从零开始学C++之运算符重载（三）：完善String类（[]、 +、 += 运算符重载）、>>和<<运算符重载

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在前面文章中使用过几次String类的例子，现在多重载几个运算符，更加完善一下，并且重载流类运算符。

[]运算符重载+运算符重载+=运算符重载<<运算符重载>>运算符重载

String.h：

#ifndef _STRING_H_#define _STRING_H_#include <iostream>using namespace std;class String{public:    String(const char *str = "");    String(const String &other);    String &operator=(const String &other);    String &operator=(const char *str);    bool operator!() const;    char &operator[](unsigned int index);    const char &operator[](unsigned int index) const;    friend String operator+(const String &s1, const String &s2);    String &operator+=(const String &other);    friend ostream &operator<<(ostream &os, const String &str);    friend istream &operator>>(istream &is, String &str);    ~String(void);    void Display() const;    int Length() const;    bool IsEmpty() const;private:    String &Assign(const char *str);    char *AllocAndCpy(const char *str);    char *str_;};#endif // _STRING_H_

String.cpp：

#pragma warning(disable:4996)#include "String.h"#include <string.h>//#include <iostream>//using namespace std;String::String(const char *str){    str_ = AllocAndCpy(str);}String::String(const String &other){    str_ = AllocAndCpy(other.str_);}String &String::operator=(const String &other){    if (this == &other)        return *this;    return Assign(other.str_);}String &String::operator=(const char *str){    return Assign(str);}String &String::Assign(const char *str){    delete[] str_;    str_ = AllocAndCpy(str);    return *this;}bool String::operator!() const{    return strlen(str_) != 0;}char &String::operator[](unsigned int index){    //return str_[index];    //non const 版本调用 const版本    return const_cast<char &>(static_cast<const String &>(*this)[index]);}const char &String::operator[](unsigned int index) const{    return str_[index];}String::~String(){    delete[] str_;}char *String::AllocAndCpy(const char *str){    int len = strlen(str) + 1;    char *newstr = new char[len];    memset(newstr, 0, len);    strcpy(newstr, str);    return newstr;}void String::Display() const{    cout << str_ << endl;}int String::Length() const{    return strlen(str_);}bool String::IsEmpty() const{    return Length() == 0;}String operator+(const String &s1, const String &s2){    //int len = strlen(s1.str_) + strlen(s2.str_) + 1;    //char* newstr = new char[len];    //memset(newstr, 0, len);    //strcpy(newstr, s1.str_);    //strcat(newstr, s2.str_);    //    //String tmp(newstr);    //delete newstr;    String str = s1;    str += s2;    return str;}String &String::operator+=(const String &other){    int len = strlen(str_) + strlen(other.str_) + 1;    char *newstr = new char[len];    memset(newstr, 0, len);    strcpy(newstr, str_);    strcat(newstr, other.str_);    delete[] str_;    str_ = newstr;    return *this;}ostream &operator<<(ostream &os, const String &str){    os << str.str_;    return os;}istream &operator>>(istream &is, String &str){    char tmp[1024];    cin >> tmp;    str = tmp;    return is;}

main.cpp：

#include "String.h"#include <iostream>using namespace std;int main(void){    String s1("abcdefg");    char ch = s1[2];    cout << ch << endl;    s1[2] = 'A';    s1.Display();    const String s2("xyzabc");    ch = s2[2];    //s2[2] = 'M'; Error    s2.Display();    String s3 = "xxx";    String s4 = "yyy";    String s5 = s3 + s4;    s5.Display();    String s6 = "aaa" + s3 + "sdfadfa" + "xxxx";    s6.Display();    s3 += s4;    s3.Display();    cout << s3 << endl;    String s7;    cin >> s7;    cout << s7 << endl;if (!s7.IsEmpty())cout<<s7.Length()<<endl;    return 0;}

这里写图片描述
需要注意的是，不能将String类的构造函数声明为explicit，否则 String s3 = “xxx”; 编译出错；operator[] 的non const 版本调用了const 版本的实现，其中使用了static_cast和 const_cast 两种类型转换操作符，可以参考这里；operator+ 调用了operator+= 的实现；只能将流类运算符重载为友元函数，因为第一个参数是流类引用，不是String 类。
通过实现这样一个字符串类，我们可以熟悉基本的内存管理与拷贝控制。

参考：
C++ primer 第四版
Effective C++ 3rd
C++编程规范

转载自http://blog.csdn.net/jnu_simba/article/details/9294115

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