Exercise 13.53:
As a matter of low-level efficiency,the HasPtr
assignment operator is not ideal. Explain why. Implement a
copy-assignment and move-assignment operator for HasPtr and compare
the operations executed in your new move-assignment operator versus
the copy-and-swap version.(P.544)

文件hasptr.h：

//! a class holding a std::string*
class HasPtr
{
    friend void swap(HasPtr&,HasPtr&);
    friend bool operator <(const HasPtr& lhs,const HasPtr& rhs);
public:
    //! default constructor.
    HasPtr(const std::string &s = std::string()):
        ps(new std::string(s)),i(0)
    { }
 
    //! copy constructor.
    HasPtr(const HasPtr& hp) :
        ps(new std::string(*hp.ps)),i(hp.i)
    { }
 
    //! move constructor.
    HasPtr(HasPtr&& hp) noexcept :
        ps(hp.ps),i(hp.i)
    { hp.ps = nullptr; }
 
    //! assignment operator
    HasPtr&
    operator = (HasPtr rhs);
 
    //! destructor.
    ~HasPtr()
    {
        delete ps;
    }
 
private:
    std::string *ps;
    int    i;
};

hasptr.cpp文件的一部分：

//! specific swap.
inline void
swap(HasPtr &lhs,HasPtr &rhs)
{
    using std::swap;
    swap(lhs.ps,rhs.ps); // swap the pointers,not the string data
    swap(lhs.i,rhs.i);   // swap the int members
 
    std::cout <<"swapping!\n";
}
 
//! operator = using specific swap
HasPtr&
HasPtr::operator = (HasPtr rhs)
{
    swap(*this,rhs);
    return *this;
}

我的问题是为什么这样做效率不高？