自定义的String类

一、简介

       C++标准库中包含一个string类，提供了一套封装好的数据以及处理这些数据的函数。为了熟悉这个类的内存管理情况，实现一下自定义的String类，涉及构造函数、复制构造函数、析构函数、重载运算符的操作，主要关注其内部指针的内存的重分配。

二、详解

1、代码实现：

（1）代码String.h：

#include <iostream>
using namespace std;

//自定义String类,系统是string类
class String
{
    public:
        String();                              //默认构造函数,用于创建空字符串
        ~String();                             //析构函数
        String(const char*const cstr);           //构造函数,带一个参数用来初始化字符串
        String(const String &rs);              //复制构造函数,默认是浅层复制,需重载
        char& operator[](unsigned int length); //重载下标运算符[]
    char operator[](unsigned int length)const; //重载下标运算符[](const版本)
        String &operator=(const String &rs);     //重载复制运算符=，用于两个字符串之间的赋值
    String operator+(const String &rs);    //重载加法运算符+
    String operator+=(const String &rs);     //重载组合运算符+=
        friend ostream &operator<<(ostream &output, const String &str); //重载输出流运算符<<
        friend istream &operator>>(istream &input, String &str);        //重载输入流运算符>>
        friend bool operator<(const String&str1,const String &str2);    //重载小于运算符<
        friend bool operator>(const String&str1,const String &str2);    //重载大于运算符>
        friend bool operator==(const String&str1,const String &str2);   //重载等于运算符==

        unsigned int getlen()const;            //获取字符串长度
    const char*getstr()const;              //获取字符串

    //int operator++(){cout<<"++i\n";tmp++;return tmp;}    //int的重载自加运算符
    //int operator++(int){cout<<"i++\n";int tmp=rs;rs++;return tmp;}
    protected:
    private:
        unsigned int len;
    char *str;
};

