C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Unsigned/Signed Mismatch          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message MiniDisc_2k2 Guest Posted: Thu Jul 10, 2003 8:34 pm    Post subject: Unsigned/Signed Mismatch Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) -- MiniDisc_2k2 To reply, replace nospam.com with cox dot net. --- [ 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 Francis Glassborow Guest Posted: Thu Jul 10, 2003 10:49 pm    Post subject: Re: Unsigned/Signed Mismatch In article <%jjPa.31$zd4.2@lakeread02>, MiniDisc_2k2 <MattDelB (AT) nospam (DOT) com> writes Quote: Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) No, the normal integral promotions are applied first Which only becomes mildly interesting on systems where all integral types are the same size. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top MiniDisc_2k2 Guest Posted: Thu Jul 10, 2003 11:10 pm    Post subject: Re: Unsigned/Signed Mismatch "Ioannis Vranos" <ivr (AT) nothis (DOT) emails.ru> wrote Quote: ""MiniDisc_2k2"" <MattDelB (AT) nospam (DOT) com> wrote in message news:%jjPa.31$zd4.2 (AT) lakeread02 (DOT) .. Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) At first keep in mind that char (differently from other "plain" integer types) can be either signed or unsigned. If you want one of the two in specific, you have to declare it explicitly (e.g. signed char a=3). Assuming you wish your above code to be: signed char a = 3; unsigned char b = 255; if (a Both unsigned char and signed char get converted to int during the expression evaluation. So in summary, a either signed char either unsigned gets converted to int. Well actually I was using chars only to save my mind from calculating it all out. Let's try again. I also intended this code to be used on a two's complement machine. Use this: signed int a = 3; unsigned int b = -1; /* thus being the highest number represented by unsigned int*/ if (b Now. If b were converted to a signed integer, b a were converted to an unsigned integer, we would get 65535 < 3 == false (in 16-bit environment). Which representation does it follow. Does it proceed like the char conversion, where it is converted to a signed int? Is this at all standardized, or does the compiler get to pick? -- MiniDisc_2k2 --- [ 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 Andy Sawyer Guest Posted: Thu Jul 10, 2003 11:48 pm    Post subject: Re: Unsigned/Signed Mismatch In article <%jjPa.31$zd4.2@lakeread02>, on Thu, 10 Jul 2003 20:34:29 +0000 (UTC), [email]MattDelB (AT) nospam (DOT) com[/email] ("MiniDisc_2k2") wrote: Quote: Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b What's the correct coversion (what is the compiler supposed to do?) 5.9p2 tells us that the usual arithmetic conversions (5p9) are performed. 4.5p1 tells us that 'a' is promoted to signed int, It also tells us that b is promoted to either signed int or unsigned int, depending on the implementation, specifically: ,---- Quote: An rvalue of type char, signed char, unsigned char, short int, or unsigned short int can be converted to an rvalue of type int if int can represent all the values of the source type; otherwise, the source rvalue can be converted to an rvalue of type unsigned int. `---- Either way, it will still have the value 255 (I assume from your magic number of 255, you're thinking of a platform with 8-bit characters and you're concerned about sign-extention? Not a problem here :) If b has been promoted to signed int (such as in an 8-bit char environment), then the comparison is performed with signed integers, so (a If b has been promoted to unsigned int, a is then also converted to unsigned int (5p9) and the comparison is performed using unsigned integers, and again (a In this last case (b promoted to unsigned, a converted to unsigned), then the result is highly dependant on the actual value of a, as 4.7p3 says: ,---- Quote: If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bitfield width); otherwise, the value is implementation-defined. `---- Since, in your example, a == 3, then it can comfortable fit inside an unsigned int and hence is unchanged. At least, that's the way _I_ read the standard :) Regards, Andy S. -- "Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." -- Terry Pratchett, Reaper Man --- [ 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 Andy Sawyer Guest Posted: Fri Jul 11, 2003 1:39 am    Post subject: Re: Unsigned/Signed Mismatch In article <3nmPa.50$zd4.42@lakeread02>, on Thu, 10 Jul 2003 23:10:49 +0000 (UTC), [email]MattDelB (AT) nospam (DOT) com[/email] ("MiniDisc_2k2") wrote: Quote: Well actually I was using chars only to save my mind from calculating it all out. Let's try again. I also intended this code to be used on a two's complement machine. Use this: signed int a = 3; unsigned int b = -1; /* thus being the highest number represented by unsigned int*/ No, it's a signed integer converted to an unsigned integer. And it's a value that the unsigned target cannot represent, so the value of b is actually implementation-defined. If you *actually* want the highest value represented by an unsigned integer, then write:: unsinged int b = std::numeric_limits<unsigned int>::max(); Quote: if (b Now. If b were converted to a signed integer Which it won't be. Quote: b a were converted to an unsigned integer It will be. Quote: we would get 65535 < 3 == false (in 16-bit environment). Which representation does it follow. Does it proceed like the char conversion, where it is converted to a signed int? Is this at all standardized, or does the compiler get to pick? It's standardized in detail, see: 4.5 Integral Promotions 4.7 Integral Conversions 5p9 "the usual suspects^H^H^H^H^H^H^H^H arithmetic conversions":) What the compiler *can* pick is the result of the conversion from signed to unigned types if the value of the signed type cannot be represented in the unsigned type (see 4.7p3) Regards, Andy S -- "Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." -- Terry Pratchett, Reaper Man --- [ 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 Andy Sawyer Guest Posted: Fri Jul 11, 2003 4:06 pm    Post subject: Re: Unsigned/Signed Mismatch In article <3f0dd160$0$245$4d4eb98e (AT) read (DOT) news.gr.uu.net>, on Thu, 10 Jul 2003 21:55:32 +0000 (UTC), [email]ivr (AT) nothis (DOT) emails.ru[/email] (Ioannis Vranos) wrote: Quote: ""MiniDisc_2k2"" <MattDelB (AT) nospam (DOT) com> wrote in message news:%jjPa.31$zd4.2 (AT) lakeread02 (DOT) .. Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) At first keep in mind that char (differently from other "plain" integer types) can be either signed or unsigned. If you want one of the two in specific, you have to declare it explicitly (e.g. signed char a=3). Remember that char, signed char and unsigned char are three disticnt types. FWIW, if I'm using the value in an arithmetic context, then I always write either signed char or unsigned char. If I'm using it to represent text elements, then I write char. And I probably wouldn't write: char a = 3; I'd write char a = '03'; or, more likely - given my current project: char a = ASCII::ETX; Quote: Assuming you wish your above code to be: signed char a = 3; unsigned char b = 255; if (a I suspect if he'd wished that, he'd have written that. Quote: Both unsigned char and signed char get converted to int during the expression evaluation. In fact, unsigned char may be promoted to unsigned int (depending on the environment). Quote: So in summary, a either signed char either unsigned gets converted to int. I had trouble parsing that - I assume you meant "So in summary, either a signed char or unsigned char gets converted to int." ? If that is what you meant, then it isn't strictly true on all platforms. Please see my other post for why :) Regards, Andy S. -- "Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." -- Terry Pratchett, Reaper Man --- [ 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 William M. Miller Guest Posted: Fri Jul 11, 2003 5:52 pm    Post subject: Re: Unsigned/Signed Mismatch "Andy Sawyer" <andys (AT) despammed (DOT) com> wrote Quote: In article <3nmPa.50$zd4.42@lakeread02>, on Thu, 10 Jul 2003 23:10:49 +0000 (UTC), [email]MattDelB (AT) nospam (DOT) com[/email] ("MiniDisc_2k2") wrote: signed int a = 3; unsigned int b = -1; /* thus being the highest number represented by unsigned int*/ No, it's a signed integer converted to an unsigned integer. And it's a value that the unsigned target cannot represent, so the value of b is actually implementation-defined. .... What the compiler *can* pick is the result of the conversion from signed to unigned types if the value of the signed type cannot be represented in the unsigned type (see 4.7p3) Wrong paragraph. 