string的实现

#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;
};

#include "string.h"

string::string(const char* str /* = NULL */)
{
    if (!str)
    {
        m_data = new char[1];
        *m_data = '