自己实现的string类

自己实现的一个string类，包括基本构造，复制构造，赋值和析构函数，比较函数，输入输出函数，锻炼一下动手能力。

#include <iostream>
#include <cstring>
#include <iomanip>

using namespace std;

class MyString{
public:
    MyString(const char *s=NULL);
    MyString(const MyString& rhs);
    MyString& operator=(const MyString& rhs);
    MyString& operator=(const char* s);
    ~MyString();
    char& operator[](int i);
    int length() const {return len; }
    char *c_str(){return data;}

    friend bool operator<(const MyString &st1, const MyString &st2);
    friend bool operator>(const MyString &st1, const MyString &st2);
    friend bool operator==(const MyString &st1, const MyString &st2);
    friend MyString operator+(const MyString &s1, const MyString &s2);

    friend ostream& operator<<(ostream &os, const MyString &st);
    friend istream& operator>>(istream &is, MyString &st);

private:
    char *data;
    int len;
};

MyString::MyString(const char *s)
{
    if(NULL==s)
    {
        len=0;
        data=new char[1];
        *data='\0';
    }
    else
    {
        len=strlen(s);
        data=new char[len+1];
        strcpy(data,s);
    }
}

MyString::MyString(const MyString &rhs)
{
    len=strlen(rhs.data);
    data=new char[len+1];
    strcpy(data,rhs.data);
}

MyString& MyString::operator=(const MyString &rhs)
{
    if(this==&rhs)
        return *this;
    delete []data;
    len=strlen(rhs.data);
    data=new char[len+1];
    strcpy(data,rhs.data);
    return *this;
}

MyString& MyString::operator=(const char* s)
{
    delete []data;
    len=strlen(s);
    data=new char[len+1];
    strcpy(data,s);
    return *this;
}

MyString::~MyString()
{
    delete []data;
}

char& MyString::operator[](int i)
{
    return data[i];
}

//注意友元函数定义时不要friend,而且不要String::
bool operator<(const MyString &st1, const MyString &st2)
{
    return (strcmp(st1.data,st2.data)<0);
}

bool operator>(const MyString &st1, const MyString &st2)
{
    return (strcmp(st1.data,st2.data)>0);
}

bool operator==(const MyString &st1, const MyString &st2)
{
    return (strcmp(st1.data,st2.data)==0);
}

MyString operator+(const MyString &st1, const MyString &st2)
{
    MyString res;
    res.len=st1.len+st2.len;
    res.data=new char[res.len+1];
    strcpy(res.data,st1.data);
    strcat(res.data,st2.data);
    return res;
}

istream &operator>>(istream &cin, MyString &str)
{
    const int limit_string_size = 4096;

    str.data = new char[limit_string_size];

    cin >> setw(limit_string_size) >> str.data;
    str.len = strlen(str.data);
    return cin;
}

ostream &operator<<(ostream &cout, MyString &str)
{
    cout<<str.c_str();
    return cout;
}

int main()
{
    MyString s1("I am s1.");
    MyString s2="Hello,This is s2.";
    cout<<s1<<endl;
    cout<<s2<<endl;
    MyString s3(s1);
    cout<<s3<<endl;
    s3=s2;
    cout<<s3<<endl;
    s3="My name is: " + s1;
    cout<<s3<<endl;
    s3=s1+s2;
    cout<<s3<<endl;
    return 0;
}