c++ 友元 运算符重载详解

友元

c++是面向对象的，目的之一：封装

封装：

优点之一，就是安全。

缺点：在某些特殊的场合，不是很方便。

华为与IBM 40亿的咨询故事

 

 

IBM需要对华为各级部门做深度咨询分析，

为了提高咨询效率，由任正非直接授权，直接获取各部门的所有权限。

使用前提：

某个类需要实现某种功能，但是这个类自身，因为各种原因，无法自己实现。

需要借助于“外力”才能实现。

友元函数

使用全局函数作为友元函数

需求：

计算机和计算机的升级

Computer.h

#pragma once
#include <string>
​
class Computer
{
public:Computer();
​// 使用全局函数作为友元函数 友元函数可以访问类的所有数据成员friend void upgrade(Computer* computer);
​std::string description();
​
private:std::string cpu; //CPU芯片
};

computer.cpp

#include "Computer.h"
#include <sstream>  
​
Computer::Computer()
{cpu = "i7";
}
​
std::string Computer::description()
{std::stringstream  ret;ret << "CPU:" << cpu;return ret.str();
}

main.cpp

#include <stdio.h>
#include <iostream>
#include <Windows.h>
​
#include "Computer.h"
​
void upgrade(Computer* computer) {computer->cpu = "i9";  //直接访问对象的私有数据成员！！！
}
​
int main(void) {Computer shanxing;std::cout << shanxing.description() << std::endl;upgrade(&shanxing);
​std::cout << shanxing.description() << std::endl;
​system("pause");return 0;
}

使用类的成员函数作为友元函数

需求：

计算机和计算机的升级

computer.h

#pragma once
#include <string>
#include <iostream>
#include  <sstream>
#include "Computerservice.h"
using namespace std;
​
class Computer
{
public:Computer();string description();friend void Computerservice::upgrade(Computer* computer);//friend void upgrade(Computer* computer);//把外部的全局函数申明为这个类的友元函数
private:string cpu;};
​

computerservice.h

#pragma once
class Computer;
​
class Computerservice
{
public:void upgrade(Computer* computer);
};

computer.cpp
​
​

#include "Computer.h"
​
Computer::Computer()
{this->cpu = "i7";
}
​
string Computer::description()
{stringstream des;des << "CPU" << cpu;return des.str();
​}
​

computerservice.cpp

#include "Computerservice.h"
#include "Computer.h"
​
void Computerservice::upgrade(Computer* computer)
{this->cpu = "i9";
}

​

main.cpp

#include "Computer.h"
#include <iostream>
#include "Computerservice.h"
​
//void upgrade(Computer * computer) {
//  computer->cpu = "i9";
//}
​
int main(void) {Computer computer;Computerservice serviece;cout << computer.description() << endl;serviece.upgrade(&computer);cout << computer.description() << endl;return 0;
}

功能上，这两种形式，都是相同，应用场合不同。

一个是，使用普通的全局函数，作为自己的朋友，实现特殊功能。

一个是，使用其他类的成员函数，作为自己的朋友，实现特殊功能。

友元类

友元类的作用

如果把A类作为B类的友元类，

那么A类的所有成员函数【在A类的成员函数内】，就可以直接访问【使用】B类的私有成员。

即，友元类可以直接访问对应类的所有成员！！！

Demo

Computer.h

#pragma once
#include <string>
#include <iostream>
#include  <sstream>
#include "Computerservice.h"
using namespace std;
class ComputerService;class Computer
{
public:friend class Computerservice;Computer();string description();private:string cpu;};Computer.cpp#include "Computer.h"
#include "Computerservice.h"
Computer::Computer()
{this->cpu = "i7";
}string Computer::description()
{stringstream des;des << "CPU" << cpu;return des.str();}

ComputerService.h

#pragma once
class Computer;class Computerservice
{
public:void upgrade(Computer* computer);void clear(Computer* computer);void kill(Computer* computer);
};

