class String{private:    char* m_str;public:    String(const char* str = NULL)    {        m_str = NULL;        if(NULL == str) return;        int len = strlen(str);        m_str = new char[len+1];        strcpy(m_str,str);    }    ~String()    {        if(m_str) delete m_str;    }    String(const String& that)    {        m_str = NULL;        if(that.m_str==NULL)        {            return;        }        int len = strlen(that.m_str);        m_str = new char[len+1];        strcpy(m_str,that.m_str);    }    String &operator=(const String& that)    {        if(&that != this)        {            String temp;            swap(m_str,temp.m_str);        }        return *this;    }    const char* c_str(){
   return m_str;}};