A generic sort program with 'functors' and 'templates'

Picked up this question from here and made a program out of it. It may be a good idea to quickly brush functors and templates if required.

The program sorts the input Vector provided regardless of the type.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

class Int
{
public:
  Int() {x_ = 0;}
  Int(const int &x) {x_ = x;}
  int getID (void) {return x_;}
  int x_;
};

class Str
{
public:
  Str() {x_ = "";}
  Str(const string &x) {x_ = x;}
  string getID (void) {return x_;}
  string x_;
};

template <typename Object> class Comparator {
  public:
    bool operator()(const Object &o1, const Object &o2) const 
    {
      return (const_cast<Object&>(o1).getID() < const_cast<Object&>(o2).getID());
    }

    bool operator()(const Object *o1, const Object *o2) const {
      return (o1->getID() < o2->getID());
    }
};

template <typename VecObject> void Display(VecObject v)
{
  VecObject::iterator it;
  for(it = v.begin(); it != v.end(); ++it)
  {
    cout<<it->getID()<<", ";
  }
  cout<<endl;
}

int main()
{
  vector<Int> objects1;
  objects1.push_back(Int(3));
  objects1.push_back(Int());
  objects1.push_back(Int(77));

  //print the output
  cout<<"objects1 before sort = ";
  Display(objects1);
  std::sort(objects1.begin(), objects1.end(), Comparator<Int> ());
  cout<<"objects1 after sort  = ";
  Display(objects1);

  std::vector<Str> objects2;
  objects2.push_back(Str("Hello Hello"));
  objects2.push_back(Str("apple?"));
  objects2.push_back(Str());
  objects2.push_back(Str("1 Jump"));

  //print the output
  cout<<"objects2 before sort = ";
  Display(objects2);
  std::sort(objects2.begin(), objects2.end(), Comparator<Str> ());
  cout<<"objects2 after sort  = ";
  Display(objects2);

  return 0;
}

The output is as follows:

Example of 'functors'

What are 'functors?'

Depending on what you prefer to read, there are many definitions and explanations of Functors.

From the function pointer tutorial:

Functors are functions with a state. In C++ you can realize them as a class with one or more private members to store the state and with an overloaded operator () to execute the function. Functors can encapsulate C and C++ function pointers employing the concepts templates and polymorphism. You can build up a list of pointers to member functions of arbitrary classes and call them all through the same interface without bothering about their class or the need of a pointer to an instance. All the functions just have got to have the same return-type and calling parameters. Sometimes functors are also known as closures. You can also use functors to implement callbacks.

From StackOverflow:

• A functor is pretty much just a class which defines the operator(). That makes it "look like" a function.
• Another advantage to a functor over a pointer to a function is that the call can be inlined in more cases.
• You can use boost::function, to create functors from functions and methods
  

Here is a simple example of functors that I have created using mishmash from various places:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Program to demonstrate the use of functors

#include<iostream>

using namespace std;

class someClass
{
public:
  someClass(int x) : someVariable(x) {}
  int operator()(int y) {return (someVariable + y);}
  int internalStateValue(){return someVariable;}

private:
  int someVariable;
};

int main()
{
  someClass class1(50);
  someClass class2(75);

  cout<<"Class1 state variable value is : "<<class1.internalStateValue()<<endl;
  cout<<"Class2 state variable value is : "<<class2.internalStateValue()<<endl;

  int test1 = class1(22);
  cout<<"Test 1 value is : "<< test1<<endl;
  int test2 = class2(22);
  cout<<"Test 2 value is : "<< test2<<endl;

  cout<<"Class1 final state variable value is : "<<class1.internalStateValue()<<endl;
  cout<<"Class2 final state variable value is : "<<class2.internalStateValue()<<endl;

  return 0;
}

The output is as follows: