Understanding C++ Functors

Functors or function objects are any objects that can be called with () operation. Creating functors is as easy as overriding the operator ().

HelloFunctor.h

#ifndef HELLOFUNCTOR_H_
#define HELLOFUNCTOR_H_

#include <string>
using namespace std;

class HelloFunctor {
public:
    HelloFunctor(string name);
    virtual ~HelloFunctor();
    void operator()(string message);
private:
    string name;
};

#endif /* HELLOFUNCTOR_H_ */

HelloFunctor.cpp

#include "HelloFunctor.h"
#include <iostream>
using namespace std;

HelloFunctor::HelloFunctor(string name) : name(name) {
}

HelloFunctor::~HelloFunctor() {
}

void HelloFunctor::operator ()(string message) {
    cout << name << ": " << message << endl;
}

Main.cpp

#include <algorithm>
#include <vector>
#include "HelloFunctor.h"
using namespace std;

int main() {
    HelloFunctor hf("Foo");
    hf("Hello World!");
    hf("Bye World!");

    vector<string> v;
    v.push_back("Hello World!");
    v.push_back("Bye World!");
    for_each(v.begin(), v.end(), hf);

    return 0;
}

The output is:

Foo: Hello World!
Foo: Bye World!
Foo: Hello World!
Foo: Bye World!

In this example, the for_each() function takes in a functor as opposed to a normal function. This makes functor a very powerful mechanism to make any class callable from within many of the STL functions.