C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  const functions and IO          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Mike P. Wagner Guest Posted: Fri Jun 27, 2003 7:13 pm    Post subject: const functions and IO I have a real world problem that I hope someone has solved. I implementing a class in a kernel that stores (and reads) data persistently. There is one underlying "raw IO" function that calls the device. This function cannot be const (unless I lie) since it changes persistent data. But that means that none of my read functions can be const, since they need to call the underlying "raw IO" function. Now that seems really silly. Is there a simple way to tell the compiler, "Look, when I call this function with read requests, it's const"? I know that I can goof around with typedefs and reinterpret_cast<> to do this, but it seems like there must be a more elegant way to do this. It's not an exotic problem, is it? Mike Back to top John Harrison Guest Posted: Fri Jun 27, 2003 7:28 pm    Post subject: Re: const functions and IO "Mike P. Wagner" <mikepwagner (AT) hotmail (DOT) com> wrote Quote: I have a real world problem that I hope someone has solved. I implementing a class in a kernel that stores (and reads) data persistently. There is one underlying "raw IO" function that calls the device. This function cannot be const (unless I lie) since it changes persistent data. But that means that none of my read functions can be const, since they need to call the underlying "raw IO" function. Now that seems really silly. Is there a simple way to tell the compiler, "Look, when I call this function with read requests, it's const"? I know that I can goof around with typedefs and reinterpret_cast<> to do this, but it seems like there must be a more elegant way to do this. It's not an exotic problem, is it? Mike Declare your persistent data to be mutable and declare the rawIO function const. Even const functions are allowed to change mutable data class X { mutable int data; }; mutable is useful on caches and the like, which if I understand you correctly is what you have. john Back to top Thomas Matthews Guest Posted: Fri Jun 27, 2003 7:28 pm    Post subject: Re: const functions and IO Mike P. Wagner wrote: Quote: I have a real world problem that I hope someone has solved. I implementing a class in a kernel that stores (and reads) data persistently. There is one underlying "raw IO" function that calls the device. This function cannot be const (unless I lie) since it changes persistent data. But that means that none of my read functions can be const, since they need to call the underlying "raw IO" function. Now that seems really silly. Is there a simple way to tell the compiler, "Look, when I call this function with read requests, it's const"? I know that I can goof around with typedefs and reinterpret_cast<> to do this, but it seems like there must be a more elegant way to do this. It's not an exotic problem, is it? Mike Most of the time, reading is not const and destroys things. If you want to make member function const and have it modify member variables, declare those variables as "mutable". -- Thomas Matthews C++ newsgroup welcome message: http://www.slack.net/~shiva/welcome.txt C++ Faq: http://www.parashift.com/c++-faq-lite C Faq: http://www.eskimo.com/~scs/c-faq/top.html alt.comp.lang.learn.c-c++ faq: http://www.raos.demon.uk/acllc-c++/faq.html Other sites: http://www.josuttis.com -- C++ STL Library book 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 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.