C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Pointer to member function - can it be done without using st          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Andrea Guest Posted: Wed Jan 14, 2004 11:02 pm    Post subject: Pointer to member function - can it be done without using st I have class that contains a member function that is to be implemented as a task. This task is a 'receive message' type of task that will pend until receipt of a message. It determines the message type and then call the appropriate function in the appropriate class to process the message. I need to get a pointer to this member function but do not want to have to declare my pointer as static. Actually, if someone can help me out here with an understand of what that would mean relative to my example. I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. What if that operation calls another operation in another class. Does that other operation have to be static? Is it enough that if I only ever have one instance of this task running, just don't let it modify any data members in the class directly. Use get/set routines, or would those have to be static too. Does that make sense? Thanks so much in advance. (PS: In case it isn't totally obvious, I am new to C++. As such, I am somewhat intimidated by some C++ syntax. No offense will be taken if you reply with simple syntax and, if possible, comment well! :-) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Birbacher Guest Posted: Thu Jan 15, 2004 10:32 am    Post subject: Re: Pointer to member function - can it be done without usin Hi! Andrea wrote: Quote: I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. What if that operation calls another operation in another class. Does that other operation have to be static? Look at this: struct Foo { static int i; int j; static void setFoosI(int i_) { i = i_; //j = i_; //not allowed, j is not static } static void setFoosJ(Foo *this_, int j_) { i = j_; //is allowed this_->j = j_; //j is not static, but we have an object } //this last method is the whole idea of member functions: //you can rewrite the above as: void setJ(int j_) { i = j_; //is allowed this->j = j_; } }; So if you have an object (or pointer to an object), then you can access non static data even from static methods. Frank [ 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 (nee Spang Guest Posted: Thu Jan 15, 2004 3:08 pm    Post subject: Re: Pointer to member function - can it be done without usin Good morning, Andrea! Andrea schrieb: [snip] Quote: I need to get a pointer to this member function but do not want to have to declare my pointer as static. Actually, if someone can help me out here with an understand of what that would mean relative to my example. I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. What if that operation calls another operation in another class. Does that other operation have to be static? The problem is, that static has several meaning, which depend on the context, the corresponding keyword is used in. The sentence "[..] but books have eluded to the fact that data that a static function operates on needs to be static also." is plainly wrong, independent on the meaning of static. What comes next to that description is a static member function, which has no access to the this pointer, so it can use only its arguments and globally accessible objects or functions. There are no operational limitations of the static member function static_foo and the non-static member function foo in the example below: class A { void foo() const; // Non-static member function static void static_foo(const A& local_this); // Static member function }; because static_foo can use its argument local_this similar to non-static member functions (implicitely) using the this pointer. Differences come into play, if you consider virtuality of functions and the type of function pointers here, but you have the possibility to get either a "normal" function to a static member function or a member function pointer to a non-static member function. Quote: Is it enough that if I only ever have one instance of this task running, just don't let it modify any data members in the class directly. Use get/set routines, or would those have to be static too. Does that make sense? I have difficulties understanding of your meaning of a "static pointer" mentioned above. Do you mean a static data member, which is a pointer in this case to reach a singleton-like behaviour? (Singletons are objects which exists exacly once in a program, in a thread, in a .. whatever). If you use such a singleton, you can call its member functions and that's it. In this case the singleton object saves the complete state its member functions act on. Quote: (PS: In case it isn't totally obvious, I am new to C++. As such, I am somewhat intimidated by some C++ syntax. No offense will be taken if you reply with simple syntax and, if possible, comment well! Each of us has once started learning C++, so I think that everyone should understand your position. Unfortunately, I am missing somewhat more information to understand your actual problem... If you can be somewhat more precise concerning your requirements I am sure, I can provide somemore useful advice. Greetings from Bremen, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Robert Kindred Guest Posted: Fri Jan 16, 2004 2:50 am    Post subject: Re: Pointer to member function - can it be done without usin "Andrea" <forwork (AT) comcast (DOT) net> wrote Quote: I have class that contains a member function that is to be implemented as a task. This task is a 'receive message' type of task that will pend until receipt of a message. It determines the message type and then call the appropriate function in the appropriate class to process the message. I can't tell for sure, but it sounds like you are using your operating system to 'activate' a member function as a thread. This member function must be static, because the operating system makes a C-style call to your function, which will not have the invisible 'this' parameter in the call. To get the 'this' in there anyway depends on which operating system you are using. Quote: I need to get a pointer to this member function but do not want to have to declare my pointer as static. Actually, if someone can help me out here with an understand of what that would mean relative to my example. I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. What if that operation calls another operation in another class. Does that other operation have to be static? It is really nice having a task or a thread treated as an instantiated class in your program. The ACE libraries pull this off. You