C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  help using function pointer at run-rime          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Jean-Daniel Gamache Guest Posted: Mon Sep 29, 2003 9:30 pm    Post subject: help using function pointer at run-rime Hi, I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? ie: char myFunctionName[] = "myFunction"; void myFunction () { } //this is the tricky part i am wondering how i could call // a function that is specified in my variable myFunctionName? functionPtr = GetFunctionAddress(myFunctionName); functionPtr(); thanks, JD Back to top Alf P. Steinbach Guest Posted: Mon Sep 29, 2003 9:33 pm    Post subject: Re: help using function pointer at run-rime On 29 Sep 2003 14:30:13 -0700, [email]jdgamache (AT) hotmail (DOT) com[/email] (Jean-Daniel Gamache) wrote: Quote: I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? C++ provides no direct facility for that. To do it using only standard C++ you'd have to make a table of all relevant functions, or generate code and call the compiler from your program. However, what is it that you're trying to achieve by doing that? Back to top Mike Wahler Guest Posted: Mon Sep 29, 2003 9:46 pm    Post subject: Re: help using function pointer at run-rime "Jean-Daniel Gamache" <jdgamache (AT) hotmail (DOT) com> wrote Quote: Hi, I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? You cannot. Once a C++ program is compiled, there are no function 'names' only memory addresses. Quote: ie: char myFunctionName[] = "myFunction"; This is just an array of characters containing a C style string. It has no connection with any function, unless you associate it with some function's address (e.g. with a std::map) Quote: void myFunction () { } //this is the tricky part i am wondering how i could call // a function that is specified in my variable myFunctionName? Change your variable's type to a pointer to a function, and assign or initialize it with the address of the desired function. void (*f)() = myFunction; Call it like this: f(); Quote: functionPtr = GetFunctionAddress(myFunctionName); functionPtr(); This could be implemented e.g. std::map<std::string, void (*)()> table; table["myFunction"] = myFunction; table["myFunction"](); /* function call */ Of course you'll need to add all the 'scaffolding' e.g. put this stuff into functions, provide the needed header(s) etc. -Mike Back to top Gianni Mariani Guest Posted: Mon Sep 29, 2003 10:13 pm    Post subject: Re: help using function pointer at run-rime Jean-Daniel Gamache wrote: Quote: Hi, I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? ie: char myFunctionName[] = "myFunction"; void myFunction () { } //this is the tricky part i am wondering how i could call // a function that is specified in my variable myFunctionName? functionPtr = GetFunctionAddress(myFunctionName); functionPtr(); The only portable way to do this is to place these in a registry. This whole discussion very quickly leads to factories. Let's start with somthig along the lines of what you did. #include <cstring> char myFunction1Name[] = "myFunction1"; void myFunction1 () { } char myFunction2Name[] = "myFunction2"; void myFunction2 () { } struct RegistryEntry { const char * name; void ( * func )(); }; RegistryEntry Registry[] = { { myFunction1Name, & myFunction1 }, { myFunction2Name, & myFunction2 }, }; int RegistryCount = sizeof( Registry ) / sizeof( * Registry ); void ( * GetFunctionAddress( const char * name ) )() { for ( int i = RegistryCount - 1; i >= 0; i -- ) { if ( std::strcmp( name, Registry[i].name ) == 0 ) { return Registry[i].func; } } return 0; } OK - Problem number 1 - only functions that you compile into the "Registry" are accessible. Most reasons for doing this is so that they can load modules at a later point in time and this system requires a recompilation of the array "Registry". The second thing, often you need some context and a collection of method - oops, this is called a class. The third thing, you'll want it to be able to do this with dynamically loaded objects (.dll's or .so's). The fourth thing, you'll want to be able to reliably unregister them as well. Anything else ? Oh yeah, network transparency - starting to look like COM and Corba. I have one of these factory frameworks which is a bunch of templates that does most of this. It's a general purpose system. Let me know if you're interested but it's far more involved than the system above. G Back to top Yamin Guest Posted: Tue Sep 30, 2003 12:09 am    Post subject: Re: help using function pointer at run-rime "Jean-Daniel Gamache" <jdgamache (AT) hotmail (DOT) com> wrote Quote: Hi, I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? ie: char myFunctionName[] = "myFunction"; void myFunction () { } //this is the tricky part i am wondering how i could call // a function that is specified in my variable myFunctionName? functionPtr = GetFunctionAddress(myFunctionName); functionPtr(); thanks, JD Hey Jean, Someone on the newsgroup had a similar situation recently. He had a nice way to handle it using std::map. Use string names "myfunction" as the key and a function pointer as the value. Insert each key/value pair once on startup, then to call a function, just use the map. Yamin Back to top Mike Wahler Guest Posted: Tue Sep 30, 2003 12:32 am    Post subject: Re: help using function pointer at run-rime "Yamin" <absdfsd (AT) sdfdasfsd (DOT) com> wrote Quote: "Jean-Daniel Gamache" <jdgamache (AT) hotmail (DOT) com> wrote in message news:dd1d9b4f.0309291330.3968234c (AT) posting (DOT) google.com... Hi, I want to be able to call a function specified at run-time (in a string). How can I do that in C++ ???? ie: char myFunctionName[] = "myFunction"; void myFunction () { } //this is the tricky part i am wondering how i could call // a function that is specified in my variable myFunctionName? functionPtr = GetFunctionAddress(myFunctionName); functionPtr(); thanks, JD Hey Jean, Someone on the newsgroup had a similar situation recently. He had a nice way to handle it using std::map. Use string names "myfunction" as the key and a function pointer as the value. Insert each key/value pair once on startup, then to call a function, just use the map. This is exactly what I showed in my reply. -Mike 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.