// ------String.h------#ifndef STRING_H#define STRING_H#include <iostream>using namespace std;class String{public:    String();    String(int n, char c);    String(const char* source);    String(const String& s);    // String& operator = (char* s);    String& operator = (const String& s);    ~String();    char& operator [] (int i){return a[i];}    const char& operator [] (int i) const {return a[i];} // Indexer to constant.    String& operator += (const String& s);    int length();    friend istream& operator >> (istream& is, String& s); // Try to figure out why set ">>" friend function member.    // friend bool operator < (const String& left,const String& right);    friend bool operator > (const String& left, const String& right); // These three operators is not necessary to set them friend. Here is just for simplification.    friend bool operator == (const String& left, const String& right);    friend bool operator != (const String& left, const String& right);private:    char* a;    int size;};#endif

// ------String.cpp------#include "String.h"#include <cstring>#include <cstdlib>String::String(){    a = new char[1];    a[0] = '\0';    size = 0;}String::String(int n, char c){    a = new char[n + 1];    memset(a, c, n);    a[n] = '\0';    size = n;}String::String(const char* source){    if(source == NULL)    {        a = new char[1];        a[0] = '\0';        size = 0;    }    else    {           size = strlen(source);        a = new char[size + 1];        strcpy(a, source);    }}String::String(const String& s){    size = strlen(s.a);    a = new char[size + 1];    strcpy(a,s.a);}String& String::operator = (const String& s){    if(this == &s)        return *this;    else    {        delete[] a;        size = strlen(s.a);        a = new char[size + 1];        strcpy(a, s.a);        return *this;    }}String::~String(){    delete[] a;}String& String::operator += (const String& s){    int j = strlen(a);    int size = j + strlen(s.a);    char* tmp = new char[size + 1];    strcpy(tmp, a);    strcpy(tmp + j, s.a);    delete[] a;    a = tmp;    return *this;}int String::length(){    return strlen(a);}

// ------Main.cpp------#include <iostream>#include "String.h"using namespace std;bool operator == (const String& left, const String& right){ int a = strcmp(left.a, right.a); if(a == 0) {  return true; } else {  return false; }}bool operator != (const String& left, const String& right){ return !(left == right);}ostream& operator << (ostream& os, String& s){ int length = s.length(); for(int i = 0; i < length; i++) {  os << s[i]; } return os;}String operator + (const String& a, const String& b){ String temp; temp = a; temp += b; return temp;}bool operator < (const String& left, const String& right){ int j = 0; while((left[j] != '\0') && (right[j] != '\0')) {  if(left[j] < right[j])  {   return true;  }  else  {   if(left[j] == right[j])   {    j++;    continue;   }   else   {    return false;   }  } } if((left[j] == '\0') && (right[j] != '\0')) {  return true; } else {  return false; }}bool operator > (const String& left, const String& right){    int a = strcmp(left.a, right.a); if(a > 0) {  return true; } else {  return false; }}istream& operator >> (istream& is, String& s){ delete[] s.a; s.a = new char[20]; int m = 20; char c; int i = 0; while(is.get(c) && isspace(c)); if(is) {  do  {   s.a[i] = c;   i++;   if(i == m - 1)   {    s.a[i] = '\0';    char* b = new char[m];    strcpy(b, s.a);    m = m * 2;    s.a = new char[m];    strcpy(s.a, b);    delete[] b;   }  }  while (is.get(c) && !isspace(c));  if(is)  {   is.unget();  } } s.size = i; s.a[i] = '\0'; return is;}int main(){ String a = "abcd"; String b = "www"; String c(6, 'l'); String d; String e = a; String f; cin >> f; String g; g = a + b; if(a < b) {  cout << "a < b" << endl; } else {  cout << "a >= b" << endl; } if(e == a) {  cout << "e == a" << endl; } else {  cout << "e != a" << endl; } b += a; cout << a << endl; cout << b << endl; cout << c << endl; cout << d << endl; cout << e << endl; cout << f << endl; cout << g << endl; cout << g[0] << endl; return 0;}// Output:/*ta < be == aabcdwwwabcdllllllabcdtabcdwwwa*/