2news.demon.co.uk X-Complaints-To: abuse@demon.net X-Mail2News-Path: news.demon.net!mulga.cs.mu.oz.au X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Authentication-Warning: "Anthony Williams" X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 5.50.4807.1700 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4807.1700 Lines: 70 Xref: archiver1.google.com comp.std.c++:8836 "Michiel Salters" wrote in message news:QFqZ7.4501$cD4.8752@www.newsranger.com... > In article <3C36019C.32739F68@sensor.com>, Ron Natalie says... > > > >Michiel Salters wrote: > >> 3) > >> What prohibits int f( volatile void )? What does it mean, if it > >> doesn't cause a program to be ill-formed? > > > >Ill-formed. 8.3.5 permits f(void) as a special case only in that > >exact form. > > Yes, that's indeed the special case. But the reasoning by which > volatile void is ill-formed was not obvious to me. 8.3.5p2 says "The parameter list (void) is equivalent to the empty parameter list. Except for this special case, void shall not be a parameter type (though types derived from void, such as void*, can)." So f(void) is permitted as the special case, f(int,void) is illegal. Fine. Is volatile-T a "type derived from T"? In any case, "The cv-qualified or cv-unqualified versions of a type are distinct types;" (3.9.3p1), so "volatile void" is not "void", and is not explicitly affected by this restriction. 8.9.1p9 introduces restrictions on void: "The void type has an empty set of values. The void type is an incomplete type that cannot be completed. It is used as the return type for functions that do not return a value. Any expression can be explicitly converted to type cv void (5.4). An expression of type void shall be used only as an expression statement (6.2), as an operand of a comma expression (5.18), as a second or third operand of ?: (5.16), as the operand of typeid, or as the expression in a return statement (6.6.3) for a function with the return type void." However, is an expression of type cv-void necessarily of type void? If so, why the distinction in the middle of the paragraph? If not, expressions can be of type "const void", and can then be treated as normal rvalue expressions --- except that non-class rvalues cannot have cv-qualified types, so there isn't a problem (though I would personally like to see that restriction removed, for consistency). Indeed, since void is not an object type, function parameters of "volatile void" (which is incomplete) are OK, as only object type parameters are required to be complete (5.2.2p4) 8.3.2p1 prevents references to cv-void, but that doesn't affect this issue. In any case, f(volatile void) can never be called, because you cannot have an lvalue of type void to copy, and rvalues of type void cannot appear in function call expressions. Anthony -- Anthony Williams Software Engineer, Nortel Networks Optical Components Ltd The opinions expressed in this message are not necessarily those of my employer --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.research.att.com/~austern/csc/faq.html ]