class String

{

public:

    String(const char* pStr = NULL);

    String(const String& str);

    virtual ~String();

    String &operator =(const String& str);

    int Length() const;

    const char* cstr() const;

    friend std::ostream& operator<<(std::ostream& os, const String& str);

private:

    char* m_pData;

};

String::String(const char* pStr)

{

    if (pStr == NULL)

    {

        m_pData = new char('\0');

    }

    else

    {

        m_pData = new char[strlen(pStr) + ];

        strcpy(m_pData, pStr);

    }

}

String::String(const String& str)

{

    m_pData = new char[str.Length() + ];

    strcpy(m_pData, str.cstr());
　　 //类的成员函数可以直接访问作为其参数的同类型对象的私有成员。
　　 //即可写为strcpy(m_pData, str.m_pData);

}

int String::Length() const

{

    return strlen(m_pData);

}

const char * String::cstr() const

{

    return m_pData;

}

String::~String()

{

    if (m_pData)

        delete[] m_pData;

}

String& String::operator =(const String& str)

{

    if (this == &str)

        return *this;

    delete[] m_pData;

    m_pData = new char[str.Length() + ];

    strcpy(m_pData, str.cstr());

    return *this;

}

std::ostream& operator<<(std::ostream& os, const String& str)

{

    return os << str.cstr();

}

int main()

{

    String s;

    cout << s << endl;

    String s1("hello");

    cout << s1 << endl;

    String s2(s1);

    cout << s2 << endl;

    String s3("hello world");

    cout << s3 << endl;

    s3 = s2;

    cout << s3 << endl;

    String s4 = "lwm";

    cout << s4 << endl;

    return ;

}