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 use of operator()?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Olumide Guest Posted: Thu Sep 23, 2004 9:20 pm    Post subject: What's the use of operator()? I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) Anyone know where can I find tutorials on this operator? I cant seem to find any online. Thanks, - Olumide Back to top Gernot Frisch Guest Posted: Thu Sep 23, 2004 10:08 pm    Post subject: Re: What's the use of operator()? "Olumide" <50295 (AT) web (DOT) de> schrieb im Newsbeitrag news:46713cbd1df997361d57d9aec0230f8a (AT) localhost (DOT) talkaboutprogramming.com... Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) Anyone know where can I find tutorials on this operator? I cant seem to find any online. Instead of creating a [][] operator I usually use the (a,b) operator. So this way I only need one class instead of n for each brace. Take a look at boost's spirit parser library. They use all operators so exesively, you don't even know it's C++ after you wrote some code with it. -Gernot Back to top Olumide Guest Posted: Thu Sep 23, 2004 10:23 pm    Post subject: Re: What's the use of operator()? Thanks Gernot. Got any links? I'm just a learning you see. - Olumide - Back to top Unforgiven Guest Posted: Thu Sep 23, 2004 10:37 pm    Post subject: Re: What's the use of operator()? "Olumide" <50295 (AT) web (DOT) de> wrote Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) You can use them to create functors, which the STL does extensively. -- Unforgiven Back to top Kai-Uwe Bux Guest Posted: Thu Sep 23, 2004 10:47 pm    Post subject: Re: What's the use of operator()? Olumide wrote: Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) One good thing about operator() is that objects of a class with operator() can have different states, i.e., you can realize different functions. For instance you could realize a class to represent quadratic polynomials like so (unchecked code): class QuadraticPolynomial { private: double a, b, c; public: QuadraticPolynomial ( double _a, double _b, double _c ) : a ( _a ) , b ( _b ) , c ( _c ) {} double operator() ( double x ) const { return( c + x*( b + x*a ) ); } }; Now different objects of this class represent different functions. How would you do that using function pointers? Best Kai-Uwe Bux Back to top Rolf Magnus Guest Posted: Fri Sep 24, 2004 12:10 am    Post subject: Re: What's the use of operator()? Olumide wrote: Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) In addition to what others wrote, they can be used for optimization. Consider the C style qsort function that takes a pointer to a comparison function. For every compare, the function needs to be called through that pointer, which means it cannot be inlined. The same is true when using the std::sort template with a function pointer. OTOH, if you use std::sort with a function object (i.e. an object that overloads operator() and thus can be used similarly to a function), the actual call is not through a pointer and the function can be inlined. Back to top Gernot Frisch Guest Posted: Fri Sep 24, 2004 7:16 am    Post subject: Re: What's the use of operator()? "Olumide" <50295 (AT) web (DOT) de> schrieb im Newsbeitrag news:5fe57c430ceced2bb3e34d0ef3e71f74 (AT) localhost (DOT) talkaboutprogramming.com... Quote: Thanks Gernot. Got any links? I'm just a learning you see. - Olumide - class CA { public: operator ()(int x, int y) { return data[x][y]; } int data[50][70]; }; CA a; a(3,4) = 7; cout << a(3,4); Google for "C++ operator overloading" Back to top Gernot Frisch Guest Posted: Fri Sep 24, 2004 7:18 am    Post subject: Re: What's the use of operator()? "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> schrieb im Newsbeitrag news:civol2$1pp$00$2 (AT) news (DOT) t-online.com... Quote: Olumide wrote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) In addition to what others wrote, they can be used for optimization. Consider the C style qsort function that takes a pointer to a comparison function. For every compare, the function needs to be called through that pointer, which means it cannot be inlined. The same is true when using the std::sort template with a function pointer. OTOH, if you use std::sort with a function object (i.e. an object that overloads operator() and thus can be used similarly to a function), the actual call is not through a pointer and the function can be inlined. Very sophisticated. Thank's a lot. -Gernot Back to top Chris Guest Posted: Sun Sep 26, 2004 6:44 am    Post subject: Re: What's the use of operator()? "Olumide" <50295 (AT) web (DOT) de> wrote Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) what would you like to do with it? Flexibility is the name of the game: you can use it for whatever you want. for example: class random { public: //////////// omit stuff for brevity..... int operator() ( int x,int y ) { return data[x][y]; ); // subscript into data const random* operator()() const { return this;} // hey look! a pointer! int operator() ( int n ) { return rand() % n; } // return random number between 0 and n private: int data[10][10]; }; -Chris Back to top Gerhard Wesp Guest Posted: Mon Sep 27, 2004 8:46 am    Post subject: Re: What's the use of operator()? Olumide <50295 (AT) web (DOT) de> wrote: Quote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) Well, basically it is what we call ``syntactic sugar''. If you have a class which represents a function, then you would want to use it as such, i.e. write z = f( x , y ) ; instead of e.g. z = f.evaluate( x , y ) ; operator() and pointers to functions are totally unrelated concepts. Cheers -Gerhard Back to top Richard Herring Guest Posted: Thu Oct 07, 2004 10:19 am    Post subject: Re: What's the use of operator()? In message <cj8k1t$738$2 (AT) esel (DOT) cosy.sbg.ac.at>, Gerhard Wesp <gwesp (AT) cosy (DOT) sbg.ac.at> writes Quote: Olumide <50295 (AT) web (DOT) de> wrote: I'm studying C++ and cant seem to find much justification for operator()? Is it good for anything? (beyond manipulating pointers to functions - which are part of the C standard anyway.) Well, basically it is what we call ``syntactic sugar''. If you have a class which represents a function, then you would want to use it as such, i.e. write z = f( x , y ) ; instead of e.g. z = f.evaluate( x , y ) ; operator() and pointers to functions are totally unrelated concepts. Not in C++ generic programming template-land. There, what counts is not what you are, but how you behave. So a templated algorithm that expects a function as its argument can be passed _anything_ that looks syntactically like a function, whether it is one or not. -- Richard Herring Back to top Gerhard Wesp Guest Posted: Wed Oct 13, 2004 4:04 pm    Post subject: Re: What's the use of operator()? Richard Herring <junk@[127.0.0.1]> wrote: Quote: So a templated algorithm that expects a function as its argument can be passed _anything_ that looks syntactically like a function, whether it is one or not. Of course. Cheers -Gerhard -- Gerhard Wesp o o Tel.: +41 (0) 43 5347636 Bachtobelstrasse 56 | http://www.cosy.sbg.ac.at/~gwesp/ CH-8045 Zuerich _/ See homepage for email address! 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