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;
}