Map and Functor :: STL

Hi.

How do you define a functor that handles elements in an STL map? For example:

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());

---

How do you define DeleteValue functor that handles elements in an STL map?

Thanks,
Kuphryn

use string :D

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;
}

---

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;
}

---

:D Yes, yours is quite relevant, isn't it? Good example, too.

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.

Thanks everyone.

Here is one answer by Joaquín M López Muñoz of CodeProject.

Code:

---

class DeleteValue : public std::unary_function<std::pair<int const,char *>&, void>
{
  public:
      void operator()(std::pair<int const,char *> &pairObject)
      {
        delete [] pairObject.second;
      }
};

---

Kuphryn

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."