How to implement Auto Pointer C++

In C++, auto_ptr<> is a smart pointer which automatically destroy itself when the program ends. Implementing an auto_ptr class is really easy and straightforward.

#include <iostream>
#include <cstdlib>

template< class T >
class AutoPointer
{
public :
    explicit AutoPointer( T* rhs = NULL )
            :is_owner( rhs != NULL ), g( rhs )
    { }

    AutoPointer( AutoPointer& rhs )
            :is_owner( rhs.is_owner )
    {
        g = rhs.release();
    }

    ~AutoPointer()
    {
        free_mem();
    }

    const AutoPointer& operator =( AutoPointer& rhs )
    {
        if ( &rhs != this )
        {
            T* other = rhs.deref();
            if ( other != g )
            {
                free_mem();
                is_owner = rhs.is_owner;
            }
            else if ( rhs.is_owner )
            {
                is_owner = true;
            }
            g = rhs.release();
        }
        return *this;
    }

    T& operator *() const
    {
        return *deref();
    }

    T* operator ->() const
    {
        return deref();
    }

    T* deref() const
    {
        return g;
    }

    T* release()
    {
        is_owner = false;
        return g;
    }

private:
    T* g;
    bool is_owner;
    void free_mem()
    {
        if ( is_owner )
            delete g;
    }
};

struct Integer
{
    Integer( int a ):x( a )
    { }

    ~Integer()
    {
        std::cout << "DESTRUCT " << x << '\n';
    }

    int x;
};


int main( )
{
    AutoPointer< Integer > p1( new Integer( 3 ) );
    AutoPointer< Integer > p2 = p1;
    AutoPointer< Integer > p3;

    std::cout << ( *p1 ).x << " " << ( *p2 ).x << '\n';
    p3 = p1;
    std::cout << p1->x << " " << p2->x << " " << p3->x << '\n';
    // 3 is owned by p2.

    AutoPointer< Integer > p4( new Integer( 4 ) );
    std::cout << p1->x << " " << p2->x << " " << p3->x << " " << p4->x << '\n';
    p2 = p4;

    std::cout << "3 is no longer owned! p1 and p3 are stale!!!" << '\n';
    std::cout << p2->x << " " << p4->x << std::endl;

    return 0;
}