// mystring.h -- class definition
#include <iostream>
#include<string>
using namespace std;
class String
{
private:
    char * str;             // pointer to string
    int len;                // length of string
    static int num_strings;  // number of objects
    static const int CINLIM = 80;   // cin input limit
public:
    // constructors and other methods
    String(const char * s);       // constructor
    String();               // default constructor
    String(const String &);   // copy constructor
    ~String();              // destructor
    int length () const { return len; }
    // overloaded operator methods   
    String & operator=(const String &);
    String & operator=(const char *);
    char & operator[](int i);
    const char & operator[](int i) const;
    // overloaded operator friends
    friend bool operator<(const String &st, const String &st2);
    friend bool operator>(const String &st1, const String &st2);
    friend bool operator==(const String &st, const String &st2);
    friend ostream & operator<<(ostream & os, const String & st);
    friend istream & operator>>(istream & is, String & st);
    // static function
    static int HowMany(); 
};

// mystring.cpp -- String class methods

// 初始化静态类成员num_strings
int String::num_strings = 0;
// static method
int String::HowMany()
{
    return num_strings;
}
// class methods，要求动态分配字符串内存空间
String::String(const char * s)     
{
    len = strlen(s);
    str = new char[len];  
    str = static_cast<char*>(s);
}
String::String()                  
{
    len = 4;
    str = new char[1];
    str[0] = '\0';         
    num_strings++;
}
String::String(const String & st)
{
    *this(st.str); 
}
String::~String()                     // necessary destructor
{
    delete str;                 
}
// overloaded operator methods   
// assign a String to a String
String & String::operator=(const String & st)
{
    if (this == &st)
        return st;
    delete [] str;
    len = st.len;
    return st;
}
// assign a C string to a String
String & String::operator=(const char * s)
{
    delete [] str;
    len = std::strlen(s);
    str = new char[len + 1];
    std::strcpy(str, s);
    return *this;
}
// read-write char access for non-const String
char & String::operator[](int i)
{
    return str[i];
}
// read-only char access for const String
const char & String::operator[](int i) const
{
    return static_cast<const char>(str[i]);  //此处与上一空内容一样
}
// overloaded operator friends
bool operator<(const String &st1, const String &st2)
{
    return (std::strcmp(st1.str, st2.str) < 0);
}
bool operator>(const String &st1, const String &st2)
{
    return st2.str < st1.str;
}
bool operator==(const String &st1, const String &st2)
{
    return st1.str == st2.str;
}
// simple String output
ostream & operator<<(ostream & os, const String & st)
{
    os <<  st.str <<" " << st.len << " " << st.num_strings << std::endl;
}
// quick and dirty String input
istream & operator>>(istream & is, String & st)
{
    is >> st.len >> st.str;
}