ComputerService.cpp

#include "Computerservice.h"
#include "Computer.h"void Computerservice::upgrade(Computer* computer)
{computer->cpu = "i9";
}void Computerservice::clear(Computer* computer)
{cout << "正在对电脑执行清理[CPU:" << computer->cpu << "]" < endl;
}void Computerservice::kill(Computer* computer)
{cout << "杀毒" << endl;
}

main.cpp

#include "Computer.h"
#include <iostream>
#include "Computerservice.h"int main(void) {Computer computer;Computerservice serviece;cout << computer.description() << endl;serviece.upgrade(&computer);cout << computer.description() << endl;serviece.clear(&computer);serviece.kill(&computer);return 0;
}

使用注意

友元类，和友元函数，使用friend关键字进行声明即可，与访问权限无关，

所以，可以放在private/pulic/protected任意区域内。

万物可运算-运算符重载

为什么要使用运算符重载

C/C++的运算符，支持的数据类型，仅限于基本数据类型。

问题：一头牛+一头马 = ？（牛马神兽？）

一个圆 +一个圆 = ？ （想要变成一个更大的圆）

一头牛 – 一只羊 = ？ (想要变成4只羊，原始的以物易物：1头牛价值5只羊)

解决方案：使用运算符重载

运算符重载基本用法

方式1-使用成员函数重载运算符

使用成员函数重载运算符

需求： // 规则：

// 一斤牛肉：2斤猪肉

// 一斤羊肉：3斤猪肉

Cow.h

#pragma onceclass Pork;
class Goat;class Cow
{
public:Cow(int weight);// 参数此时定义为引用类型，更合适，避免拷贝Pork operator+(const Cow& cow);  //同类型进行运算，很频繁Pork operator+(const Goat& goat); //不同类型进行运算，比较少见
private:int weight = 0;
};

Cow.cpp

#include "Cow.h"
#include "Pork.h"
#include "Goat.h"Cow::Cow(int weight)
{this->weight = weight;
}// 规则：
// 一斤牛肉：2斤猪肉
// 一斤羊肉：3斤猪肉
Pork Cow::operator+(const Cow &cow)
{int tmp = (this->weight + cow.weight) * 2;return Pork(tmp);
}Pork Cow::operator+(const Goat& goat)
{// 不能直接访问goat.weight//int tmp = this->weight * 2 + goat.weight * 3;int tmp = this->weight * 2 + goat.getWeight() * 3;return Pork(tmp);
}

Goat.cpp

#include "Goat.h"Goat::Goat(int weight) {this->weight = weight;
}int Goat::getWeight(void) const
{return weight;
}

Goat.h

#pragma once
class Goat
{
public:Goat(int weight);int getWeight(void) const;
private:int weight = 0;
};

Pork.h

#pragma once
#include <iostream>class Pork
{
public:Pork(int weight);std::string  description(void);private:int weight = 0;
};

Pork.cpp

#include "Pork.h"
#include <sstream>Pork::Pork(int weight)
{this->weight = weight;
}std::string Pork::description(void)
{std::stringstream ret;ret << weight << "斤猪肉";return ret.str();
}

main.cpp

#include <iostream>
#include "Pork.h"
#include "Cow.h"
#include "Goat.h"int main(void) {Cow c1(100);Cow c2(200);// 调用c1.operator+(c2);//相当于：Pork p = c1.operator+(c2);Pork p = c1 + c2;std::cout << p.description() << std::endl;Goat g1(100);p = c1 + g1;std::cout << p.description() << std::endl;system("pause");return 0;
}

