From 5387284604766136264 X-Google-Thread: f78e5,1d1109cd86668825 X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local02.nntp.dca.giganews.com!nntp.speakeasy.net!news.speakeasy.net.POSTED!not-for-mail NNTP-Posting-Date: Wed, 16 Aug 2006 11:30:02 -0500 Return-Path: X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu From: "Kristof Zelechovski" Newsgroups: comp.std.c++ Subject: Re: strtol const-ness problem Organization: Internet Partners Message-ID: References: <1155392893.026182.121030@h48g2000cwc.googlegroups.com> <1155445528.956928.185450@p79g2000cwp.googlegroups.com> <1155587670.517846.8770@h48g2000cwc.googlegroups.com> X-Complaints-To: abuse@ipartners.pl X-RFC2646: Format=Flowed; Original X-Newsreader: Microsoft Outlook Express 6.00.2900.2869 X-MSMail-Priority: Normal X-Priority: 3 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2962 X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Date: Wed, 16 Aug 2006 11:28:50 CST Lines: 118 NNTP-Posting-Host: 65.182.171.162 X-Trace: sv3-0MHIsbH3xda18+SHhXtvEsHkW6ESQCQh7pQTCOevucuQjhzA2f2jzcBQ4zib7Fv6hiW3th0xVsLGPPf!fKVIW/qsXj0o25NvtexhiSe6nrIltkiGwxE58Zj4iPa6lUVdeuiCW9SxLAX8GD9/XbvN4i1Xrx+K!wSsA3Mto/QnLr+s= X-Complaints-To: abuse@speakeasy.net X-DMCA-Complaints-To: abuse@speakeasy.net X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.32 Xref: g2news2.google.com comp.std.c++:3293 Uzytkownik "Manfred von Willich" napisal w wiadomosci news:1155587670.517846.8770@h48g2000cwc.googlegroups.com... > > Greg Herlihy wrote: >> dstevel wrote: >> > The signature for strtol is: >> > >> > strtol( const char*, char**, int) >> > > [snip] >> > >> > void func( const char* a ) >> > { >> > char* tmp; >> > int i = strtol( a, &tmp, 10 ); // OK >> > a = tmp; >> > } >> > >> > This indirectly allows us to modify the original const char* a through >> > the new pointer tmp since tmp will point into the character array a. It >> > doesn't involve a hard cast, but it seems just as dangerous or even >> > more so because it's not obvious what just happened. The calling >> > function could see a change in the string pointed to by a, even though >> > it's passed as pointer to const. >> >> The called function can still use a const_cast to change the value >> referenced by the const pointer. So about the only variables that a >> program can treat as unmodifiable are those defined as const. >> >> > Can someone help me figure out why this is OK? >> >> First it would be rather unusual (not to mention inefficient) for a >> program to spend its cycles scanning text for numbers - when the >> contents of text being scanned (and by extension, the number values >> contained therein) are both constant and were known at compile time. >> Therefore, in the vast majority of cases, it is likely that strtol will >> be called to process non-const character values. > > This is not the only context - it is perfectly reasonable (and *not* > unusual) for a function to have char const * formal parameter and a > const * actual parameter, meaning that the function guarantees not to > modify the (potentially non-const) string, and for this function then > to pass the pointer to strtol. IMHO the signature is not const-correct > - see below. > >> Second, most C++ programmers are accustomed to be able to pass a >> pointer to non-const (such as char *) whenever a function declares a >> pointer to const parameter. So, by extension, it is likely that C++ >> programmers would expect to be able to pass a pointer to a pointer to >> non const whenever a pointer to a pointer to const parameter is >> declared (in other words to pass char** for a a const char** >> parameter). > > The more often C/C++ programmers get disabused of this > misconception/expectation, the faster they'll learn correct usage. No > point in supporting incorrect usage simply because the correct usage is > counterintuitive to the less experienced. > >> [snip] >> >> Finally, since a cast will be necessary either in one case or the other >> - it seems that a decision favoring the less unusual cast in the less >> likely scenario is, on the whole, nothing other than a decision to >> annoy fewer C++ programmers, the fewer number of times. >> >> Greg > > On the contrary, the existing signature can be used without casts for > all uses, though safety is sacrificed: > > void func( const char* a ) > { > char* tmp; > int i = strtol( a, &tmp, 10 ); > const char* b = tmp; // any other use of tmp is unsafe, b is > safe (no cast). > } > > A const-correct library would have had the signature: > > long strtol (const char *nptr, const char **endptr, int base); > > However, given that a function may wish to scan and then modify data > from the point scanned to (I expect this use to be far less common), > the following additional signature would have been appropriate: > > long strtol (char *nptr, char **endptr, int base); > > In C++, two signatures would have been fine. Given that the signature > is in , the linkage is extern"C", an overloaded signature is > precluded. I surmise that the provided signature is a compromise, > supporting both uses, but relying on the programmer not to do > "something silly" with the returned pointer. > Suppose you have extern "C" strtol(char const [], char const **, int). How do you make a non-const overload? A: First, we should use a reference instead of a pointer. static inline long strtol(char const buf[], char const *&end, int base) { return strtol(buf, &end, base); } Next, observe that we can do pointer arithmetic on pointers belonging to the same buffer: static inline long strtol(char buf[], char *&end, int base) { auto char const *gotyou; auto long result(strtol(buf, gotyou, base)); end = gotyou - buf + buf; return result; } Ready. Chris --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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