（2）代码String.cpp：

#include <string.h>
#include "String.h"
#include <stdio.h>
//默认构造函数,用于创建空字符串
String::String()
{
    cout<<"---String constructor---"<<endl;
    len = 0;
    str = new char[1];
    str[0] = '\0';
}
//析构函数
String::~String()
{
      //cout<<"---String destructor---"<<endl;
    delete []str;
    len=0;
}
//构造函数,带一个参数用来初始化字符串
String::String(const char*const cstr)
{
    /*len = strlen(cstr);
    str = new char[len+1];
    for (unsigned int i =0; i < len; i++) {
      str[i] = cstr[i];
    }
    str[len]='\0';*/
    cout<<"---String constructor:char*---"<<endl;
    if (cstr == NULL) {
        len = 0;
    str = new char[1];
    memset(str, 0, len+1);
    if (str == NULL) return;
  }
  else {
     len = strlen(cstr);
     str = new char[len + 1];
     memset(str, 0, len+1);
     if (str == NULL) return;
     strncpy(str, cstr, len);
   }
}
//复制构造函数,默认是浅层复制,需重载
String::String(const String &rs)
{
    cout<<"---String copy constructor---"<<endl;
    len = rs.getlen();
  str = new char[len + 1];
  for(unsigned int i = 0 ;i < len; i++) {
    str[i] = rs.str[i];
  }
  str[len] = '\0';
  /*len = rs.getlen();
  str = new char[len+1];
  if (str == NULL) return;
  strcpy(str, rs.str);*/
}
//重载下标运算符[]
char&String::operator[](unsigned int length)
{
  if(length>len) return str[len-1];
  else return str[length];
}
//重载下标运算符[](const版本)
char String::operator[](unsigned int length)const
{
    //cout<<"String::operator[] const"<<endl;
  if(length>len) return str[len-1];
  else return str[length];
}
//重载复制运算符=，用于两个字符串之间的赋值
String &String::operator=(const String &rs)
{
    cout<<"String::operator="<<endl;
    if (this == &rs) return *this;
  delete []str;
  len = rs.getlen();
  str = new char[len + 1];
  for(int i = 0; i < len; i++) {
    str[i] = rs[i];     //重载下标运算符[]才可使用
    //str[i] = rs.str[i];
  }
  str[len] = '\0';
  return *this;
}
//重载加法运算符+
String String::operator+(const String &rs)
{
    /*char *st= new char[len + rs.getlen() + 1];
  memset(st, 0, len + rs.getlen() + 1);
  strcat(st, str);
  strcat(st, rs.str);
  return String(st);*/
  cout<<"String::operator+"<<endl;
  unsigned int total = len + rs.getlen();
  char *tmpstr = new char[total + 1];
  unsigned int i, j;
  for (i = 0; i < len; i++) {
    tmpstr[i] = str[i];
  }
  for( j = 0; j < rs.getlen(); j++, i++) {
    tmpstr[i] = rs[j];
  }
  tmpstr[total] ='\0';
  String st(tmpstr);
  delete tmpstr;
  tmpstr = NULL;
  return st;
}
//重载组合运算符+=
String String::operator+=(const String &rs)
{
  cout<<"String::operator+="<<endl;
    int total = len + rs.getlen();
  //delete []str;
  char *tmpstr = new char[total + 1];
  unsigned int i,j;
  for(i = 0; i < len; i++) {
    tmpstr[i] = str[i];
  }
  for(j = 0; j < rs.getlen(); j++,i++) {
    tmpstr[i] = rs[j];
  }
  tmpstr[total]='\0';
  delete []str;
  str = new char[total + 1];
  strncpy(str, tmpstr, total + 1);
  return *this;
  /*
  char *temp = str;
  len = len + rs.getlen();
  str = new char[len + 1];
  strcpy(str, temp);
  strcat(_str, rs.getstr());
  delete[] temp;
  return *this;*/
}
//重载输出流运算符<<
ostream &operator<<(ostream &output, const String &str)
{
    output<<str.str;
  return output;
}
//重载输入流运算符>>
istream &operator>>(istream &input, String &str)
{
    input>>str.str;
  return input;
}
//重载小于运算符<
bool operator<(const String&str1,const String &str2)
{
    if(strcmp(str1.str, str2.str) < 0) return 1;
  else return 0;
}
//重载大于运算符>
bool operator>(const String&str1,const String &str2)
{
    if(strcmp(str1.str, str2.str) > 0) return 1;
  else return 0;
}
//重载等于运算符==
bool operator==(const String&str1,const String &str2)
{
    if(strcmp(str1.str, str2.str) == 0) return 1;
  else return 0;
}
//获取字符串长度
unsigned int String::getlen()const
{
    return len;
}
//获取字符串
const char*String::getstr()const
{
    return str;
}

（3）代码main.cpp：

#include <iostream>
#include <stdio.h>
#include "String.h"
using namespace std;

int main()
{
    String str1;                  //默认构造函数
    String str2(NULL);            //带参构造函数
    String str3("helloworld");    //带参构造函数
    cout<<"str3="<<str3.getstr()<<endl;

    String str4(str3);            //复制构造函数
    //String str4 = str3;         //复制构造函数
    cout<<"str4="<<str4.getstr()<<endl;


    const String str5("tai");     //重载下标运算符[](const版本)
    cout<<"str5[0]="<<str5[0]<<endl;

    String str6;
    str6 = str3;                  //重载复制运算符=
    cout<<"str6="<<str6.getstr()<<endl;

    String str7 = str3 + str5;    //重载加法运算符+
    cout<<"str7="<<str7.getstr()<<endl;

    String str8("123");
    str8 += str7;                 //重载组合运算符+=
    cout<<"str8="<<str8.getstr()<<endl;

    String str9;
    cin>>str9;                    //重载输入流运算符>>
    cout<<"str9="<<str9<<endl;    //重载输出流运算符<<

    String str10("compare String");
    String str11("compare string");
    int comp = str10 > str11;
    cout<< "str10大于str11:" << comp <<endl;
    return 0;
}