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

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

     C++ Code 
  

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#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;

private:
    String &Assign( const char *str);
     char *AllocAndCpy( const char *str);
     char *str_;
};

#endif // _STRING_H_

     C++ Code 
  

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#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;
}

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;
     //return tmp;
    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];
    is >> tmp;
    str = tmp;
     return is;
}

     C++ Code 
  

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#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;

     return 0;
}

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。