C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  howto convert form byte[4] to Int32 while retaining the bina          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message jeff@foundrymusic.com Guest Posted: Tue Dec 21, 2004 10:20 am    Post subject: howto convert form byte[4] to Int32 while retaining the bina How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matthew Hanna Guest Posted: Wed Dec 22, 2004 8:56 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the wouldn't memcpy work? [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Wed Dec 22, 2004 9:01 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 Presumably byte is a type alias for char (or unsigned char?) which you assume to have 8 bits and Int32 is a type alias for a 32-bit integer? There are several ways of converting between multiple bytes and larger integers; which do you want? Quote: while retaining the binary value of the byte array? What do you mean by that? -- Ben Hutchings I'm not a reverse psychological virus. Please don't copy me into your sig. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Dumais Guest Posted: Wed Dec 22, 2004 9:03 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the You could try something like... { union { unsigned char c[4]; long l; } u ; u.c[0] = 1; u.c[1] = 2; u.c[2] = 3; u.c[3] = 4; // use the integer value std::cout << u.l << std::endl; } [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Wed Dec 22, 2004 9:04 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? Depends on the system. There's no portable way to do this (depending on what you are really asking for). [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Wed Dec 22, 2004 9:05 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? First, why should anything happen to the byte array? Second, what order are these bytes in? -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Jørgensen Guest Posted: Wed Dec 22, 2004 9:05 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the <jeff (AT) foundrymusic (DOT) com> wrote Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? What do you mean by "binary value of the byte array"? Well, this is a matter of interpretation, better known as "endianness". You have four bytes, byte[0], byte[1], byte[2], and byte[3]. If the bytes are ordered in little-endian, then the result is int32_t value = (byte[3] << 24) + (byte[2] << 16) + (byte[1] << + byte[0]; // little-endian. If the bytes are ordered in big-endian, then the result is: int32_t value = (byte[0] << 24) + (byte[1] << 16) + (byte[2] << + byte[3]; // big-endian. Note, this has nothing to do with the endianness of the CPU; it depends solely on the semantics *you* impose on the byte array. The above is the cheap, simple, and portable. However, it does not perform very well. There are techniques that are substantially quicker, at the expense of losing portability. Many networking protocols - for instance - use big endian byte arrays. One approach is to reinterpret the byte array into the CPUs endianness, and then convert to big-endian: int32_t value = ntohl( *(int32_t *) &byte[0] ); // big-endian. -Michael. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Dumais Guest Posted: Thu Dec 23, 2004 3:31 pm    Post subject: Re: howto convert form byte[4] to Int32 while retaining the After looking at this, it seems you may using .Net. If so, check out the BitConverter class... http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfSystemBitConverterClassToInt32Topic.asp [email]jeff (AT) foundrymusic (DOT) com[/email] wrote: Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rade Guest Posted: Thu Dec 23, 2004 9:16 pm    Post subject: Re: howto convert form byte[4] to Int32 while retaining the <jeff (AT) foundrymusic (DOT) com> wrote Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? But what if OP has mixed C++ and C# - there are two rather basic C# types called byte and Int32 ? If this is the case, the whole discussion is off-topic (both here and in c.l.c++) <OT>, and I'd recommend the OP to look at the System.BitConverter class in the MSDN</OT>. Rade [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Dec 23, 2004 9:47 pm    Post subject: Re: howto convert form byte[4] to Int32 while retaining the Michael Jørgensen wrote: Quote: jeff (AT) foundrymusic (DOT) com> wrote in message news:1103573700.383982.27200 (AT) z14g2000cwz (DOT) googlegroups.com... How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? What do you mean by "binary value of the byte array"? Well, this is a matter of interpretation, better known as "endianness". Endianness is really only the tip of the iceberg. First, of course, with regards to four byte ints (which are far from universal -- I've seen 2 and 4 personally, and have heard of machines using 1 and 6, and maybe one with 3), there are 24 different orders possible. Some are pretty improbable, but I've seen at least 3. Then, there is a question of representation : most modern machines are two's complement, but at least one is one's complement, and a signed magnitude machine was sold not so long ago. Not to mention that depending on where the bytes came from, they may not correspond to any hardware format what so ever. Quote: You have four bytes, byte[0], byte[1], byte[2], and byte[3]. If the bytes are ordered in little-endian, then the result is int32_t value = (byte[3] << 24) + (byte[2] << 16) + (byte[1] << + byte[0]; // little-endian. If the bytes are ordered in big-endian, then the result is: int32_t value = (byte[0] << 24) + (byte[1] << 16) + (byte[2] << + byte[3]; // big-endian. Note, this has nothing to do with the endianness of the CPU; it depends solely on the semantics *you* impose on the byte array. Or more often, that someone else outside the project has decided to impose, without asking you:-). Quote: The above is the cheap, simple, and portable. However, it does not perform very well. There are techniques that are substantially quicker, at the expense of losing portability. Are you sure? I've never had any performance problems with it. -- James Kanze GABI Software http://www.gabi-soft.fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel M. Pfeffer Guest Posted: Mon Jul 18, 2005 8:25 am    Post subject: Re: howto convert form byte[4] to Int32 while retaining the <jeff (AT) foundrymusic (DOT) com> wrote Quote: How do you convert form byte[4] to Int32 while retaining the binary value of the byte array? You can't do this in any portable way. However, if I make the assumptions 1. byte is a synonym for unsigned char (e.g. typedef unsigned char byte;) 2. Int32 is a synonym for an (n*CHAR_BITS)-bit signed integer (see <climits> for the value of CHAR_BITS) 3. There are no unused bits or trap values in the representation of Int32 then the following code should work for reading and writing values in a particular runtime environment (=CPU + operating system + compiler + compiler settings): union x { byte b[sizeof(Int32)]; Int32 i; }; writing a value into the i member and reading it out of the b member (or vice versa) should provide the desired conversion. HTH Daniel Pfeffer [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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