Using smart_ptr for user defined class objects

As a C++ neophyte trying to understand smart pointers. I have written below code to check.

It did compile and run but I was expecting the destructor of my class to be invoked and print the cout's from the destructor but it didn't .

Do we need to overload any function in the user defined class so that its destructor is called when the smart_ptr object of that class gets destroyed.

Why is that it did not invoke the object destructor. What is that i am missing?

#include <iostream>
#include <cstdlib>
#include <tr1/memory> 
#include <string>

//using namespace std;

class myclass
{
public:
  myclass();
  myclass(int);
  ~myclass();
private:
  int *ptr;
  std::string *cptr;
};

myclass::myclass()
{
    std::cout << "Inside default constructor\n";
}

myclass::myclass(int a)
{
   std::cout << "Inside user defined constructor\n" ;
   ptr = new int[10];
   cptr = new std::string("AD");
}

myclass::~myclass()
{
    std::cout << "Inside destructor..\n";
    delete [] ptr;
    delete cptr;

    std::cout << "Freed memory..\n";
}

int main()
{


   int i;
   std::cin >> i;       
 std::tr1::shared_ptr<std::string> smartstr(new std::string);
 std::tr1::shared_ptr<myclass> smart_a(new myclass(i));
   if(i == 0)
   {
      std::cout << "Exiting...\n";
      exit(-1);
   }


}