String实现

// 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);
	if(!s) str = 0;
	else
	{
		str = new char[len+1];  
		strcpy(str,s);
	}
}

String::String()                  
{
	len = 4;
	str = new char[1];
	str[0] = '\0';         
	num_strings++;
}

String::String(const String & st):len(st.len)
{
	if(!st.str) str = 0;
	else
	{
		str = new char[len+1];  
		strcpy(str,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 *this;
	delete [] str;
	len = st.len;
	if(!st.str) str = 0;
	else
	{
		str = new char[len + 1];
		std::strcpy(str, st.str);
	}
	return *this;
}

// assign a C string to a String
String & String::operator=(const char * s)
{
	delete [] str;
	len = std::strlen(s);
	if(!s) str = 0;
	else
	{
		str = new char[len + 1];
		std::strcpy(str, s);
	}
	return *this;
}

// read-write char access for non-const String
char & String::operator[](int i)
{
	if(i >=0 && i <= strlen(str))
	   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 (std::strcmp(st1.str, st2.str) > 0);
}

bool operator==(const String &st1, const String &st2)
{
	if(strlen(st1.str) != strlen(st2.str))
		return false;
	return strcmp(st1.str,st2.str) ? false:true;
}

// simple String output
ostream & operator<<(ostream & os, const String & st)
{
	os <<  st.str <<" " << st.len << " " << st.num_strings << std::endl;
	return os;
}

// quick and dirty String input
istream & operator>>(istream & is, String & st)
{
	is >> st.len >> st.str;
	return is;
}

int main()
{
   String s1 = "abd";
   String s2 = s1;
   cout << s2[1];
}