~cpp 
//ptr.h
#ifndef GUARD_Ptr_h
#define GUARD_Ptr_h

#include <cstddef>
#include <stdexcept>

template <class T> class Ptr {
public:
	// new member to copy the object conditionally when needed
	void make_unique() {
		if (*refptr != 1) {
			--*refptr;
			refptr = new size_t(1);
			p = p? clone(p): 0;
		}
	}
	/*
	이 함수는 프로그래머가 필요할때 현재 가리키는 내부객체와 
	동일한 내용의 객체를 복사한 객체를 만들어준다.

	대신에 가리키는 대상의 핸들이 1개인 경우에는 이런 복사를 행하지 않는다.
	*/
	// the rest of the class looks like `Ref_handle' except for its name
	Ptr(): p(0), refptr(new size_t(1)) { }
	Ptr(T* t): p(t), refptr(new size_t(1)) { }
	Ptr(const Ptr& h): p(h.p), refptr(h.refptr) { ++*refptr; }

	Ptr& operator=(const Ptr&);    // implemented analogously to 14.2/261
	~Ptr();                        // implemented analogously to 14.2/262
	operator bool() const { return p; }
	T& operator*() const;          // implemented analogously to 14.2/261
	T* operator->() const;         // implemented analogously to 14.2/261

private:
	T* p;
	std::size_t* refptr;
};

template <class T> T* clone(const T* tp)
{
	return tp->clone();
}

template<class T>
T& Ptr<T>::operator*() const { if (p) return *p; throw std::runtime_error("unbound Ptr"); }

template<class T>
T* Ptr<T>::operator->() const { if (p) return p; throw std::runtime_error("unbound Ptr"); }

template<class T>
Ptr<T>& Ptr<T>::operator=(const Ptr& rhs)
{
        ++*rhs.refptr;
        // \f2free the lhs, destroying pointers if appropriate\fP
        if (--*refptr == 0) {
                delete refptr;
                delete p;
        }

        // \f2copy in values from the right-hand side\fP
        refptr = rhs.refptr;
        p = rhs.p;
        return *this;
}

template<class T> Ptr<T>::~Ptr()
{
        if (--*refptr == 0) {
                delete refptr;
                delete p;
        }
}
#endif

~cpp 
Student_info s1;
s1.read(cin);
Student_info s2 = s1;
s2.read(cin)

~cpp 
void Student_info::regrade(double final, double thesis) {
	cp.make_unique();

	if(cp)
		cp->regrade(final, thesis);
	else throw run_time_error("regrade of unknown student");
}

~cpp 
//Str.h - 치기가 귀찮아서 그냥 복사함. -_-
#ifndef GUARD_Str_h
#define GUARD_Str_h

#include <algorithm>
#include <iostream>
#include "Ptr.h"
#include "Vec.h"

template<>
Vec<char>* clone(const Vec<char>*);

// does this version work?
class Str {
	friend std::istream& operator>>(std::istream&, Str&);
	friend std::istream& getline(std::istream&, Str&);

public:
	Str& operator+=(const Str& s) {
		data.make_unique();
		std::copy(s.data->begin(), s.data->end(),
		          std::back_inserter(*data));
		return *this;
	}

	// interface as before
	typedef Vec<char>::size_type size_type;

	// reimplement constructors to create `Ptr's
	Str(): data(new Vec<char>) { }
	Str(const char* cp): data(new Vec<char>)  {
		std::copy(cp, cp + std::strlen(cp),
		          std::back_inserter(*data));
	}

	Str(size_type n, char c): data(new Vec<char>(n, c)) { }
	template <class In> Str(In i, In j): data(new Vec<char>) {
		std::copy(i, j, std::back_inserter(*data));
	}

	// call `make_unique' as necessary
	char& operator[](size_type i) {
		data.make_unique();
		return (*data)[i];
	}
	const char& operator[](size_type i) const { return (*data)[i]; }
	size_type size() const { return data->size(); }

	typedef char* iterator;
	typedef const char* const_iterator;

	iterator begin() { return data->begin(); }
	const_iterator begin() const { return data->begin(); }

	iterator end() { return data->end(); }
	const_iterator end() const { return data->end(); }

private:
	// store a `Ptr' to a `vector'
	Ptr< Vec<char> > data;
};
// as implemented in 12.3.2/216 and 12.3.3/219 
std::ostream& operator<<(std::ostream&, const Str&);
Str operator+(const Str&, const Str&);
inline bool operator<(const Str& lhs, const Str& rhs)
{
        return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
}
inline bool operator>(const Str& lhs, const Str& rhs)
{
        return std::lexicographical_compare(rhs.begin(), rhs.end(), lhs.begin(), lhs.end());
}
inline bool operator<=(const Str& lhs, const Str& rhs)
{
        return !std::lexicographical_compare(rhs.begin(), rhs.end(), lhs.begin(), lhs.end());

}
inline bool operator>=(const Str& lhs, const Str& rhs)
{
        return !std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());

}
inline bool operator==(const Str& lhs, const Str& rhs)
{
        return lhs.size() == rhs.size() &&
                std::equal(lhs.begin(), lhs.end(), rhs.begin());
}
inline bool operator!=(const Str& lhs, const Str& rhs)
{
        return !(lhs == rhs);
}
#endif