C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What's the best way to define global variable          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message David WOO Guest Posted: Mon Mar 01, 2004 10:24 am    Post subject: What's the best way to define global variable Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. I am wondering is this the best way to do it (regarding the convenience and safety), as this is such a fundamental thing, I believe most of you have a say based on your experiences. any comment or replay will be much appreciated. David. Back to top Karl Heinz Buchegger Guest Posted: Mon Mar 01, 2004 12:43 pm    Post subject: Re: What's the best way to define global variable David WOO wrote: Quote: Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. A singleton surfes a completely different problem. Quote: I am wondering is this the best way to do it (regarding the convenience and safety), The best way is to not have the global variables at all. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Buster Guest Posted: Mon Mar 01, 2004 12:47 pm    Post subject: Re: What's the best way to define global variable David WOO wrote: Quote: Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. I am wondering is this the best way to do it (regarding the convenience and safety), as this is such a fundamental thing, I believe most of you have a say based on your experiences. any comment or replay will be much appreciated. David. Split the global data into categories. Declare a struct to store each category's data. In your classes, store a const reference to each struct whose data the class will need to access. The actual objects can be created in main () and passed to your class constructors. If you think you might accidentally instantiate any of these structs more than once, perhaps while you're sleeping, then the singleton pattern is for you; otherwise I don't think you need to bother with it. This isn't the best way to design classes, but I hope it helps. Regards, Buster. Back to top Julie Guest Posted: Mon Mar 01, 2004 4:32 pm    Post subject: Re: What's the best way to define global variable And with a response like that, we have one less newbie to worry about. Karl Heinz Buchegger wrote: Quote: David WOO wrote: Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. A singleton surfes a completely different problem. I am wondering is this the best way to do it (regarding the convenience and safety), The best way is to not have the global variables at all. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Karl Heinz Buchegger Guest Posted: Tue Mar 02, 2004 1:24 pm    Post subject: Re: What's the best way to define global variable Julie wrote: Quote: And with a response like that, we have one less newbie to worry about. 2 things * please don't top post. Thank you. * If you know better then me, then why haven't you provided an answer on your own? -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Gary Guest Posted: Tue Mar 02, 2004 2:27 pm    Post subject: [OT] How to read posts (based on What's the best way to defi "Karl Heinz Buchegger" <kbuchegg (AT) gascad (DOT) at> wrote Quote: Julie wrote: And with a response like that, we have one less newbie to worry about. 2 things * please don't top post. Thank you. * If you know better then me, then why haven't you provided an answer on your own? When I read this post (quickly, and the first time) I said to myself: "Boy is Karl getting cranky. There was no need to answer like that." Then I read it again. After a little thought I said to myself: "Right on. That is surely a reasonable response to the interchange." What I think I learned from this is: Without the personal reactions of body language, facial response, voice tone, etc. just about anything written can be taken two ways --- as straight and helpful, or as angry and criticizing. It was the pause and rereading that let me see that my first impulse wasn't the only, and maybe not even the right, reaction. Being too lazy to look it up (!) I now ask: does or should the FAQ contain the advice to read responses that make you angry at least twice with an eye toward understanding what the response might mean in a more reasonable light? I'm sure general netiquette FAQ's do. -- Gary Back to top Julie Guest Posted: Tue Mar 02, 2004 4:47 pm    Post subject: Re: What's the best way to define global variable I don't recall saying that I knew better than you, perhaps you can point it out to me. I don't have an answer for the op. Karl Heinz Buchegger wrote: Quote: Julie wrote: And with a response like that, we have one less newbie to worry about. 2 things * please don't top post. Thank you. * If you know better then me, then why haven't you provided an answer on your own? -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Thomas Matthews Guest Posted: Tue Mar 02, 2004 5:31 pm    Post subject: Re: What's the best way to define global variable Julie wrote: Quote: And with a response like that, we have one less newbie to worry about. Karl Heinz Buchegger wrote: David WOO wrote: Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. A singleton surfes a completely different problem. I am wondering is this the best way to do it (regarding the convenience and safety), The best way is to not have the global variables at all. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Julie, Karl was not insulting the OP. One of the credos of programming is to have no global variables. Many programmers work very hard to eliminate global variables. Global variables a a huge source of problems in large projects. Encapsulation, data hiding and Singletons are solutions to the problems imposed by global variables. Global variables become very dangerous in multi-tasking and multiprocessor applications. Search the Web for "mutex", "resource locking" and "signals" for some techniques. In some code shops, global variables must go through rigorous testing. The policing agencies that review the code of asked "have you tested every piece of code that access the variable to make sure there are no collisions?" That one takes a lot of work. After a couple of times of researching and supplying proof to those questions, one learns not to use globals. In summary, globals are evil. Most applications can be developed without any global variables. Those that can't are subject to rigorous testing. -- 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 Ron Natalie Guest Posted: Tue Mar 02, 2004 7:46 pm    Post subject: Re: What's the best way to define global variable "Thomas Matthews" <Thomas_MatthewsSpitsOnSpamBots (AT) sbcglobal (DOT) net> wrote Quote: Karl was not insulting the OP. One of the credos of programming