might look in their source to see how they do it. <snip> Quote: Thanks so much in advance. hth, Robert Kindred [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Fri Jan 16, 2004 2:57 am    Post subject: Re: Pointer to member function - can it be done without usin Andrea wrote: Quote: I have class that contains a member function that is to be implemented as a task. Do you mean that it will be the main function of a thread? Quote: This task is a 'receive message' type of task that will pend until receipt of a message. It determines the message type and then call the appropriate function in the appropriate class to process the message. I need to get a pointer to this member function but do not want to have to declare my pointer as static. Supposing the thread API has this thread creation function: handle start_thread(int (* func)(void *), void * arg); then you can have it call your non-static member function using a wrapper function: class message_processor { void start() { thread_handle_ = start_thread(run_wrapper, this); if (!thread_handle_) throw ...; } // Wrapper function - does the necessary pointer conversion // then calls the required function. static int run_wrapper(void * obj) { return static_cast<foo *>(obj)->run(); } // The function you're really interested in. int run(); handle thread_handle_; ... }; If the thread creation function doesn't take an argument that it passes into the thread function, then it's broken! Quote: Actually, if someone can help me out here with an understand of what that would mean relative to my example. I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. A static member function does not operate on an instance of its class (it does not have an implicit "this" parameter) so there are no non-static member variables for it. Quote: What if that operation calls another operation in another class. Does that other operation have to be static? No. If it can get a pointer to a class instance from a static variable, an explicit parameter or "new" then it can use that to call non-static functions. Quote: Is it enough that if I only ever have one instance of this task running, just don't let it modify any data members in the class directly. Use get/set routines, or would those have to be static too. Does that make sense? snip If the thread creation function doesn't take the extra argument, and if only one instance needs to run, then you can use a static variable in place of the function argument: class message_processor { void start() { thread_handle_ = start_thread(run_wrapper); if (!thread_handle_) throw ...; } static int run_wrapper() { return instance_->run(); } int run(); handle thread_handle_; // The class will be a singleton, and this will point // to its instance. static message_processor * instance_; ... }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrea Guest Posted: Fri Jan 16, 2004 2:59 am    Post subject: Re: Pointer to member function - can it be done without usin Thanks Frank for answering so quickly. But, either I mis-stated my problem or I'm not understanding your solution. This isn't complete but an idea of what I'm trying to do: (BTW, we're using Greenhills/Integrity) ---------- class Message_Receiver { public: Message_Receiver(); ~Message_Receiver(); void message_receiver_task(); } ---------- void Message_Receiver::message_receiver_task() { while (true) { message = wait_for_message switch (message.ID) { case 1: Case1MessageClass::processCase1Message(); break; case 2: Case2MessageClass::processCase2Message(); break; Back to top Xenos Guest Posted: Fri Jan 16, 2004 1:03 pm    Post subject: Re: Pointer to member function - can it be done without usin "Andrea" <forwork (AT) comcast (DOT) net> wrote Quote: I have class that contains a member function that is to be implemented as a task. This task is a 'receive message' type of task that will pend until receipt of a message. It determines the message type and then call the appropriate function in the appropriate class to process the message. I need to get a pointer to this member function but do not want to have to declare my pointer as static. Actually, if someone can help me out here with an understand of what that would mean relative to my example. I understand what static means but books have eluded to the fact that data that a static function operates on needs to be static also. What if that operation calls another operation in another class. Does that other operation have to be static? Is it enough that if I only ever have one instance of this task running, just don't let it modify any data members in the class directly. Use get/set routines, or would those have to be static too. Does that make sense? If the function your particular system uses to spawn the task allows it to give the task parameters, then you are in luck. You can create a static wrapper function that takes the this pointer as a parameter and use it to call the actual function member. In this case, what I usually do is define an abstract task class like such: class task { public: // whatever ctors that are appriopriate virtual ~task(); void start(); // uses OS specific call to spawn task via wrapper, passing it the this pointer virtual func() = 0; // actual task function, override to provide functionality private: static void wrapper(task* p) {p->func();}// actual function spawned by OS }; For each task you need, define a child class of task and override func. You may have to use different parameter type for the wrapper depending on your system (i.e., a lot of systems only pass integers to the task function, so you will have to make use of reinterpret_cast. You will also have to think about such things as whether the destuctor should terminate the task (including if it has already died or is unresponsive), protecting the object from copy construction and assignment, etc. It is also important NOT to spawn the task from the constructor, because the task may start execution and possibly use class data before construction is complete in the case of childern of class task! Hope this helps, DrX [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Mang Guest Posted: Fri Jan 16, 2004 6:07 pm    Post subject: Re: Pointer to member function - can it be done without usin Andrea schrieb: Quote: I need to have a pointer to my task (pointer to member function message_receiver_task). According to "Effective C++", pointers to member functions need to be declared as static. Where did you read this (Chapter / Item)? Maybe this is a cause of confusion? regards, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] 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 (Moderated) 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.