C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Re: why is a bool 4 bytes in g++? Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message stephan beal Guest Posted: Thu Jul 17, 2003 4:25 pm    Post subject: Re: why is a bool 4 bytes in g++? Jakob Bieling wrote: Quote: If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? The bool members are the same size, either way (unless there's a specialization for vector<bool>, i suppose, which accomodates them in bits). i'll admit that vector is generally a better choice, i'm just curious why you say it would take less space. -- ----- stephan beal Registered Linux User #71917 http://counter.li.org I speak for myself, not my employer. Contents may be hot. Slippery when wet. Reading disclaimers makes you go blind. Writing them is worse. You have been Warned. Back to top Ron Natalie Guest Posted: Thu Jul 17, 2003 4:32 pm    Post subject: Re: why is a bool 4 bytes in g++? "stephan beal" <stephan (AT) wanderinghorse (DOT) net> wrote Quote: Jakob Bieling wrote: If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? Because vector<bool> is a specializtion of vector. Rather than just making an array of bool types like vector does normally, it packs them together. Of course, some of the vector semantics behave oddly as a result. Back to top Jakob Bieling Guest Posted: Thu Jul 17, 2003 4:42 pm    Post subject: Re: why is a bool 4 bytes in g++? "stephan beal" <stephan (AT) wanderinghorse (DOT) net> wrote Quote: Jakob Bieling wrote: If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? The bool members are the same size, either way (unless there's a specialization for vector<bool>, i suppose, which accomodates them in bits). i'll admit that vector is generally a better choice, i'm just curious why you say it would take less space. "The vector<bool> Class is a full specialization of the template class vector for elements of type bool with an allocator for the underlying type used by the specialization. "The vector<bool> reference Class is a nested class whose objects are able to provide references to elements (single bits) within a vector<bool> object." This is an excerpt from the documentation of the implementation I use. There is also info about this in the Standard (23.2.5). In other words, there is a specialization for bool that uses the space efficiently, like you guessed already. regards -- jb (replace y with x if you want to reply by e-mail) Back to top Roel Schroeven Guest Posted: Thu Jul 17, 2003 5:17 pm    Post subject: Re: why is a bool 4 bytes in g++? Tim Partridge wrote: Quote: Sorry to ask a question that I'm sure has been answered, but I had trouble finding it. Why would a bool not be 1 bit, or at most 1 byte? Is it for reasons of instruction efficiency? In g++ 2.95.2, a bool is 4 bytes and a char is 1 byte. I have to store a 90,000 entry 2D array of bools. Would it not make more sense for me to store chars instead of bools, saving me 270,000 bytes? Not here though: $ for cpp in /usr/bin/g++-* ; do $cpp sizeof.cpp -o sizeof-$(basename $cpp) ; done $ for s in sizeof-* ; do echo $s; ./$s; done sizeof-g++-2.95 bool: 1 byte(s) sizeof-g++-3.0 bool: 1 byte(s) sizeof-g++-3.2 bool: 1 byte(s) sizeof-g++-3.3 bool: 1 byte(s) (2.95 is 2.95.4 here, but it would surprise there would be a difference between 2.95.2 and 2.95.4 in that respect) I suppose consecutive elements are aligned at 4-byte boundaries though, for efficiency reasons: it's most efficient for a processor to operate at its native word length, which is 4 bytes for 386-style processors. See the other responses for alternative ways to handle large numbers of bools. -- "Codito ergo sum" Roel Schroeven Back to top David Hall Guest Posted: Thu Jul 17, 2003 9:04 pm    Post subject: Re: why is a bool 4 bytes in g++? If I recall correctly, bool is implemented as enum bool {false =0, true =1}; and enums, by default, are of size int, which is 4 bytes normally. Back to top Jakob Bieling Guest Posted: Thu Jul 17, 2003 9:12 pm    Post subject: Re: why is a bool 4 bytes in g++? "David Hall" <dlwh (AT) aol (DOT) com> wrote Quote: If I recall correctly, bool is implemented as enum bool {false =0, true =1}; and enums, by default, are of size int, which is 4 bytes