4.7p3 deals with conversion to signed types ("if the destination type is signed..."), and the result is implementation-defined, as you say. The description of conversion to an unsigned type is in 4.7p2, and the result is defined by the Standard, not by the implementation: If the designation type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2**n where n is the number of bits used to represent the unsigned type). [Note: In a two's complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation). ] -- William M. Miller --- [ 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 Ron Natalie Guest Posted: Sat Jul 12, 2003 12:12 am    Post subject: Re: Unsigned/Signed Mismatch ""MiniDisc_2k2"" <MattDelB (AT) nospam (DOT) com> wrote Quote: Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) The usual aritmentic conersions are performd (5.1/9) which means the integral promotions (4.5) are performed which means both a and b are converted to int values of 3 and 255 respectively. The value of a therefore is true. --- [ 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 Ben Hutchings Guest Posted: Sat Jul 12, 2003 12:15 am    Post subject: Re: Unsigned/Signed Mismatch In article <llv6szff.fsf (AT) evo6 (DOT) com>, Andy Sawyer wrote: Quote: In article <3nmPa.50$zd4.42@lakeread02>, on Thu, 10 Jul 2003 23:10:49 +0000 (UTC), [email]MattDelB (AT) nospam (DOT) com[/email] ("MiniDisc_2k2") wrote: Well actually I was using chars only to save my mind from calculating it all out. Let's try again. I also intended this code to be used on a two's complement machine. Use this: signed int a = 3; unsigned int b = -1; /* thus being the highest number represented by unsigned int*/ No, it's a signed integer converted to an unsigned integer. And it's a value that the unsigned target cannot represent, so the value of b is actually implementation-defined. snip No, this is well-defined (standard 4.7p2). It's the reverse that is implementation-defined. --- [ 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 Ioannis Vranos Guest Posted: Sat Jul 12, 2003 12:16 am    Post subject: Re: Unsigned/Signed Mismatch "Andy Sawyer" <andys (AT) despammed (DOT) com> wrote Quote: Remember that char, signed char and unsigned char are three disticnt types. char is implemented either as signed char or as unsigned char. FWIW, if I'm using the value in an arithmetic context, then I Quote: always write either signed char or unsigned char. If I'm using it to represent text elements, then I write char. And I probably wouldn't write: char a = 3; I'd write char a = '03'; It's a personal issue. I am thinking in integral form, so i would write 3 if i had not a specific character in mind. Quote: In fact, unsigned char may be promoted to unsigned int (depending on the environment). If the involved values fit in int, they get promoted to int. It's in the standard. Quote: So in summary, a either signed char either unsigned gets converted to int. I had trouble parsing that - I assume you meant "So in summary, either a signed char or unsigned char gets converted to int." ? "So in summary the variable a, either it is signed char or unsigned char gets converted to int (in real world scenarios where sizeof(int)>1)". -- Ioannis * Programming pages: http://www.noicys.freeurl.com * Alternative URL 1: http://run.to/noicys * Alternative URL 2: http://www.noicys.cjb.net --- [ 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 Kanze Guest Posted: Sun Jul 13, 2003 3:58 pm    Post subject: Re: Unsigned/Signed Mismatch [email]ivr (AT) nothis (DOT) emails.ru[/email] ("Ioannis Vranos") writes: Quote: ""MiniDisc_2k2"" <MattDelB (AT) nospam (DOT) com> wrote in message news:3nmPa.50$zd4.42 (AT) lakeread02 (DOT) .. Well actually I was using chars only to save my mind from calculating it all out. Let's try again. I also intended this code to be used on a two's complement machine. Which doesn't change anything here. Quote: Use this: signed int a = 3; unsigned int b = -1; /* thus being the highest number represented by unsigned int*/ if (b Now. If b were converted to a signed integer, b == true. If a were converted to an unsigned integer, we would get 65535 < 3 == false (in 16-bit environment). Which representation does it follow. Does it proceed like the char conversion, where it is converted to a signed int? Is this at all standardized, or does the compiler get to pick? The C++ standard says: [conv.prom] 4.5 Integral promotions Wrong section. There are no promotions here. Quote: 1 An rvalue of type char, signed char, unsigned char, short int, or unsigned short int can be converted to an rvalue of type int if int can represent all the values of the source type; otherwise, the source rvalue