含有指针对象的级联赋值为什么会出问题？

#include <iostream>

class num
{
public:
    num( int n=0 ) : n_( new int(n) )
    {
        std::cout << "构造\n";
    }
    num( const num& rhs ) : n_( new int(*rhs.n_) )
    {
        std::cout << "拷贝\n";
    }
    num& operator=( const num& rhs )
    {
        std::cout << "赋值\n";
        *n_ = *rhs.n_;
        return *this;
    }
    ~num()
    {
        std::cout << "析构\n";
        delete n_;
    }

    //
    //int get() const
    //{
    //    return *n_;
    //}
    //void set( int x )
    //{
    //    *n_ = x;
    //}

private:
    int* n_;

    friend std::ostream& operator<<( std::ostream& os, const num& obj );
};

std::ostream& operator<<( std::ostream& os, const num& obj )
{
    return os << *obj.n_;
}

using namespace std;

int main()
{
    num one(1), two;
    two = one;
    cout << two << endl;

    num three(two);
    cout << three << endl;

    return 0;
}