Hi.
How do you define a functor that handles elements in an STL map? For example:
How do you define DeleteValue functor that handles elements in an STL map?Code:std::map<int, char *> testMap; char *testChar = new char[10]; _strcpy(testChar, "November"); testMap.insert(std::pair<int, char *>(0, testChar)); ... // Now I want to deallocate all values in testMap. std::for_each(testMap.begin(), testMap.end(), DeleteValue());
Thanks,
Kuphryn
use string
if there is a reason you're not using string i'd like to hear it..
if you have some legitimate need to use char:
search your local STL reference for map::first and map::second, IIRC these are the names of the keys and values.
The question is about functors, not char *.
std::map<int, std::string *> testMap;
std::string *pText = new std::string;
...
How do you define a functor that handles elements from a map?
Kuphryn
Well, without knowing much about the STL (and not caring either), it's hard to say exactly how it may be implemented. Basically, to build a functor, you need:
- a structure to hold a pointer to an object, and a pointer to the function it will call.
- a function to assign the above to a given structure.
- for added genericity, derive the functor from a non-templated base-class.
Code:class functor_container { public: virtual void invoke() = 0; }; template <class type> class functor : public functor_container { public: functor(type* instance, void (type::* function)(void)){ object = instance; invocation = function; } void invoke(){ (*object.*invocation)(); } private: type * object; void (type::*invocation)(); }; class test { public: String text; test(char * this_text) :text(this_text) { /* */ } void print( ){ cout << text << endl; getch(); } }; int main(){ test attempt("Hello from a functor..."); functor <test> test_functor(&attempt, &test::print); system("pause"); test_functor.invoke(); return 0; }
Code:#include <cmath> #include <complex> bool euler_flip(bool value) { return std::pow ( std::complex<float>(std::exp(1.0)), std::complex<float>(0, 1) * std::complex<float>(std::atan(1.0) *(1 << (value + 2))) ).real() < 0; }
And gasp.. an actual answer to the question, without a rant about the STL :).
Code:#include <iostream> #include <map> #include <algorithm> typedef std::pair<const int, int> mapitem; void printitem(mapitem& m) { std::cout << m.first << ' ' << m.second << std::endl; } int main() { std::map<int, int> squares; for (int i = 0; i < 10; ++i) { squares[i] = i*i; } std::for_each(squares.begin(), squares.end(), printitem); return 0; }
Yes, yours is quite relevant, isn't it? Good example, too.
Code:#include <cmath> #include <complex> bool euler_flip(bool value) { return std::pow ( std::complex<float>(std::exp(1.0)), std::complex<float>(0, 1) * std::complex<float>(std::atan(1.0) *(1 << (value + 2))) ).real() < 0; }
Sorry guys but neither of those are functors.
You see a functor is a class that has an overloaded operator () and neither of yours have.
You need to overload operator () to do a map.erase() kuphryn erasing the mapitem pointed to by a passed iterator.
Free the weed!! Class B to class C is not good enough!!
And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi
Thanks everyone.
Here is one answer by Joaquín M López Muñoz of CodeProject.
KuphrynCode:class DeleteValue : public std::unary_function<std::pair<int const,char *>&, void> { public: void operator()(std::pair<int const,char *> &pairObject) { delete [] pairObject.second; } };
I always thought of a functor as anything that acts like a function... they do too.
http://www.codeproject.com/csharp/cs_functors.asp
"One of the tools that an STL-programmer must learn to master is the functor concept. A functor is any valid expression that can be syntactically written as a function call. Those expressions can be functions, function pointers and operator() overloading."
