/* functor is a class that behave like a function. It's done by overloading () operator. Q: Why do I need functor? A: If the number of our function parameters is limited to 1, how do you use the other variables? You may think of global variable, and yeah that's a solution. But global variable is considered a bad practice and should be used only if there isn't any other way. Well there is, the functor. Q: How does a functor use more information? A: By storing them as the members, and we can use them later. Q: How does a functor exatcly work? A: Check out the code below! */ //There are 2 codes you can choose: BASIC, ADVANCED. Type either one after #define #define BASIC #ifdef BASIC #include #include //std::vector #include #include //std::transform() class Add { public: Add(int x): val(x) {} int operator()(int element) //overloading operator() { std::cout << "Element: " << element << " added " << val << "\n"; return element + val; } private: int val; }; void print(std::vector& vec) { for(const int i : vec) //foreach loop { std::cout << i << " "; } std::cout << "\n"; } int main() { std::vector a{1, 3, 5, 7, 9}; //C++11 uniform initialization Add add_10(10); std::cout << "Before: "; print(a); std::transform(a.begin(), a.end(), a.begin(), add_10); //Add(10) == add_10 //Add(10) returns an Add object with val 10, and then tranform() make every vector elements pass to operator() of this object. For more be sure to check out std::transform() std::cout << "After: "; print(a); return 0; } #elif defined(ADVANCED) #include #include //std::vector #include #include //std::transform() #include //std::pow() //at BASIC code we make a functor that can increase any value to every elements in vector. Now let's go up to 6 operations from 1. class Math { public: enum class Mode //+-*/%power() { add, sub, mul, div, mod, pow }; Math(const Math::Mode m, const int n): val(n), mode(m) {} int operator()(const int element) const { std::cout << "Element: " << element << " is doing "; switch(mode) { case Mode::add: std::cout << "addition\n"; return element + val; case Mode::sub: std::cout << "substraction\n"; return element - val; case Mode::mul: std::cout << "multiplication\n"; return element * val; case Mode::div: std::cout << "division\n"; return element / val; case Mode::mod: std::cout << "modulation\n"; return element % val; case Mode::pow: std::cout << "power to " << val << "\n"; return std::pow(element, val); } return -1; } private: int val; Mode mode; }; void print(const std::vector& x) { for(const int i : x) std::cout << i << " "; std::cout << "\n"; } int main() { typedef Math::Mode Mode; Math add10(Mode::add, 10); std::vector a{1,4,9,16,25}; std::vector b{"Nick", "Jack"}; std::cout << "Before: "; print(a); std::transform(a.begin(), a.end(), a.begin(), Math(Mode::mod, 3)); std::cout << "After: "; print(a); return 0; } //code below is just handling invalid option. #else #include int main() {std::cout << "Invalid option at line 16 #define.\nDo either #define BASIC or #define ADVANCED";return 0;} #endif