String 实现

#include<iostream>using namespace std;class String{    friend ostream& operator<< (ostream&,String&);public:    String(const char* str=NULL); //赋值构造兼默认构造函数(char)    String(const String &other); //赋值构造函数(String)    String& operator=(const String&other);       //operator=    String operator+(const String &other)const; //operator+    bool operator==(const String&);              //operator==    char& operator[](unsigned int);              //operator[]    size_t size(){return strlen(m_data);};    ~String(void) {delete[] m_data;}private:    char *m_data;};inline String::String(const char* str){    if (!str) m_data=0;    else    {        m_data = new char[strlen(str)+1];        strcpy(m_data,str);    }}inline String::String(const String& other){    if(!other.m_data) m_data=0;//在类的成员函数内可以访问同种对象的私有成员（同种类肯定是友元关系）    else    {        m_data=new char[strlen(other.m_data)+1];        strcpy(m_data,other.m_data);    }}inline String& String::operator=(const String& other){    if (this!=&other)    {        delete[] m_data;        if(!other.m_data) m_data=0;        else        {            m_data = new char[strlen(other.m_data)+1];            strcpy(m_data,other.m_data);        }    }    return *this;}inline String String::operator+(const String &other)const{    String newString;    if(!other.m_data)        newString = *this;    else if(!m_data)        newString = other;    else    {        newString.m_data = new char[strlen(m_data)+strlen(other.m_data)+1];        strcpy(newString.m_data,m_data);        strcat(newString.m_data,other.m_data);    }    return newString;}inline bool String::operator==(const String &s)    {    if ( strlen(s.m_data) != strlen(m_data) )        return false;    return strcmp(m_data,s.m_data)?false:true;}inline char& String::operator[](unsigned int e){    if (e>=0&&e<=strlen(m_data))        return m_data[e];}ostream& operator<<(ostream& os,String& str){    os << str.m_data;    return os;}