normally. In C++ bool is a built-in type. No enum needed. -- jb (replace y with x if you want to reply by e-mail) Back to top Ron Natalie Guest Posted: Thu Jul 17, 2003 9:29 pm    Post subject: Re: why is a bool 4 bytes in g++? "David Hall" <dlwh (AT) aol (DOT) com> wrote Quote: If I recall correctly, bool is implemented as enum bool {false =0, true =1}; and enums, by default, are of size int, which is 4 bytes normally. You don't recall correctly. bool is a fundemental type in C++ (as is _Bool in C99). Further, enums are not, by default the sizeof int. The rule just says they won't be bigger than int unless you have enumerators outside the range that int can support. Further, it's a bad assumption that sizeof(int) == 4. Back to top E. Robert Tisdale Guest Posted: Thu Jul 17, 2003 10:08 pm    Post subject: Re: why is a bool 4 bytes in g++? Tim Partridge wrote: Quote: Sorry to ask a question that I'm sure has been answered, but I had trouble finding it. Why would a bool not be 1 bit, or at most 1 byte? Is it for reasons of instruction efficiency? In g++ 2.95.2, a bool is 4 bytes and a char is 1 byte. I have to store a 90,000 entry 2D array of bools. Would it not make more sense for me to store chars instead of bools, saving me 270,000 bytes? cat bool.cc #include int main(int argc, char* argv[]) { bool array[33]; std::cout << sizeof(bool) << " = sizeof(bool)" << std::endl; std::cout << sizeof array << " = sizeof array" << std::endl; return 0; } Quote: g++ -Wall -ansi -pedantic -O2 -o bool bool.cc ./bool 1 = sizeof(bool) 33 = sizeof array Quote: g++ --version g++ (GCC) 3.2 20020903 (Red Hat Linux 8.0 3.2-7) You should upgrade your compiler. Back to top David Hall Guest Posted: Fri Jul 18, 2003 1:16 am    Post subject: Re: why is a bool 4 bytes in g++? "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "David Hall" <dlwh (AT) aol (DOT) com> wrote If I recall correctly, bool is implemented as enum bool {false =0, true =1}; and enums, by default, are of size int, which is 4 bytes normally. You don't recall correctly. bool is a fundemental type in C++ (as is _Bool in C99). Further, enums are not, by default the sizeof int. The rule just says they won't be bigger than int unless you have enumerators outside the range that int can support. Further, it's a bad assumption that sizeof(int) == 4. Well then, I stand corrected. I just remember something somewhere about implementation being different from specification. And my enumerated types always seem to be sizeof(int),* though I haven't tried for that many enumerated types before. *As to sizeof(int), "normally" means well, normally, or rather the type of system one finds on desktops and the one that he is probably using. *ducks from objects flung by embedded and 64-bit programmers* Back to top Evan Guest Posted: Fri Jul 18, 2003 2:11 am    Post subject: Re: why is a bool 4 bytes in g++? stephan beal <stephan (AT) wanderinghorse (DOT) net> wrote Quote: Jakob Bieling wrote: If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? The bool members are the same size, either way (unless there's a specialization for vector<bool>, i suppose, which accomodates them in bits). i'll admit that vector is generally a better choice, i'm just curious why you say it would take less space. The other replies to this are only half right... Yes, vector<bool> is a specialization of vector<T> that the standard created in order to make it possible to pack, say, 8 bools into one byte. However, the standard does not require this to be done, and at least accoring to some resources (e.g. More Exceptional C++ by Sutter, a compilation of expanded versions of his Guru of the Week) many implementations do not make this optimization. Back to top Jakob Bieling Guest Posted: Fri Jul 18, 2003 8:09 am    Post subject: Re: why is a bool 4 bytes in g++? "Evan" <eed132 (AT) psu (DOT) edu> wrote Quote: stephan beal <stephan (AT) wanderinghorse (DOT) net> wrote Jakob Bieling wrote: If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? The bool members are the same size, either way (unless there's a specialization for vector<bool>, i suppose, which accomodates them in bits). i'll admit that vector is generally a better choice, i'm just curious why you say it would