C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  size of char array vs chars          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Rick Holloman Guest Posted: Sat Dec 11, 2004 3:22 pm    Post subject: size of char array vs chars If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Sat Dec 11, 2004 11:55 pm    Post subject: Re: size of char array vs chars Rick Holloman wrote: Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? Not by the standard, but popular compilers offer switches and options to control the layout and padding of structure elements. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrew Koenig Guest Posted: Sat Dec 11, 2004 11:56 pm    Post subject: Re: size of char array vs chars "Rick Holloman" <richard.t.holloman (AT) boeing (DOT) com> wrote Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? No. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Dec 12, 2004 12:02 am    Post subject: Re: size of char array vs chars Rick Holloman wrote: Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? Not by the standard. As a quality of implementation issue, however, I would certainly hope that that would be the case in practice. -- James Kanze home: www.gabi-soft.fr Conseils en informatique orient=E9e objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre S=E9mard, 78210 St.-Cyr-l'=C9cole, 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 Bo Rydberg Guest Posted: Sun Dec 12, 2004 10:02 am    Post subject: Re: size of char array vs chars "Rick Holloman" <richard.t.holloman (AT) boeing (DOT) com> wrote Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? It could be but this is compiler setup dependent. You need to tell your compiler the struct member alignment. Usually this is 2, 4, or 8-byte depending on the environment you are working in. But it should also let you have a 1-byte alignment. -- mvh // Bo Rydberg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Hairgrove Guest Posted: Sun Dec 12, 2004 10:04 am    Post subject: Re: size of char array vs chars On 11 Dec 2004 10:22:28 -0500, "Rick Holloman" <richard.t.holloman (AT) boeing (DOT) com> wrote: Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? The standard does make this guarantee for POD-struct types (see section 9.2, paragraph 14 of the C++ standard). It is unclear to me whether or not the same holds true for non-static members of a non-POD type ... the standard only says that "later members have higher addresses" (see paragraph 12 of the same section). But since sizeof(char) is guaranteed to be 1 by the standard, I cannot see how any implementation-defined packing or alignment would cause them not to be allocated contiguously. I would recommend using a std::vector<char> instead of an array because the standard does guarantee that std::vector must allocate storage in contiguous memory blocks (see section 23.2.4 of the standard). -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jack Klein Guest Posted: Sun Dec 12, 2004 10:10 am    Post subject: Re: size of char array vs chars On 11 Dec 2004 10:22:28 -0500, "Rick Holloman" <richard.t.holloman (AT) boeing (DOT) com> wrote in comp.lang.c++.moderated: Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? No, there is not. I do not think there is any implementation which would not do it this way, but there is no such requirement or guarantee in the C++ language standard. Even in a POD structure with no access specifiers, the implementation is allowed to insert padding bytes at the end of the structure and after any member. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Dec 12, 2004 11:03 pm    Post subject: Re: size of char array vs chars In article <5lsmr09fm49ver9mrltabeotnlukq20h8p (AT) 4ax (DOT) com>, Bob Hairgrove <invalid (AT) bigfoot (DOT) com> writes Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? The standard does make this guarantee for POD-struct types (see section 9.2, paragraph 14 of the C++ standard). I completely fail to see how that text (or anything else in the Standard) makes the guarantee you seem to think it does. AFAIK the compiler is always free to add padding between members. All 9.2, para 14 requires is that it do it the same way for all POD class types that contain the same sequence of member types. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Sun Dec 12, 2004 11:06 pm    Post subject: Re: size of char array vs chars [email]invalid (AT) bigfoot (DOT) com[/email] (Bob Hairgrove) wrote (abridged): Quote: But since sizeof(char) is guaranteed to be 1 by the standard, I cannot see how any implementation-defined packing or alignment would cause them not to be allocated contiguously. An implementation might use padding bytes for error detection, as a way of detecting buffer overruns or other memory corruption. I believe VC++7.1 will do this for the stack, given the right commandline options. The idea is to set the padding bytes to some known value and periodically check whether they have been changed. -- Dave Harris, Nottingham, UK [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Sun Dec 12, 2004 11:08 pm    Post subject: Re: size of char array vs chars Bob Hairgrove wrote: Quote: The standard does make this guarantee for POD-struct types (see section 9.2, paragraph 14 of the C++ standard). No. This paragraph tells you whether *two* POD-struct types are layout compatible, not whether one POD-struct type is compatible with any array. Quote: But since sizeof(char) is guaranteed to be 1 by the standard, I cannot see how any implementation-defined packing or alignment would cause them not to be allocated contiguously. Some implementations may prefer to align members to the word-boundary for performance reasons. In any case, the padding between members is unspecified, but as stated elsewhere, popular implementations allow to control it. Quote: I would recommend using a std::vector<char> instead of an array because the standard does guarantee that std::vector must allocate storage in contiguous memory blocks And see 8.3.4/1 for the same guarantee for arrays. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jack Klein Guest Posted: Sun Dec 12, 2004 11:09 pm    Post subject: Re: size of char array vs chars On 12 Dec 2004 05:04:01 -0500, Bob Hairgrove <invalid (AT) bigfoot (DOT) com> wrote in comp.lang.c++.moderated: Quote: On 11 Dec 2004 10:22:28 -0500, "Rick Holloman" [email]richard.t.holloman (AT) boeing (DOT) com[/email]> wrote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? The standard does make this guarantee for POD-struct types (see section 9.2, paragraph 14 of the C++ standard). It is unclear to me whether or not the same holds true for non-static members of a non-POD type ... the standard only says that "later members have higher addresses" (see paragraph 12 of the same section). I am curious as to how you come to this interpretation of the wording in the standard: "Two POD struct (clause 9) types are layout compatible if they have the same number of members, and corresponding members (in order) have layout compatible types (3.9)." Where do you see anything at all about layout compatibility with an array, or any other non struct/class type? -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Hairgrove Guest Posted: Mon Dec 13, 2004 10:32 am    Post subject: Re: size of char array vs chars On 12 Dec 2004 18:03:20 -0500, Francis Glassborow <francis (AT) robinton (DOT) demon.co.uk> wrote: Quote: In article <5lsmr09fm49ver9mrltabeotnlukq20h8p (AT) 4ax (DOT) com>, Bob Hairgrove [email]invalid (AT) bigfoot (DOT) com[/email]> writes If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? The standard does make this guarantee for POD-struct types (see section 9.2, paragraph 14 of the C++ standard). I completely fail to see how that text (or anything else in the Standard) makes the guarantee you seem to think it does. AFAIK the compiler is always free to add padding between members. All 9.2, para 14 requires is that it do it the same way for all POD class types that contain the same sequence of member types. OK, I think I have overlooked something. Consider the following POD structs: struct A { char a1; char a2; // snip... char a128; }; struct B { char a[128]; }; Now I see that I overlooked the fact that both A and B must have "the same number" of non-static data members, and one array just isn't the same as 128 single items ... IOW, exactly what the OP also wanted to know! It would be interesting to know what the C standard has to say about this ... with POD types, I would assume that the C++ standard would have to go with with the C standard here. Unfortunately, although I have a C++ standard reference, I don't have access to the C standard. -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Rafter Guest Posted: Mon Dec 13, 2004 10:37 am    Post subject: Re: size of char array vs chars Quote: I would recommend using a std::vector<char> instead of an array because the standard does guarantee that std::vector must allocate storage in contiguous memory blocks (see section 23.2.4 of the standard). One of the reasons for this is so they are backwardly compatible with arrays - which are also guarenteed to be allocated in contiguous memory. But, I too, would recommend using vector over arrays. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue Dec 14, 2004 3:33 am    Post subject: Re: size of char array vs chars In article <32kpr01uaei9n7bif5lr9pnppgs01rntp4 (AT) 4ax (DOT) com>, Bob Hairgrove <invalid (AT) bigfoot (DOT) com> writes Quote: It would be interesting to know what the C standard has to say about this ... with POD types, I would assume that the C++ standard would have to go with with the C standard here. Unfortunately, although I have a C++ standard reference, I don't have access to the C standard. The C Standard has exactly the same requirement. An array must be contiguous (no padding), separate members do not have to be. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Wu Yongwei Guest Posted: Thu Dec 23, 2004 3:22 pm    Post subject: Re: size of char array vs chars Bo Rydberg wrote: Quote: If I had a structure that contained 128 individual consecutive char elements are they guaranteed to lay out in memory the same as an array of 128 chars? It could be but this is compiler setup dependent. You need to tell your compiler the struct member alignment. Usually this is 2, 4, or 8-byte depending on the environment you are working in. But it should also let you have a 1-byte alignment. I do not think this is correct. Struct alignment has nothing to do with it. Try it with your compiler, and you'll see the size of the above kind of struct won't change when the alignment changes. Alignment affects the size of the struct in the following case: struct foo { int n; char ch; }; The size will be 5 with alignment 1, 6 with alignment 2, and 8 with alignment 4 or 8 (assuming int has a size of 4). Best regards, Yongwei [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.