From -5573826933315481866 X-Google-Thread: 7894ca11fe,e2347147b98ac8c4,start X-Google-Attributes: gid7894ca11fe,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII Path: g2news2.google.com!news1.google.com!news.glorb.com!news.alt.net!frodo.cs.rpi.edu!not-for-mail From: =?ISO-8859-1?Q?Joaqu=EDn_M_L=F3pez_Mu=F1oz?= Newsgroups: comp.std.c++ Subject: Adding 0 to the null pointer Date: Tue, 26 May 2009 10:37:43 CST Organization: http://groups.google.com Lines: 60 Sender: cppmods@cs.rpi.edu Approved: stephen.clamage@sun.com Message-ID: <7c71cedf-287b-4fed-a4b0-807160d2e59e@g1g2000yqh.googlegroups.com> NNTP-Posting-Host: netlab.cs.rpi.edu Content-Type: text/plain; charset=ISO-8859-1 To: (Usenet) Return-Path: X-Original-Date: Mon, 25 May 2009 12:54:18 -0700 (PDT) X-Submission-Address: std-c++@netlab.cs.rpi.edu Xref: g2news2.google.com comp.std.c++:725 Hello, Is the following a legal C++ program (note we're passing 0 as the third argument to std::copy)? #include int main() { int x,*p=&x; std::copy(p,p,(int*)0); } The (C++03) standard does not say whether result in std::copy(first,last,result) must be a valid pointer, and only states that the algorithm returns result+ (last-first), which in our particular case (first==last) is result+0. Regarding this latter expression, 5.7/8 [expr.add] says: "If the value 0 is added to or subtracted from a pointer value, the result compares equal to the original pointer value [...]" without regard to whether the pointer being added or subtracted 0 is the null pointer or not. So, it seems to me that result+0 is indeed valid and yields result, i.e. the null value of int*. To summarize, I think the program above is indeed valid, but some stdlib implementers dissent: the implementation of std::copy for MSVC 9.0 has roughly the following checking code (when parameter debugging is activated): _DEBUG_RANGE(first, last); if (first != last) _DEBUG_POINTER(result); which accepts the program above as valid, whereas the implementer changed her mind in MSVC 10.0 and modified the checking code as _DEBUG_RANGE(first, last); _DEBUG_POINTER(result); where _DEBUG_POINTER triggers and assertion when result is a null pointer, thus failing on the program above. So, who's right, I (and MSVC 9.0) or MSVC 10.0? Thank you, Joaqu�n M L�pez Mu�oz Telef�nica, Investigaci�n y Desarrollo -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]