take less space. The other replies to this are only half right... Yes, vector<bool> is a specialization of vector<T> that the standard created in order to make it possible to pack, say, 8 bools into one byte. However, the standard does not require this to be done, and at least accoring to some resources (e.g. More Exceptional C++ by Sutter, a compilation of expanded versions of his Guru of the Week) many implementations do not make this optimization. If the Standard says that vector <bool> is provided to optimize space allocation, then I can be sure that a) a specialization exists and b) that specialization will occupy less memory than a vector <T> for T = bool (i.e. the non-specialized version). Further, the Standard says that vector <bool>::reference "is a class that simulates the behavior of references of a single bit in vector<bool>". regards -- jb (replace y with x if you want to reply by e-mail) Back to top Tim Partridge Guest Posted: Fri Jul 18, 2003 2:50 pm    Post subject: Re: why is a bool 4 bytes in g++? Actually, the matrix of bools I have to store is an upper triangular matrix. For anyone who might read this later, I found this site: http://www.nist.gov/dads/HTML/uppertriangm.html which describes the how to convert an upper triangular matrix to a 1D array. Entry (i,j) in the matrix corresponds to entry (2n-i)(i-1)/2 + j in the array, where n is the number of rows (or columns), and 1 <= i,j <= n. On Thu, 17 Jul 2003, Jakob Bieling wrote: Quote: "Tim Partridge" news:Pine.SOL.4.44.0307171203470.6965-100000 (AT) lassar (DOT) math.uwaterloo.ca... Sorry to ask a question that I'm sure has been answered, but I had trouble finding it. Why would a bool not be 1 bit, or at most 1 byte? Is it for reasons of instruction efficiency? In g++ 2.95.2, a bool is 4 bytes and a char is 1 byte. I have to store a 90,000 entry 2D array of bools. Would it not make more sense for me to store chars instead of bools, saving me 270,000 bytes? If you are after saving space, you should go for a vector< vector bool> >. And if you want to save even more, then just use a single vector bool> and calculate the 2D offsets yourself (not too difficult). Actually, I would even prefer the vector <bool> over vector< vector . hth -- jb (replace y with x if you want to reply by e-mail) Back to top Tim Partridge Guest Posted: Mon Jul 21, 2003 4:11 pm    Post subject: Re: why is a bool 4 bytes in g++? On Fri, 18 Jul 2003, Tim Partridge wrote: *snip* Quote: which describes the how to convert an upper triangular matrix to a 1D array. Entry (i,j) in the matrix corresponds to entry (2n-i)(i-1)/2 + j in the array, where n is the number of rows (or columns), and 1 <= i,j <= n. *snip* Note the additional restriction of i >= j. Back to top Frank Schmitt Guest Posted: Tue Jul 22, 2003 7:56 am    Post subject: Re: why is a bool 4 bytes in g++? Tim Partridge <tjpartri (AT) lassar (DOT) math.uwaterloo.ca> writes: Quote: Sorry to ask a question that I'm sure has been answered, but I had trouble finding it. Why would a bool not be 1 bit, or at most 1 byte? Is it for reasons of instruction efficiency? In g++ 2.95.2, a bool is 4 bytes and a char is 1 byte. I have to store a 90,000 entry 2D array of bools. Would it not make more sense for me to store chars instead of bools, saving me 270,000 bytes? Apart from the other answers, you might want to have a look at std::bitset iff your matrix has a fixed size. I don't know if g++ 2.95.2 already had it, though. HTH & kind regards frank -- Frank Schmitt 4SC AG phone: +49 89 700763-0 e-mail: frank DOT schmitt AT 4sc DOT com Back to top Christoph Rabel Guest Posted: Wed Jul 30, 2003 8:41 am    Post subject: Re: why is a bool 4 bytes in g++? stephan beal wrote: Quote: Forgive my ignorance, but why would a vector<bool> use less space than (e.g.) an array of bool? The bool members are the same size, either way (unless there's a specialization for vector<bool>, i suppose, which accomodates them in bits). There is a specialization vector<bool>. And this specialization stores bits not bools. Christoph 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 (comp.lang.c++) All times are GMT Goto page 1, 2  Next Page 1 of 2   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.