自定义String类

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String类:
1.无参数、带参数、对象浅拷贝、对象深拷贝等构造函数
2.运算符= += == != > < >= <=、输入输出流istream ostream的重载
//实现Stinrg 类中 构造函数、运算符= += == != > < >= <=、输入输出流istream ostream的重载#include <iostream>//#include <String>    //不加入此头文件#include <string.h>#include <stdio.h>using std::cout;using std::cin;using std::endl;class String {public:    String();    String(const char *);    String(const String&);    ~String();    String &operator=(const String &);    String &operator=(const char *);    String &operator+=(const String &);    String &operator+=(const char *);    char &operator[](std::size_t index);    const char &operator[](std::size_t index) const;          //const对象只能调用const成员函数;普通对象均能调用    std::size_t size() const;    const char* c_str() const;    friend bool operator==(const String &, const String &);    friend bool operator!=(const String &, const String &);    friend bool operator<(const String &, const String &);    friend bool operator>(const String &, const String &);    friend bool operator<=(const String &, const String &);    friend bool operator>=(const String &, const String &);    friend std::ostream &operator<<(std::ostream &, const String &);    friend std::istream &operator>>(std::istream &, String &);private:    char * _pstr;};String operator+(const String &, const String &);String operator+(const String &, const char *);String operator+(const char *, const String &);String::String():_pstr(new char[1]){    *_pstr='\0';}String::String(const char* pstr):_pstr(new char[strlen(pstr)+1]){    strcpy(_pstr,pstr);}String::String(const String & rhs):_pstr(new char[strlen(rhs._pstr)+1]){    strcpy(_pstr,rhs._pstr);}String::~String(){    delete [] _pstr;}String & String::operator=(const String &rhs){    if(this != &rhs){        //避免自复制问题 左右均为指针        delete [] _pstr;        _pstr=new char [strlen(rhs._pstr)+1];        strcpy(_pstr,rhs._pstr);    }    return *this;}String & String::operator=(const char * pstr){    if(!strcmp(this->_pstr,pstr)){         //判断字符串内容是否相同;pstr在文字常量区,_pstr在栈区,首地址肯定不相同        delete [] _pstr;        _pstr=new char [strlen(pstr)+1];        strcpy(_pstr,pstr);    }    return *this;}String & String::operator+=(const String& rhs){    int need_length=strlen(this->_pstr)+strlen(rhs._pstr)+1;    char *temp=new char [strlen(_pstr)+1];    strcpy(temp,_pstr);    delete [] _pstr;    _pstr = new char [need_length];    strcpy(_pstr,temp);    delete [] temp;    strcat(_pstr,rhs._pstr);    return *this;}String & String::operator+=(const char* pstr){    int need_length=strlen(this->_pstr)+strlen(pstr)+1;    char *temp=new char [strlen(_pstr)+1];    strcpy(temp,_pstr);    delete [] _pstr;    _pstr = new char [need_length];    strcpy(_pstr,temp);    delete [] temp;    strcat(_pstr,pstr);    return *this;}char & String::operator[](std::size_t index){    static char nullchar = '\0';    if(index>=0 && index<strlen(_pstr)){        return _pstr[index];    }    return nullchar;    //return '\0';     //引用不要返回局部变量}const char & String::operator[](std::size_t index) const{          //const对象只能调用const成员函数;普通对象均能调用    static char nullchar = '\0';    if(index>=0 && index<strlen(_pstr)){        return _pstr[index];    }    return nullchar;}std::size_t String::size() const{    if(*(this->_pstr)=='\0') return 0;    return strlen(this->_pstr);}const char* String::c_str() const{    return _pstr;                    //返回char* }//A类的友元函数 能直接访问A类的private成员变量bool operator==(const String &left, const String &right){    return !strcmp(left._pstr,right._pstr);}bool operator!