string的实现

面试常常用到string类的实现，自己总结了一下：

　　

#pragma once

#include <iostream>
#include <cassert>
#include <iomanip>
using namespace std;

class string{

    friend ostream& operator <<(ostream&,string&); 
    friend istream& operator >>(istream&,string&);

public:
    string(const char* str = NULL);
    //assignment constructor 
    string(const string& str); 
    ~string(){
        delete [] m_data;
    }

    //get the length of string
    int size(){
        return mystrlen(m_data);
    }
    //return substring  这里存在一些问题待改善
    //当字符串的长度稍大一些时会出错;
    string substr(int pos);
    string substr(int pos,int len);
    //overload  operator =
    string& operator =(const string& other);
    //overload operator +
    string operator +(const string& other)const;
    //overload operator +=
    string& operator +=(const string& other);
    //overload operator ==
    bool operator ==(const string& other)const;
    //overload operator []
    char& operator [](unsigned int);

private:
    char* m_data;
};

cpp

#include "string.h"

string::string(const char* str /* = NULL */)
{
    if (!str)
    {
        m_data = new char[1];
        *m_data = '';
    } 
    else
    {
        int nLen = strlen(str);
        m_data = new char[nLen + 1];
        strcpy(m_data,str);
    } 
}

string::string(const string& str)
{
    
    m_data = new char[strlen(str.m_data)+1];
    strcpy(m_data,str.m_data);
}

ostream& operator <<(ostream& oscout,string& str)
{
    oscout<<str.m_data;
    return oscout;
}

istream& operator >>(istream& iscin,string& str)
{
    char temp[255];
    iscin>>setw(255)>>temp;
    str = temp;
    return iscin;
}

string string::substr(int pos)
{
    /*
    int nLen = strlen(m_data);
    char* res = new char(nLen - pos +1);
    char* src = m_data + pos;
    strncpy(res,src,nLen-pos);
    string strres(res);
    return strres;
    */

    int nLen = strlen(m_data);
    if (nLen < pos)
    {
        pos = nLen;
    }
    int nCopyLen = nLen - pos;
    char* res = new char(nLen - pos +1);
    char* dest =  res;
    char* src = m_data + pos;

    for (int i = 0;i < nCopyLen; i++)
    {
        *dest++ = *src++;
    }

    *dest = '';
    string strres(res);
    return strres;
}

string string::substr(int pos,int len)
{
    int maxlen = strlen(m_data)-pos;
    if (maxlen < 0) maxlen = 0;
    int newLen = maxlen < len ? maxlen : len;
    char* res = new char(newLen+1);
    char* src = m_data + pos;
    strncpy(res,src,newLen);
    *(res+newLen) = ''; 
    string strres(res);
    return strres;
}

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; 
}

string& string::operator =(const string& other)
{
    if (this==&other) 
        return *this;
     
    delete [] m_data;  

    m_data = new char[strlen(other.m_data)+1];  
    strcpy(m_data,other.m_data);  
    
    return *this;
}

string& string::operator +=(const string& other)
{

    char* data = new char[strlen(m_data)+strlen(other.m_data)+1];
    strcpy(data,m_data);
    delete [] m_data;
    m_data = data;
    strcat(m_data,other.m_data);
    
    return *this;
}

//overload operator ==
bool string::operator ==(const string& other)const
{
    if ( strlen(other.m_data) != strlen(m_data) )  
        return false;  
    return strcmp(m_data,other.m_data)?false:true; 
}
//overload operator []
char& string::operator [](unsigned int index)
{
    if (index >= 0 && index <= strlen(m_data))  
        return m_data[index];

    return m_data[strlen(m_data)+1];
}

另外转置输出string例子

#include <iostream>
#include <string>
using namespace std;



//只限英文字符
void ReverseString(string& str) 
{
    int nLen = str.size();

    for (int i = 0; i < nLen/2; i++)
    {    
        char cTmp = str[nLen-i-1];
        str[nLen-i-1] = str[i];
        str[i] = cTmp;
    }
}
//中英字符
void ReverseString1(string& str)
{
    int nLen = str.size();
    
    string strTmp;
    strTmp.resize(nLen,' ');

    for (int i =0; i < nLen; i++)
    {    
        if ( str[i] < 255 && str[i] > 0)
        {
            strTmp[nLen-i-1] = str[i];
        } 
        else
        {
            strTmp[nLen-i-2] = str[i];
            strTmp[nLen-i-1] = str[i+1];
            ++i;
        }
    }
    str = strTmp;
}


int main(int argc, char* argv[])
{
    int nflag = 1;

    while (nflag)
    {
        cout<<"enter a string including both lowercase and upercase letters!"<<endl;
        string str;
        cin.clear();
        cin.sync();
        getline(cin,str);
        ReverseString1(str);
        cout<<str<<endl;
        cout<<"enter 0 to exit or 1 to continue"<<endl;    
        cin>>nflag;
    }

    return 0;
}