C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What are the header defines for?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Marco Aschwanden Guest Posted: Mon Dec 29, 2003 10:53 am    Post subject: What are the header defines for? I am re-learning c++. And I see lots of code. All the time I stumble over the following pattern (in header files): #ifndef some_filename_h #define some_filename_h .. .. .. #endif What is this "filename" define good for? I found this pattern in many examples and in many books, but it is nowhere explained what it is good for? Can anyone explain this to me? Thanks a lot, Marco Back to top Sam Holden Guest Posted: Mon Dec 29, 2003 11:00 am    Post subject: Re: What are the header defines for? On 29 Dec 2003 02:53:19 -0800, Marco Aschwanden <PPNTWIMBXFFC (AT) spammotel (DOT) com> wrote: Quote: I am re-learning c++. And I see lots of code. All the time I stumble over the following pattern (in header files): #ifndef some_filename_h #define some_filename_h . . . #endif What is this "filename" define good for? I found this pattern in many examples and in many books, but it is nowhere explained what it is good for? Can anyone explain this to me? They are include guards and are used so that a header being included multiple times doesn't cause compilation errors (by defining something twice, for example). -- Sam Holden Back to top John Carson Guest Posted: Mon Dec 29, 2003 11:53 am    Post subject: Re: What are the header defines for? "Marco Aschwanden" <PPNTWIMBXFFC (AT) spammotel (DOT) com> wrote Quote: I am re-learning c++. And I see lots of code. All the time I stumble over the following pattern (in header files): #ifndef some_filename_h #define some_filename_h . . . #endif What is this "filename" define good for? I found this pattern in many examples and in many books, but it is nowhere explained what it is good for? Can anyone explain this to me? Thanks a lot, Marco Suppose that you #include two files, "a.h" and "b.h" into the file "filename.cpp". Suppose that "b.h" itself #includes "a.h". That means that "a.h" is being #included into "filename.cpp" twice: once directly and once indirectly via "b.h". The scheme you have described stops this from happening. The first time "a.h" is #included, some_filename_h is not yet defined. Accordingly #ifndef some_filename_h returns true, some_filename_h is then defined, and the code that follows is read. The second time "a.h" is #included, some_filename_h has already been defined, #ifndef some_filename_h returns false and execution skips to the #endif, so none of the header code is read. -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top EventHelix.com Guest Posted: Tue Dec 30, 2003 3:31 am    Post subject: Re: What are the header defines for? The following article explains duplicate inclusion and other rules related to header file inclusion in C++. http://www.eventhelix.com/RealtimeMantra/HeaderFileIncludePatterns.htm Sandeep -- http://www.EventHelix.com/EventStudio EventStudio 2.0 - Go Beyond UML Use Case and Sequence Diagrams 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.