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实现了 MyString 类，同时实现了运算符重载，重载的运算符包括 <、>、==、!=、<<、>>、[] 等。

如果一个类重载了某个运算符，那么对这个类的对象进行操作的时候便会使用类重载的运算符。比如下边代码 MyString 类中重载了 <、>、==、!=、<<、>> 运算符，那么使用这些运算符操作对象的时候，便会使用对象重载之后的运算符。

(1) <、>、==、!=

① 4 个运算符重载，可以声明为友元函数，也可以声明为类的成员函数

② 声明友元函数时，形参是两个；声明为成员函数时，形参是一个

③ 对象 O1 和 O2 进行比较，成员函数是 O1.operator < (O1)；友元函数是 operator < (O1, O2)

④ 声明为成员函数时，默认会带一个 this 指针，*this 是左操作数

(2) <<，>> 流操作符，需要声明为友元函数，不能声明为成员函数

(3) 不能创建新的运算符，只能重载已有的运算符

如下代码，<、>、==、!=、<<、>> 均声明为友元函数。

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <string>class MyString {
public:MyString(); // 默认构造函数MyString(int n, char c); // 普通构造函数MyString(const char *str); // 普通构造函数MyString(const MyString &str); // 拷贝构造函数MyString& operator = (char *str); // 重载 =，使用字符串赋值MyString& operator = (const MyString &s); // 重载 =，使用对象赋值~MyString(); // 析构函数char& operator[] (int i); // 重载 [], 实现数组操作const char& operator[] (int i) const; // 重载 []，事项数组操作，对象为常量MyString& operator += (const MyString& str); // 重载 +=，与对象相加MyString& operator += (const char *str); // 重载 +=，与字符串相加friend std::ostream& operator << (std::ostream& out, MyString& str); // 重载 <<，实现输出流friend std::istream& operator >> (std::istream& in, MyString& str); // 重载 >>，实现输入流friend bool operator < (const MyString& left, const MyString& right); // 重载 <friend bool operator > (const MyString& left, const MyString& right); // 重载 >friend bool operator == (const MyString& left, const MyString& right); // 重载 ==friend bool operator != (const MyString& left, const MyString& right); // 重载 !=char *GetData() const;// 对象被声明为 const，那么这个对象调用的函数也需要是 const 类型int Size() const;private:int size;char *data = nullptr;
};MyString::MyString() {// 默认构造函数，只包括一个字符 '\0'printf("MyString()\n");data = new char[1];*data = '\0';size = 0;
}MyString::MyString(int n, char c) {printf("MyString(int n, char c), n = %d, c = %c\n", n, c);data = new char[n + 1];for (int i = 0; i < n; i++) {*(data + i) = c;}*(data + n) = '\0';size = n;
}MyString::MyString(const char *str) {printf("MyString(const char *str), str = %s\n", str);if (str == nullptr) {data = new char[1];*data = '\0';size = 0;} else {data = new char[strlen(str) + 1];strcpy(data, str);size = strlen(str);}
}MyString::MyString(const MyString &str) {printf("MyString(const MyString &str), str = %s\n",  str.GetData());data = new char[str.size + 1];strcpy(data, str.data);size = str.size;
}MyString& MyString::operator = (char *str) {printf("operator =, MyString = char *, str = %s\n", str);if (data != nullptr) {delete []data;}data = new char[strlen(str) +1];strcpy(data, str);return *this;
}MyString&MyString::operator = (const MyString& str) {printf("operator =, MyString = MyString, str = %s\n", str.GetData());if (this == &str) {return *this;}if (data != nullptr) {delete []data;}data = new char[str.size + 1];strcpy(data, str.data);return *this;
}MyString::~MyString() {if (data != nullptr) {delete []data;data = nullptr;size = 0;}
}char& MyString::operator [](int i) {return data[i];
}const char& MyString::operator[] (int i) const {printf("operator [], const\n");return data[i];
}MyString& MyString::operator +=(const MyString& s) {printf("operator += MyString, str = %s\n", s.GetData());int length = size + s.size + 1;char *tmp = data;data = new char[length];size = length - 1;strcpy(data, tmp);strcat(data, s.data);delete []tmp;return *this;
}MyString& MyString::operator += (const char *str) {printf("operator += char *, str = %s\n", str);if (str == nullptr) {return *this;}int length = size + strlen(str) + 1;char *tmp = data;data = new char[length];size = length - 1;strcpy(data, tmp);strcat(data, str);delete []tmp;return *this;
}int MyString::Size() const {return size;
}char* MyString::GetData() const {return data;
}std::ostream& operator << (std::ostream &out, MyString &str) {for (int i = 0; i < str.Size(); i++) {out << str[i] << " ";}return out;
}std::istream& operator >> (std::istream& in, MyString& str) {char p[50];in.getline(p, 50);str = p;return in;
}bool operator < (const MyString& left, const MyString& right) {int i = 0;while (left[i] != '\0' && right[i] != '\0' && left[i] == right[i]) {i++;}if (left[i] < right[i]) {return true;}return false;
}bool operator > (const MyString& left, const MyString& right) {int i = 0;while (left[i] != '\0' && right[i] != '\0' && left[i] == right[i]) {i++;}if (left[i] > right[i]) {return true;}return false;
}bool operator == (const MyString& left, const MyString& right) {if (left.Size() != right.Size()) {return false;}int i = 0;while (left[i] != '\0' && right[i] != '\0') {if (left[i] != right[i]) {return false;}i++;}return true;
}bool operator != (const MyString& left, const MyString& right) {if (left.Size() != right.Size()) {return true;}int i = 0;while (left[i] != '\0' && right[i] != '\0') {if (left[i] != right[i]) {return true;}i++;}return false;
}int main() {printf("构造 --------------------------------\n");MyString s1(3, 'a'); // 默认构造函数MyString s2(s1); // 拷贝构造函数MyString s3("abcdefghijklmn"); // 通过 char * 来构造函数MyString s4; // 空的字符串printf("\noperator << --------------------------------\n");std::cout << "s1: " << s1 << std::endl; // 输出字符串，使用重载的运算符 <<std::cout << "s2: " << s2 << std::endl;std::cout << "s3: " << s3 << std::endl;std::cout << "s4: " << s4 << std::endl;printf("\noperator = --------------------------------\n");s4 = s3; // 字符串赋值，使用字符串对象赋值std::cout << "s4: " << s4 << std::endl;s4 = "abcd"; // 使用 char * 赋值std::cout << "s4: " << s4 << std::endl;std::cout << "s4[2] = " << s4[2] << std::endl;printf("\noperator += --------------------------------\n");s4 += "mmmm";std::cout << "s4: " << s4 << std::endl;s4 += s2;std::cout << "s4: " << s4 << std::endl;printf("\noperator >> --------------------------------\n");std::cin >> s2;std::cout << "s2: " << s2 << std::endl;printf("\n<, >, ==, != --------------------------------\n");MyString t1 = "1234";MyString t2 = "1234";MyString t3 = "12345";MyString t4 = "12335";std::cout << "t1 == t2 ? " << (t1 == t2) << std::endl;std::cout << "t1 < t3 ? " << (t1 < t3) << std::endl;std::cout << "t1 > t4 ?  " << (t1 > t4) << std::endl;std::cout << "t1 != t4 ? " << (t1 != t4) << std::endl;return 0;
}

