C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Traits::compare vs signed char type vs memcmp Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Paolo Carlini Guest Posted: Wed May 05, 2004 5:05 pm    Post subject: Traits::compare vs signed char type vs memcmp Hi, in the GNU libstdc++-v3 project we have this PR: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15276 which basically points out that, on machines characterized by a signed char type, implementing traits<char>::compare in terms of memcmp is incorrect on the face of Table 37. The letter of the standard appear to support this, but, on the other hand, all the implementations that I have checked, actually rely on memcmp (and wmemcmp) for the specializations. What do you think? Thanks, Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Alberto Barbati Guest Posted: Thu May 06, 2004 4:30 am    Post subject: Re: Traits::compare vs signed char type vs memcmp Paolo Carlini wrote: Quote: Hi, in the GNU libstdc++-v3 project we have this PR: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15276 which basically points out that, on machines characterized by a signed char type, implementing traits<char>::compare in terms of memcmp is incorrect on the face of Table 37. The letter of the standard appear to support this, but, on the other hand, all the implementations that I have checked, actually rely on memcmp (and wmemcmp) for the specializations. What do you think? According to my intepretation of table 37 alone, it would be perfectly reasonable to define char_traits<char>::lt like this: bool char_traits<char>::lt(char c, char d) { return static_cast<unsigned char>(c) < static_cast } With this definition, implementing char_traits<char>::compare in terms of memcmp would be perfectly consistent. However, 21.1.3.1/6 explicitly states that lt() must be identical to the built-in operator "<" (without casts) :-( With this constraint, it's clearly impossible to implement compare() in terms of memcmp() without violating the semantic of table 37, if char is signed. In order to avoid the explicit loop, a conforming implementation should use a different function, hopefully resolving to a compiler intrinsic if available. OTOH, a DR could be issued with one of two possible solutions: 1) relax the semantic of compare() (at least for the char and wchar_t specialization) 2) remove the restriction on lt() for char and wchar_t specialization Alberto --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Thu May 06, 2004 5:02 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp Alberto Barbati wrote: Quote: However, 21.1.3.1/6 explicitly states that lt() must be identical to the built-in operator "<" (without casts) Exactly, :-( Quote: OTOH, a DR could be issued with one of two possible solutions: 1) relax the semantic of compare() (at least for the char and wchar_t specialization) 2) remove the restriction on lt() for char and wchar_t specialization In my personal opinion a DR is in order: personally, I find unreasonable being forced by the C++ standard to compare /the very same/ char type as signed whereas all the C standard functions compare it as unsigned. Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James Kuyper Guest Posted: Thu May 06, 2004 5:06 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp [email]AlbertoBarbati (AT) libero (DOT) it[/email] (Alberto Barbati) wrote in message news:<u3gmc.170690$Kc3.5429610 (AT) twister2 (DOT) libero.it>... ... Quote: According to my intepretation of table 37 alone, it would be perfectly reasonable to define char_traits<char>::lt like this: bool char_traits<char>::lt(char c, char d) { return static_cast<unsigned char>(c) < static_cast } With that definition, if c is negative and d is not, then std::char_traits<char>::lt(c,d) return false. How would you argue that this counts as a conforming implementation of the specification that it "yields: whether c is to be treated as less than d"? --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Alberto Barbati Guest Posted: Fri May 07, 2004 12:42 am    Post subject: Re: Traits::compare vs signed char type vs memcmp James Kuyper wrote: Quote: AlbertoBarbati (AT) libero (DOT) it (Alberto Barbati) wrote in message news:<u3gmc.170690$Kc3.5429610 (AT) twister2 (DOT) libero.it>... According to my intepretation of table 37 alone, it would be perfectly reasonable to define char_traits<char>::lt like this: bool char_traits<char>::lt(char c, char d) { return static_cast<unsigned char>(c) < static_cast } With that definition, if c is negative and d is not, then std::char_traits<char>::lt(c,d) return false. How would you argue that this counts as a conforming implementation of the specification that it "yields: whether c is to be treated as less