C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Question about macros          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message vineoff Guest Posted: Sun Nov 27, 2005 10:45 pm    Post subject: Question about macros How can macro take a number in this way (or can it) : #define MYMACRO(x) (x) int val = 5; MYMACRO(val); seems it produces val. I need to have a macro that takes a string and a size_t and produces a function name. If I call it : MYMACRO(function, val)(); it should call function5(); Is it possible? Back to top John Harrison Guest Posted: Sun Nov 27, 2005 10:56 pm    Post subject: Re: Question about macros vineoff wrote: Quote: How can macro take a number in this way (or can it) : #define MYMACRO(x) (x) int val = 5; MYMACRO(val); seems it produces val. Remember that macros work before you program is compiled so they cannot know the value of a variable. Quote: I need to have a macro that takes a string and a size_t and produces a function name. If I call it : MYMACRO(function, val)(); it should call function5(); Is it possible? No. Macros know nothing about the values of variables. Now whatever problem you are trying to solve, macros aren't the way to do it. So explain what you are really trying to do, and someone will be able to tell you the right way to do it. John Back to top vineoff Guest Posted: Sun Nov 27, 2005 11:01 pm    Post subject: Re: Question about macros Then I think it would be impossible to do but I'd need to read function names from file and call them. May be a bug in design process but anyway. Back to top John Harrison Guest Posted: Sun Nov 27, 2005 11:06 pm    Post subject: Re: Question about macros vineoff wrote: Quote: Then I think it would be impossible to do but I'd need to read function names from file and call them. May be a bug in design process but anyway. In C++ you can't do much better than this if (name == "something") { something(); } else if (name == "wotsit") { wotsit(); } etc. You can tart it up a little but essentially you have to check the name and call the function. Now if you were programming Java it would be a different matter, but C++ is not very good at this sort of thing. john Back to top Bob Hairgrove Guest Posted: Sun Nov 27, 2005 11:09 pm    Post subject: Re: Question about macros On 27 Nov 2005 14:45:58 -0800, "vineoff" <vineoff (AT) gmail (DOT) com> wrote: Quote: How can macro take a number in this way (or can it) : #define MYMACRO(x) (x) int val = 5; MYMACRO(val); seems it produces val. I need to have a macro that takes a string and a size_t and produces a function name. If I call it : MYMACRO(function, val)(); it should call function5(); Is it possible? You can do this (sort of), but something tells me it isn't what you need: // test_macro.cpp #define MYMACRO(function,val) function##val##(##) #include <iostream> #include <ostream> void func1() { std::cout << "Called func1" << std::endl; } void func2() { std::cout << "Called func2" << std::endl; } void func3() { std::cout << "Called func3" << std::endl; } void func4() { std::cout << "Called func4" << std::endl; } void func5() { std::cout << "Called func5" << std::endl; } int main() { MYMACRO(func,1); MYMACRO(func,2); MYMACRO(func,3); MYMACRO(func,4); MYMACRO(func,5); } Actually, you could do this much better with a template function, and it would have the tremendous advantage over a macro of being typesafe! The problem is probably that you need to choose a function dynamically according to some runtime value. This macro can't do that because it performs text substitution before the compiler ever gets around to compiling your code. To do this, you need function pointers and some kind of mapping from strings (or integers, or whatever you use for a key) to function pointers. std::map<> would serve you well, I believe. -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] Back to top Markus Moll Guest Posted: Sun Nov 27, 2005 11:11 pm    Post subject: Re: Question about macros Hi vineoff wrote: Quote: Then I think it would be impossible to do but I'd need to read function names from file and call them. May be a bug in design process but anyway. That's possible. Use a map from function names to function pointers: #include <iostream> #include <string> #include <map> using namespace std; void f1(); void f2(); void f3(); void f4(); int main() { map<string, void (*)()> func_map; func_map["f1"] = &f1; func_map["f2"] = &f2; func_map["f3"] = &f3; func_map["f4"] = &f4; string name; while(cin >> name) { (*func_map[name])(); } } Markus 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.