如下代码，<、>、==、!= 声明为成员函数，当成员函数和友元函数同时存在时，优先选用成员函数。

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <string>class MyString {
public:MyString(); // 默认构造函数MyString(int n, char c); // 普通构造函数MyString(const char *str); // 普通构造函数MyString(const MyString &str); // 拷贝构造函数MyString& operator = (char *str); // 重载 =，使用字符串赋值MyString& operator = (const MyString &s); // 重载 =，使用对象赋值~MyString(); // 析构函数char& operator[] (int i); // 重载 [], 实现数组操作const char& operator[] (int i) const; // 重载 []，事项数组操作，对象为常量MyString& operator += (const MyString& str); // 重载 +=，与对象相加MyString& operator += (const char *str); // 重载 +=，与字符串相加friend std::ostream& operator << (std::ostream& out, MyString& str); // 重载 <<，实现输出流friend std::istream& operator >> (std::istream& in, MyString& str); // 重载 >>，实现输入流friend bool operator < (const MyString& left, const MyString& right); // 重载 <friend bool operator > (const MyString& left, const MyString& right); // 重载 >friend bool operator == (const MyString& left, const MyString& right); // 重载 ==friend bool operator != (const MyString& left, const MyString& right); // 重载 !=bool operator < (const MyString& str); // 重载 <bool operator > (const MyString& str); // 重载 >bool operator == (const MyString& str); // 重载 ==bool operator != (const MyString& str); // 重载 !=char *GetData() const;// 对象被声明为 const，那么这个对象调用的函数也需要是 const 类型int Size() const;private:int size;char *data = nullptr;
};MyString::MyString() {// 默认构造函数，只包括一个字符 '\0'printf("MyString()\n");data = new char[1];*data = '\0';size = 0;
}MyString::MyString(int n, char c) {printf("MyString(int n, char c), n = %d, c = %c\n", n, c);data = new char[n + 1];for (int i = 0; i < n; i++) {*(data + i) = c;}*(data + n) = '\0';size = n;
}MyString::MyString(const char *str) {printf("MyString(const char *str), str = %s\n", str);if (str == nullptr) {data = new char[1];*data = '\0';size = 0;} else {data = new char[strlen(str) + 1];strcpy(data, str);size = strlen(str);}
}MyString::MyString(const MyString &str) {printf("MyString(const MyString &str), str = %s\n",  str.GetData());data = new char[str.size + 1];strcpy(data, str.data);size = str.size;
}MyString& MyString::operator = (char *str) {printf("operator =, MyString = char *, str = %s\n", str);if (data != nullptr) {delete []data;}data = new char[strlen(str) +1];strcpy(data, str);return *this;
}MyString&MyString::operator = (const MyString& str) {printf("operator =, MyString = MyString, str = %s\n", str.GetData());if (this == &str) {return *this;}if (data != nullptr) {delete []data;}data = new char[str.size + 1];strcpy(data, str.data);return *this;
}MyString::~MyString() {if (data != nullptr) {delete []data;data = nullptr;size = 0;}
}char& MyString::operator [](int i) {return data[i];
}const char& MyString::operator[] (int i) const {return data[i];
}MyString& MyString::operator +=(const MyString& s) {printf("operator += MyString, str = %s\n", s.GetData());int length = size + s.size + 1;char *tmp = data;data = new char[length];size = length - 1;strcpy(data, tmp);strcat(data, s.data);delete []tmp;return *this;
}MyString& MyString::operator += (const char *str) {printf("operator += char *, str = %s\n", str);if (str == nullptr) {return *this;}int length = size + strlen(str) + 1;char *tmp = data;data = new char[length];size = length - 1;strcpy(data, tmp);strcat(data, str);delete []tmp;return *this;