than d"? The fact is that the numeric values of the characters have nothing to do with the notion of "order". For example, it is perfectly reasonable to consider character U+A0 NO-BREAK SPACE "greater" than U+41 LATIN CAPITAL LETTER A, despite the fact that the first is stored with negative value and the second with a positive one (assuming char is 8-bit and signed). That is also consistent with the C library, as, for example, strcmp("xa0", "x41") returns 1. Alberto --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top llewelly Guest Posted: Fri May 07, 2004 4:24 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp [email]pcarlini (AT) suse (DOT) de[/email] (Paolo Carlini) writes: Quote: Alberto Barbati wrote: However, 21.1.3.1/6 explicitly states that lt() must be identical to the built-in operator "<" (without casts) :-( Exactly, :-( OTOH, a DR could be issued with one of two possible solutions: 1) relax the semantic of compare() (at least for the char and wchar_t specialization) 2) remove the restriction on lt() for char and wchar_t specialization In my personal opinion a DR is in order: personally, I find unreasonable being forced by the C++ standard to compare /the very same/ char type as signed whereas all the C standard functions compare it as unsigned. [snip] But from a user perspective it is unreasonable that the standard allow lt() and compare() to be inconsistent. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James W. McKelvey Guest Posted: Fri May 07, 2004 4:24 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp Paolo Carlini wrote: Quote: Alberto Barbati wrote: However, 21.1.3.1/6 explicitly states that lt() must be identical to the built-in operator "<" (without casts) :-( Exactly, :-( OTOH, a DR could be issued with one of two possible solutions: 1) relax the semantic of compare() (at least for the char and wchar_t specialization) 2) remove the restriction on lt() for char and wchar_t specialization In my personal opinion a DR is in order: personally, I find unreasonable being forced by the C++ standard to compare /the very same/ char type as signed whereas all the C standard functions compare it as unsigned. Why then allow char to be signed at all, if the signs are to be ignored? The C standard functions are irrelevant here anyway. -- What portion in the world can the artist have Who has awakened from the common dream But dissipation and despair? -- William Butler Yeats --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James Kuyper Guest Posted: Fri May 07, 2004 4:52 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp [email]AlbertoBarbati (AT) libero (DOT) it[/email] (Alberto Barbati) wrote in message news:<dXzmc.41690$Qc.1633658 (AT) twister1 (DOT) libero.it>... Quote: James Kuyper wrote: [email]AlbertoBarbati (AT) libero (DOT) it[/email] (Alberto Barbati) wrote in message news:<u3gmc.170690$Kc3.5429610 (AT) twister2 (DOT) libero.it>... According to my intepretation of table 37 alone, it would be perfectly reasonable to define char_traits<char>::lt like this: bool char_traits<char>::lt(char c, char d) { return static_cast<unsigned char>(c) < static_cast } With that definition, if c is negative and d is not, then std::char_traits<char>::lt(c,d) return false. How would you argue that this counts as a conforming implementation of the specification that it "yields: whether c is to be treated as less than d"? The fact is that the numeric values of the characters have nothing to do with the notion of "order". For example, it is perfectly reasonable to consider character U+A0 NO-BREAK SPACE "greater" than U+41 LATIN CAPITAL LETTER A, despite the fact that the first is stored with negative value and the second with a positive one (assuming char is 8-bit and signed). What you're talking about is a locale-specific feature that is covered by the collate locale category. That's not what the 'lt' member of char_traits is for. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Fri May 07, 2004 4:53 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp llewelly wrote: Quote: But from a user perspective it is unreasonable that the standard allow lt() and compare() to be inconsistent. Quite to the contrary, the goal is consistency between lt() and compare(): it could be easily achieved in many ways, for instance as sketched by Alberto. Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Fri May 07, 2004 4:53 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp James W. McKelvey wrote: Quote: Why then allow char to be signed at all, if the signs are to be ignored? The same could be asked in the case of the C standard, however. Quote: The C standard functions are irrelevant here anyway. Well, do you really think so? Do you like comparing two char arrays via memcmp then passing the arrays to a basic_string constructor and "voila'" starting comparing /the very same/ data in a different way? Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Fri May 07, 2004 6:06 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp llewelly wrote: Quote: But from a user perspective it is unreasonable that the standard allow lt() and compare() to be inconsistent. Quite to the contrary, the goal is consistency between lt() and compare(): it could be easily achieved in many ways, for instance as sketched by Alberto. Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top llewelly Guest Posted: Fri May 07, 2004 9:06 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp [email]pcarlini (AT) suse (DOT) de[/email] (Paolo Carlini) writes: Quote: llewelly wrote: But from a user perspective it is unreasonable that the standard allow lt() and compare() to be inconsistent. Quite to the contrary, Not contrary at all. Quote: the goal is consistency between lt() and compare(): This is what I want to hear, thank you. Quote: it could be easily achieved in many ways, for instance as sketched by Alberto. Alberto made two suggestions. Only the second seems to guarantee consistency between lt() and compare(). --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Fri May 07, 2004 9:37 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp llewelly wrote: Quote: Alberto made two suggestions. Only the second seems to guarantee consistency between lt() and compare(). Indeed, you are right. Actually, the plain char case is the one really troublesome, since, for wchar_t, C99, 7.24.4.4,p1 holds. Paolo. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James W. McKelvey Guest Posted: Sun May 09, 2004 3:29 am    Post subject: Re: Traits::compare vs signed char type vs memcmp Paolo Carlini wrote: Quote: James W. McKelvey wrote: Why then allow char to be signed at all, if the signs are to be ignored? The same could be asked in the case of the C standard, however. The C standard functions are irrelevant here anyway. Well, do you really think so? Do you like comparing two char arrays via memcmp then passing the arrays to a basic_string constructor and "voila'" starting comparing /the very same/ data in a different way? Suppose I use memcmp to compare SIGNED char arrays; the same arrays passed to a basic_string constructor will compare the very same data in a different way. Are you saying that you want all char types to compare as if unsigned? But memcmp doesn't work with chars anyway. It works with unsigned bytes; so if I had to use it, I would be very careful and would know the limitations. std::string and char_traits are not C, but C++, and can do whatever they are defined to do. If there is a requirement that they follow C in some manner, I don't see it in the standard. memcmp and strcmp are C functions that do something similar to C++ functionality; they are not exact replacements, and must be used with care. This is the way I approach it: C++ is trying to be better than C; it can't do that if everything has to be done the way C does things. Jim -- What portion in the world can the artist have Who has awakened from the common dream But dissipation and despair? -- William Butler Yeats --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Paolo Carlini Guest Posted: Sun May 09, 2004 6:43 pm    Post subject: Re: Traits::compare vs signed char type vs memcmp Hi, James W. McKelvey wrote: Quote: But memcmp doesn't work with chars anyway. It works with unsigned bytes; so if I had to use it, I would be very careful and would know the limitations. This one *could* be a very good point ;) Unfortunately, the C Standard *always* talks about *characters* in 7.21.4... Quote: std::string and char_traits are not C, but C++, and can do whatever they are defined to do. If there is a requirement that they follow C in some manner, I don't see it in the standard. Interestingly for the historian the 1995 draft had the requirement that memcmp be used in the implementation of compare. Quote: memcmp and strcmp are C functions that do something similar to C++ functionality; they are not exact replacements, and must be used with care. This is the way I approach it: C++ is trying to be better than C; it can't do that if everything has to be done the way C does things. This is all very reasonable. My personal point of view is very similar. However, remember that C++ was born to be as compatible as possible with C and that there is an ongoing effort to *improve* that. Also, the established behavior in this area is very strong and pervasive. Not considering the performance side of the issue... Paolo. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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