实现string类

#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 ");
    MyString s2="Hello";
    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;
}