}int MyString::Size() const {return size;
}char* MyString::GetData() const {return data;
}std::ostream& operator << (std::ostream &out, MyString &str) {for (int i = 0; i < str.Size(); i++) {out << str[i] << " ";}return out;
}std::istream& operator >> (std::istream& in, MyString& str) {char p[50];in.getline(p, 50);str = p;return in;
}bool operator < (const MyString& left, const MyString& right) {int i = 0;while (left[i] != '\0' && right[i] != '\0' && left[i] == right[i]) {i++;}if (left[i] < right[i]) {return true;}return false;
}bool operator > (const MyString& left, const MyString& right) {int i = 0;while (left[i] != '\0' && right[i] != '\0' && left[i] == right[i]) {i++;}if (left[i] > right[i]) {return true;}return false;
}bool operator == (const MyString& left, const MyString& right) {if (left.Size() != right.Size()) {return false;}int i = 0;while (left[i] != '\0' && right[i] != '\0') {if (left[i] != right[i]) {return false;}i++;}return true;
}bool operator != (const MyString& left, const MyString& right) {if (left.Size() != right.Size()) {return true;}int i = 0;while (left[i] != '\0' && right[i] != '\0') {if (left[i] != right[i]) {return true;}i++;}return false;
}//
bool MyString::operator < (const MyString& str) {int i = 0;while (data[i] != '\0' && str[i] != '\0' && data[i] == str[i]) {i++;}if (data[i] < str[i]) {return true;}return false;
}bool MyString::operator > (const MyString& str) {int i = 0;while (data[i] != '\0' && str[i] != '\0' && data[i] == str[i]) {i++;}if (data[i] > str[i]) {return true;}return false;
}bool MyString::operator == (const MyString& str) {if (size != str.Size()) {return false;}int i = 0;while (data[i] != '\0' && str[i] != '\0') {if (data[i] != str[i]) {return false;}i++;}return true;
}bool MyString::operator != (const MyString& str) {if (size != str.Size()) {return true;}int i = 0;while (data[i] != '\0' && str[i] != '\0') {if (data[i] != str[i]) {return true;}i++;}return false;
}int main() {printf("构造 --------------------------------\n");MyString s1(3, 'a'); // 默认构造函数MyString s2(s1); // 拷贝构造函数MyString s3("abcdefghijklmn"); // 通过 char * 来构造函数MyString s4; // 空的字符串printf("\noperator << --------------------------------\n");std::cout << "s1: " << s1 << std::endl; // 输出字符串，使用重载的运算符 <<std::cout << "s2: " << s2 << std::endl;std::cout << "s3: " << s3 << std::endl;std::cout << "s4: " << s4 << std::endl;printf("\noperator = --------------------------------\n");s4 = s3; // 字符串赋值，使用字符串对象赋值std::cout << "s4: " << s4 << std::endl;s4 = "abcd"; // 使用 char * 赋值std::cout << "s4: " << s4 << std::endl;std::cout << "s4[2] = " << s4[2] << std::endl;printf("\noperator += --------------------------------\n");s4 += "mmmm";std::cout << "s4: " << s4 << std::endl;s4 += s2;std::cout << "s4: " << s4 << std::endl;printf("\noperator >> --------------------------------\n");std::cin >> s2;std::cout << "s2: " << s2 << std::endl;printf("\n<, >, ==, != --------------------------------\n");MyString t1 = "1234";MyString t2 = "1234";MyString t3 = "12345";MyString t4 = "12335";std::cout << "t1 == t2 ? " << (t1 == t2) << std::endl;std::cout << "t1 < t3 ? " << (t1 < t3) << std::endl;std::cout << "t1 > t4 ?  " << (t1 > t4) << std::endl;std::cout << "t1 != t4 ? " << (t1 != t4) << std::endl;return 0;
}