=(const String &left, const String &right){    return strcmp(left._pstr,right._pstr);}bool operator<(const String &left, const String &right){    return strcmp(left._pstr,right._pstr)<0?true:false;}bool operator>(const String &left, const String &right){    return strcmp(left._pstr,right._pstr)>0?true:false;}bool operator<=(const String &left, const String &right){    return strcmp(left._pstr,right._pstr)<=0?true:false;}bool operator>=(const String &left, const String &right){    return strcmp(left._pstr,right._pstr)>=0?true:false;}std::ostream &operator<<(std::ostream &os, const String &rhs){       //对于String类 输出流重载    os << rhs._pstr<<endl;    return os;}std::istream &operator>>(std::istream &is, String &rhs){    delete [] rhs._pstr;    rhs._pstr=new char[256]{0};    is >> rhs._pstr;              return is;}    String operator+(const String &, const String &);    String operator+(const String &, const char *);    String operator+(const char *, const String &);String operator+(const String &left, const String &right){   //String += String    String temp1(left);    String temp2(right);    //初始化两个实例    temp1 +=temp2;    return temp1;           //返回String 对象}String operator+(const String &left, const char *right){    //String += char*    String temp1(left);    //String temp2(right);        temp1 +=right;    return temp1;           //返回String 对象}String operator+(const char *left, const String &right){     //要求 left在前    String temp1(left);    String temp2(right);        temp1 +=temp2;    return temp1;           //返回String 对象}   int main(int argc,char ** argv){    //**********************构造函数测试    String str1;    String str2("hello,world");    String str3(str2);    cout<<str1;    cout<<str2;    cout<<str3;    cout<< String("---------------------");    //***********************  = 运算符测试    const char *pstr="hello,world,cui";    String str4=pstr;    String str5=str4;    cout<<str4;    cout<<str5;    cout<< String("---------------------");    //********************** += 运算符测试    String str6("hello,");    str6 +=str6;    cout<<str6;    str6 += "cuixuange";    cout<<str6;    cout<< String("---------------------");    //********************** [] 运算符测试    String str7("Hello");    const String str8("CUIXUANGE");    cout<<str7[0]<<endl;             //单个字符输出不会调用 重载的string输出;  额外加上换行符    cout<<str8[8]<<endl;    cout<< String("---------------------");    //********************** size c_str 函数测试    cout << "size():"<<str8.size()<<endl;    cout<<str8.c_str()<<endl;    cout<< String("---------------------");    //********************** == != > < >= <= 运算符测试    String str9="hello";    String str10="Hello";    cout<<"NOTE: true is 1 ; false is 0"<<endl;    cout<<"==? "<<(str9==str10)<<endl;    cout<<"!=? "<<(str9!=str10)<<endl;    cout<<"> ? "<<(str9> str10)<<endl;    cout<<"< ? "<<(str9< str10)<<endl;    cout<<">=? "<<(str9>=str10)<<endl;    cout<<"<=? "<<(str9<=str10)<<endl;    cout<< String("---------------------");    //********************** << >>运算符测试     String str11;    cout<<"input something...."<<endl;    cin>>str11;    cout<<"cin:"<<str11;    cout<< String("---------------------");   //测试 << 流    //********************** + 运算符测试     String str12;    String str13("hello");    String str14(",world");    const char *pstr2=",world";    str12 = str13 + str14;    cout<<"string + string:       "<<str12;    str12 =str13 + pstr2;    cout<<"string + const char*: " << str12;    str12 = pstr2 + str13 ;    cout<<"const char* + string: " << str12;    return 0;}/*1.友元函数有三种类型 自由函数，其他类的成员函数, 友元类 。 被定义的友元内容 能够直接访问当前类的private 私有成员变量2.运算符重载原则操作数必须为用户自定义的类、枚举类；优先性、结合性是固定的；一般不重载 && || 等逻辑运算符3.不能重载的运算符成员访问符 .     成员指针访问运算符 .*    域运算符 ::   长度运算符 sizeof      条件运算符号 ?:4.运算符重载形式自由函数重载operator  例如 + ，-。返回一个新的实例 友元函数重载operator   能访问类私有成员成员函数重载operator  例如 ++ += 。返回一个实例的引用,因为左值一般已经存在,避免返回新实例 浅拷贝出错、 也提高了效率5.前置++ 后置++区别前置++ 返回this的引用   ;   后置++ 返回this的复制对象前置++ 在容器的迭代器中使用能提高使用效率前置++和后置++参数和返回值类型不同后置++有一个(int)类型参数，调用时编译器传递一个0作为该(int)的值//  http://blog.csdn.net/randyjiawenjie/article/details/6747720    返回类型不同    形参不同    效率不同*/
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