is to have no global variables. Must be one of those fundamentalist religions. Back to top David WOO Guest Posted: Wed Mar 03, 2004 12:13 pm    Post subject: Re: What's the best way to define global variable As the original poster of "What's the best way to define global variable", I thank all of you who contributed to the post. As I started in the beginning of the first message that I am a newbie, As a matter of fact, I am trying to understand your personal views, in the meantime, I am trying to find a solution as well. Buster's response does give me some straight forward solutions when I solve simple problems, which will likely be used in my current work. In the meantime, I understand the point of view about "The best way is not have global variable at all", but it will work in the price of sacrifying the convenience under some situations. At the moment, I still can not understand the point put forward by Karl regarding "singleton surfes a complete different problem", as far as I understnad, the singleton can functions like a static class in Java, therefore, I works like a global, and likely it can be a solution to this problem, anyone knows about why it is not appropriate, could you please give some comments on this. Thanks again to everyone. David. Back to top Howie Guest Posted: Thu Mar 04, 2004 6:57 am    Post subject: Re: What's the best way to define global variable On Mon, 1 Mar 2004 10:24:58 -0000, "David WOO" <ixuw (AT) yahoo (DOT) com> wrote: Quote: Hi, I am a newbie on C++, I need to define some global variables which should be accessible to most classes. In the mean time, I don't won't the global variables be modified freely at most of these classes. I know there is a pattern called singleton can more or less do such a trick. I am wondering is this the best way to do it (regarding the convenience and safety), as this is such a fundamental thing, I believe most of you have a say based on your experiences. any comment or replay will be much appreciated. David. I my old ways i uses global variables a lot. But to avoid them there a a lot of strategies to do so. Look in the book: "Large Scale C++ Software Design" from Lakos. Instead of global Variables i would use: //*.h class CXX { static long aGlobal; public: static void SetaGlobla(long a){ aGlobal = a;} static long GetaGlobal() { return aGlobal;} }; //*.cpp long CXX::aGlobal= -1; /////////////// // use them like this: CXX::SetaGlobal(7); long x = CXX::GetaGlobla(); Howie Back to top Karl Heinz Buchegger Guest Posted: Fri Mar 05, 2004 2:17 pm    Post subject: Re: What's the best way to define global variable David WOO wrote: Quote: As the original poster of "What's the best way to define global variable", I thank all of you who contributed to the post. As I started in the beginning of the first message that I am a newbie, As a matter of fact, I am trying to understand your personal views, in the meantime, I am trying to find a solution as well. Buster's response does give me some straight forward solutions when I solve simple problems, which will likely be used in my current work. In the meantime, I understand the point of view about "The best way is not have global variable at all", but it will work in the price of sacrifying the convenience under some situations. That's hard to believe. Global variables are almost always, and I really believe what I write here, *almost always*, a source of a problem. You can eliminate them easily by passing things around, at least that's what function arguments are for and why they were invented. Quote: At the moment, I still can not understand the point put forward by Karl regarding "singleton surfes a complete different problem", as far as I understnad, the singleton can functions like a static class in Java, The purpose of a singleton is to make sure, that a specific object exists only once (or a specific number of times) in your whole system. The purpose of a singleton is *not* to be a replacement of global variables. It can be used like that, but by doing so you have gained nothing against the evilness of global variables. A singleton simply doesn't solve the problems which make global variables evil, that is: can be accessed and, more important, changed by any lousy function in your system. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Julie Guest Posted: Fri Mar 05, 2004 4:21 pm    Post subject: Re: What's the best way to define global variable Karl Heinz Buchegger wrote: Quote: Global variables are almost always, and I really believe what I write here, *almost always*, a source of a problem. You can eliminate them easily by passing things around, at least that's what function arguments are for and why they were invented. I gotta tell you that blanket statements can do much more harm than good. I wish that we all could get into the habit of justifying statements w/ authoritive sources or real-world experiences. If you don't have sources to attribute to, say that 'in my opinion' or 'personal experience has shown' and then go off and explain why you feel that way. --- In my experience, globals can be the source of problems, bugs, race conditions, etc. when implemented and used improperly or in a non-standard way without supporting documentation and pre-meditated design. One of the biggest problems that I've seen is when globals are used to hold read/write data -- it comes down to what can/should write to the global, and how are potential readers notified by the change? A global that changes value a lot, or is nothing more than a lazy way to get out of passing parameters should definitely be a cause for concern and a thorough code review. I've seen and used cases where a global (read-only) variable makes perfect sense, and is appropriate. For instance, some operating systems have a notion of a process or instance identifier that is unique for that process. That identifier is passed into main on program startup, but is otherwise unavailable. Storing the value of the process id in a global variable can be justified, but I'd stress that you attempt to guard it from modification by making access to it read-only. In this case, problems can arise if you inadvertantly write to it, which can be much easier than it seems if you are using equality such as if (global_instance = 123) when if (global_instance == 123) was intended. I'm sure that Karl was speaking of this type of bug which can be difficult at best to narrow down. There are numerous authorative resources out there on the subject of globals. Research and understand the ramifications of global now while you are still learning as it can be much harder to change bad habits down the road. 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