C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  write bool to ostream Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Mehturt@gmail.com Guest Posted: Wed Oct 19, 2005 8:50 am    Post subject: write bool to ostream Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. Thanks.. m. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stefan Näwe Guest Posted: Thu Oct 20, 2005 3:51 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; bool b=true; // always initialize. Quote: // ofstream open(), etc. of<<"b="< of << "b=" << (b?"true":"false") << 'n'; -- Stefan Naewe naewe.s_AT_atlas_DOT_de [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Thu Oct 20, 2005 10:35 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? Yes, accessing the value of a uninitialised variable yields undefined behaviour. Uli [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Thu Oct 20, 2005 11:15 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of << "b=" << b << 'n'; to print anything else than 0 or 1? Yes, sure. Quote: I am getting random integer numbers; using gcc 4.0.2. You are invoking UB (Undefined Behavior) by writing a value that was not initialized. Whatever happened to be on the stack location occupied by b, is being displayed. The UB *might* be more predictable if you do something like this: bool b = false; *(reinterpret_cast std::cout << "b=" << b << std::endl; On my system this consistently writes the value 2... there's nothing in the standard that guarantees this. Initialize b to false, and see if you stop getting "Random integer numbers". [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Markus Moll Guest Posted: Thu Oct 20, 2005 11:15 am    Post subject: Re: write bool to ostream Hi [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. Yes, as the behaviour is undefined. Markus [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler Guest Posted: Thu Oct 20, 2005 11:19 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. This snipped is an example of a program causing undefined behaviour (UB). Reason is: You are trying to read a non-initialized variable (b). The consequences of UB are neither guaranteed to be documentable nor to be reproducible. One of the pictorial examples is that a compliant compiler is allowed to create code, which formats your harddisc or writes an email to your boss... So: Don't do that! Btw.: Although the standard allows practically everything as a possible result of UB, it mentiones your example in a footnote: "Using a bool value in ways described by this International Standard as ‘‘undefined,’’ such as by examining the value of an uninitialized automatic variable, might cause it to behave as if it is neither true nor false." After setting an initial value to b, the output should be either 0 or 1 depending on the actual value of b. Greetings from Bremen, Daniel Krügler [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sebastian Redl Guest Posted: Thu Oct 20, 2005 11:20 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. You didn't initialize your boolean. Reading it results in undefined behaviour - in your case, in the bool having a value that shouldn't be possible. -- Sebastian Redl [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nicola Musatti Guest Posted: Thu Oct 20, 2005 11:22 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. Yes, because b is not initialized. See the standard, 3.9.1-6, where note 42 says: "Using a bool value in ways described by this International Standard as ''undefined,'' such as by examining the value of an uninitialized automatic variable, might cause it to behave as if is neither true nor false." Cheers, Nicola Musatti [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gene Bushuyev Guest Posted: Thu Oct 20, 2005 11:23 am    Post subject: Re: write bool to ostream <Mehturt (AT) gmail (DOT) com> wrote Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. Because you are invoking undefined behavior. Your bool is not initialized. As to printing, you can print anything you want: of << (b ? "good day" : "the sky is falling"); -- Gene ---------------------------------------------------------------- There is no greatness where there is no simplicity, goodness and truth. ~ Leo Tolstoy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ferdi Smit Guest Posted: Thu Oct 20, 2005 11:24 am    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? I am getting random integer numbers; using gcc 4.0.2. You can use the std::boolalpha manipulator. Ie. std::cout << std::boolalpha << (1 == 0) << std::endl; will print "false". [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sumit Rajan Guest Posted: Thu Oct 20, 2005 11:25 am    Post subject: Re: write bool to ostream <Mehturt (AT) gmail (DOT) com> wrote Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? Sorry, I did not mean to say "Yes" in the previous post in this thread. Please disregard it. Thanks. Sumit. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Thu Oct 20, 2005 11:26 am    Post subject: Re: write bool to ostream "Mehturt (AT) gmail (DOT) com" <Mehturt (AT) gmail (DOT) com> writes: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< This reads an uninitialized variable. The program has thus undefined behavior. Anything can happen, including a program crash and the computer catching fire. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sumit Rajan Guest Posted: Thu Oct 20, 2005 11:26 am    Post subject: Re: write bool to ostream <Mehturt (AT) gmail (DOT) com> wrote Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? Yes. Quote: I am getting random integer numbers; using gcc 4.0.2. Possibly because you have not initialized the variable. Regards, Sumit. -- Sumit Rajan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mehturt@gmail.com Guest Posted: Thu Oct 20, 2005 11:26 am    Post subject: Re: write bool to ostream Ok, I have found the problem.. I thought I have initialized the bool variable in a function called before the output to ostream, but I did not pass by reference as I intended to.. Therefore the bool variable was uninitialized.. [ 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: Thu Oct 20, 2005 12:22 pm    Post subject: Re: write bool to ostream [email]Mehturt (AT) gmail (DOT) com[/email] wrote: Quote: Is it possible for: std::ofstream of; bool b; // ofstream open(), etc. of<<"b="< to print anything else than 0 or 1? It's possible for it to do just about anything, including format your hard disk. Reading an uninitialized variable is undefined behavior, and all bets are off. Quote: I am getting random integer numbers; using gcc 4.0.2. Technically, they're within their rights. From a quality of implementation point of view: on one hand, I'm inclined to say that you should just get 0 or 1, regardless. On the other, the problem only occurs if you have an error in your code, and the earlier you detect the error, the better. Perhaps the best solution would be an assertion failure, with what g++ is doing second best. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre 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. 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