C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Where should global variables be placed ?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message vivekian Guest Posted: Sun Nov 27, 2005 7:37 pm    Post subject: Where should global variables be placed ? Have a couple of global variables of a class type which i have declared and defined in the application. 1. Not sure where should the global variables be placed ? 2. Would i need to forward declare the class before the global variable is declared ? Thanks, vivekian Back to top Phlip Guest Posted: Sun Nov 27, 2005 7:42 pm    Post subject: Re: Where should global variables be placed ? vivekian wrote: Quote: Have a couple of global variables of a class type which i have declared and defined in the application. 1. Not sure where should the global variables be placed ? In the .cpp file of their class type. Quote: 2. Would i need to forward declare the class before the global variable is declared ? Include the .h file of their class type at the top of that module. Then, before everyone here starts yelling about "global" variables, make them static variables of their class type, and see about making them private. -- Phlip http://www.greencheese.org/ZeekLand <-- NOT a blog!!! Back to top Moonlit Guest Posted: Sun Nov 27, 2005 7:45 pm    Post subject: Re: Where should global variables be placed ? Hi, Well apart from the fact that you should try to avoid global variables (sometimes they do come in handy however). I usually put them in the module containing main or winmain. Since I only use globals for things that really should be global (like a class to log error messages) I use "extern" in the header file of the global class and declare it in the main module. For instance for my logging class (usually the only global I use). -> Log.h class CLog { .... }; extern CLog Log; main.c CLog Log( "/tmp/program.log" ); int main() { } But this is just one way of doing it ofcourse. -- Regards, Ron AF Greve http://moonlit.xs4all.nl "vivekian" <viveklinux (AT) gmail (DOT) com> wrote Quote: Have a couple of global variables of a class type which i have declared and defined in the application. 1. Not sure where should the global variables be placed ? 2. Would i need to forward declare the class before the global variable is declared ? Thanks, vivekian Back to top vivekian Guest Posted: Sun Nov 27, 2005 7:54 pm    Post subject: Re: Where should global variables be placed ? Quote: Well apart from the fact that you should try to avoid global variables (sometimes they do come in handy however). I usually put them in the module containing main or winmain. Writing a small simulator which schedules tasks. In this case the pointer to the current running task descriptor is the global variable and should be visible across files. So as mentioned above i would have something like class task { .... } ; extern task * current ; Is making it global a good choice ? or is static fine ( have my doubts though since the variable is too be present across files ? thanks, vivekian Back to top vivekian Guest Posted: Sun Nov 27, 2005 8:46 pm    Post subject: Re: Where should global variables be placed ? Quote: I would not use a global for this. I would add a "context" object that is shared by other objects for this kind of thing. Not sure what a "context" object is :) Back to top roberts.noah@gmail.com Guest Posted: Sun Nov 27, 2005 8:47 pm    Post subject: Re: Where should global variables be placed ? vivekian wrote: Quote: Well apart from the fact that you should try to avoid global variables (sometimes they do come in handy however). I usually put them in the module containing main or winmain. Writing a small simulator which schedules tasks. In this case the pointer to the current running task descriptor is the global variable and should be visible across files. So as mentioned above i would have something like class task { ... } ; extern task * current ; Is making it global a good choice ? or is static fine ( have my doubts though since the variable is too be present across files ? A better design might be: class task { private: task * current; public: static task& current_task() { return *current; } // there better be one too... }; Now, you could make that code more robust by making sure there IS a current task but the point is that no global is required and the class itself keeps track of the things it should be tracking. One major benefit is that you can place the checks to see if there is a current task in ONE place instead of all over the damn place (any time you access the global pointer you would need to check). You could also have some sort of "task manager" class that maybe contains all the tasks to be performed and manages them in some way...it might be a better place to track the _current_ task. Your task manager could very well be a singleton. Back to top Gianni Mariani Guest Posted: Sun Nov 27, 2005 8:55 pm    Post subject: Re: Where should global variables be placed ? vivekian wrote: Quote: Have a couple of global variables of a class type which i have declared and defined in the application. Globals are Evil(TM). Okay okay, if they're read-only then they're not evil. If you MUST. Quote: 1. Not sure where should the global variables be placed ? For NON TEMPLATE variables. - Declared in header files, defined in non headers. For templates, you can define them in header files as well. Quote: 2. Would i need to forward declare the class before the global variable is declared ? You don't NEED to if you don't access it other than in the compilation unit you use it. If you don't put them in a header file, it's also good idea to place them in an anonymous namespace to avoid conflicts with other compilation units. Back to top Gianni Mariani Guest Posted: Sun Nov 27, 2005 9:40 pm    Post subject: Re: Where should global variables be placed ? vivekian wrote: Quote: extern task * current ; Is making it global a good choice ? or is static fine ( have my doubts though since the variable is too be present across files ? I would not use a global for this. I would add a "context" object that is shared by other objects for this kind of thing. Back to top Ian Guest Posted: Sun Nov 27, 2005 10:23 pm    Post subject: Re: Where should global variables be placed ? [email]roberts.noah (AT) gmail (DOT) com[/email] wrote: Quote: vivekian wrote: Well apart from the fact that you should try to avoid global variables (sometimes they do come in handy however). I usually put them in the module containing main or winmain. Writing a small simulator which schedules tasks. In this case the pointer to the current running task descriptor is the global variable and should be visible across files. So as mentioned above i would have something like class task { ... } ; extern task * current ; Is making it global a good choice ? or is static fine ( have my doubts though since the variable is too be present across files ? A better design might be: class task { private: task * current; You forgot 'static' here, static task* current; Ian Back to top roberts.noah@gmail.com Guest Posted: Sun Nov 27, 2005 10:32 pm    Post subject: Re: Where should global variables be placed ? Ian wrote: Quote: roberts.noah (AT) gmail (DOT) com wrote: vivekian wrote: Well apart from the fact that you should try to avoid global variables (sometimes they do come in handy however). I usually put them in the module containing main or winmain. Writing a small simulator which schedules tasks. In this case the pointer to the current running task descriptor is the global variable and should be visible across files. So as mentioned above i would have something like class task { ... } ; extern task * current ; Is making it global a good choice ? or is static fine ( have my doubts though since the variable is too be present across files ? A better design might be: class task { private: task * current; You forgot 'static' here, static task* current; I sure did. Thank you. 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