can be converted to an rvalue of type unsigned int. [...] Quote: The (1) is your case. Not at all. That paragraph only applies if the type is char, signed char, unsigned char, short int or unsigned short int. There are none of those here. Quote: The answer is that everything is promoted to unsigned int and you must get a compiler warning about comparing signed with unsigned values. Right answer, wrong reason. See 5/9 -- it gives the complete list of conversions (call the usual arithmetic conversions) which apply to binary operators. In particular, the last point "Otherwise, if either operand is unsigned, the other shall be converted to unsigned." Quote: Try to compile and see. That's not always a valid solution. If he were comparing a long and an unsigned int, for example, the conversions used will depend on the implementation. -- James Kanze mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France +33 1 41 89 80 93 --- [ 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 Kanze Guest Posted: Sun Jul 13, 2003 3:58 pm    Post subject: Re: Unsigned/Signed Mismatch [email]francis (AT) robinton (DOT) demon.co.uk[/email] (Francis Glassborow) writes: Quote: In article <%jjPa.31$zd4.2@lakeread02>, MiniDisc_2k2 [email]MattDelB (AT) nospam (DOT) com[/email]> writes Okay, here's a question about the standard. What does it say about unsigned/signed mismatches in a comparison statement: char a = 3; unsigned char b = 255; if (a<b) Now what's the real answer here? If a is converted to unsigned, then b>a. But, if b is converted to signed,then a>b. What's the correct coversion (what is the compiler supposed to do?) No, the normal integral promotions are applied first Which only becomes mildly interesting on systems where all integral types are the same size. But on such systems, 255 is within the range of char, so the problem isn't interesting either. (For that matter, as formulated, it isn't interesting on a system with 9 bit bytes either.) -- James Kanze mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France +33 1 41 89 80 93 --- [ 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 Andy Sawyer Guest Posted: Mon Jul 14, 2003 5:26 pm    Post subject: Re: Unsigned/Signed Mismatch In article <3f0ef04e$0$242$4d4eb98e (AT) read (DOT) news.gr.uu.net>, on Sat, 12 Jul 2003 00:16:53 +0000 (UTC), [email]ivr (AT) nothis (DOT) emails.ru[/email] ("Ioannis Vranos") wrote: Quote: "Andy Sawyer" <andys (AT) despammed (DOT) com> wrote in message news:ptkiszvi.fsf (AT) evo6 (DOT) com... Remember that char, signed char and unsigned char are three disticnt types. char is implemented either as signed char or as unsigned char. Please cite your reference. I suggest you read 3.9.1p1, which says in part: ,---- Quote: Plain char, signed char, and unsigned char are three distinct types. `---- It's in the standard (almost word-for-word - I omitted "plain"). Quote: In fact, unsigned char may be promoted to unsigned int (depending on the environment). If the involved values fit in int, they get promoted to int. It's in the standard. Again, cite your reference. I suggest you read 4.5p1, which says: ,---- Quote: An rvalue of type char, signed char, unsigned char, short int, or unsigned short int can be converted to an rvalue of type int if int can represent all the values of the source type; otherwise, the source rvalue can be converted to an rvalue of type unsigned int. `---- It's in the standard. Note the "if int can represent *ALL* values of the source type". (my emphasis). I suspect you're thinking of integral _conversions_, which (in this context) happen after promotions - see 5p9. It's in the standard. Quote: So in summary, a either signed char either unsigned gets converted to int. I had trouble parsing that - I assume you meant "So in summary, either a signed char or unsigned char gets converted to int." ? "So in summary the variable a, either it is signed char or unsigned char gets converted to int Not necessarily - it's in the standard. Quote: (in real world scenarios where sizeof(int)>1)". There are real world scenarios where sizeof(int)==sizeof(char). Neither your being unfamilair with them, nor the fact that they are less common than those where sizeof(int)>sizeof(char) does not in any way disprove their existence. Regards, Andy S. -- "Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." -- Terry Pratchett, Reaper Man --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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