方式二 - 使用非成员函数【友元函数】重载运算符

Cow.h

#pragma onceclass Pork;
class Goat;class Cow
{
public:Cow(int weight);// 有友元函数实现运算符重载friend Pork operator+(const Cow& cow1, const Cow& cow2);friend Pork operator+(const Cow& cow1, const Goat& goat);
private:int weight = 0;
};

main.cpp

其他文件不变

两种方式的区别

区别：

1. 使用成员函数来实现运算符重载时，少写一个参数，因为第一个参数就是this指针。

两种方式的选择：

1. 一般情况下，单目运算符重载，使用成员函数进行重载更方便（不用写参数）

2. 一般情况下，双目运算符重载，使用友元函数

   #include <iostream>
   #include "Pork.h"
   #include "Cow.h"
   #include "Goat.h"Pork operator+(const Cow &cow1, const Cow &cow2)
   {int tmp = (cow1.weight + cow2.weight) * 2;return Pork(tmp);
   }Pork operator+(const Cow& cow1, const Goat& goat)
   {int tmp = cow1.weight * 2 + goat.getWeight() * 3;return Pork(tmp);
   }int main(void) {Cow c1(100);Cow c2(200);Goat g1(100);Pork p = c1 + c2;std::cout << p.description() << std::endl;p = c1 + g1;  // 思考：如何实现：p = g1 + c1;std::cout << p.description() << std::endl;system("pause");return 0;
   }

   更直观

方便实现a+b和b+a相同的效果，成员函数方式无法实现。

例如： 100 + cow; 只能通过友元函数来实现

cow +100; 友元函数和成员函数都可以实现

特殊情况：

（1） = （） [ ] -> 不能重载为类的友元函数！！！（否则可能和C++的其他规则矛盾），只能使用成员函数形式进行重载。

（2）如果运算符的第一个操作数要求使用隐式类型转换，则必须为友元函数（成员函数方式的第一个参数是this指针）

注意：

同一个运算符重载， 不能同时使用两种方式来重载，会导致编译器不知道选择哪一个（二义性）

运算符重载的禁区和规则

1. 为了防止对标准类型进行运算符重载，

   C++规定重载运算符的操作对象至少有一个不是标准类型，而是用户自定义的类型

   比如不能重载 1+2

   但是可以重载 cow + 2 和 2 + cow // cow是自定义的对象

   2.不能改变原运算符的语法规则， 比如不能把双目运算符重载为单目运

2. 不能改变原运算符的优先级

   4.不能创建新的运算符，比如 operator*就是非法的, operator是可以的

3. 不能对以下这四种运算符，使用友元函数进行重载

   = 赋值运算符，（）函数调用运算符，[ ]下标运算符，->通过指针访问类成员

4. 不能对禁止重载的运算符进行重载

不能被重载的运算符

成员访问	.
域运算	：：
内存长度运算	sizeof
三目运算	? : :
预处理	#

可以被重载的运算符

双目运算符	+ - * / %
关系运算符	== != < <= > >=
逻辑运算符	&& || !
单目运算符	+(正号) -（负号） *（指针） &（取地址） ++ --
位运算	& | ~ ^ <<（左移） >>（右移）
赋值运算符	= += -= *= /= %= &= |= ^= <<= >>=
内存分配	new delete new[ ] delete[ ]
其他	( ) 函数调用-> 成员访问 [ ] 下标， 逗号

重载运算符实例

重载赋值运算符=

Boy.h

#pragma once
#include <string>
using namespace std;
class Boy
{
public:Boy(const char* name = NULL, int age = 0,int salary = 0,int darkHorse = 0);~Boy();string description();Boy& operator= (const Boy & boy);
private:char* name;int age;int salary;int darkHorse;//潜力系数unsigned int id;//编号static int LAST_ID;};

Boy.cpp

#include "Boy.h"
#include <string.h>
#include <sstream>
using namespace std;
int Boy::LAST_ID = 0;Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {;name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) +1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;}Boy::~Boy()
{if (name) {delete name;}
}string Boy::description()
{stringstream des;des << "ID:" << id << "\t姓名:" << name << "\t年龄" << age << "\t薪资" << salary << "/t黑马系数" << darkHorse;return des.str();
}Boy& Boy::operator=(const Boy& boy)
{// TODO: 在此处插入 return 语句if (name) {delete name;}this->name = new char[strlen(boy.name) + 1];strcpy_s(this->name, strlen(boy.name) + 1,boy.name);this->age = boy.age;this->darkHorse = boy.darkHorse;this->salary = boy.salary;//this->idreturn *this;//返回这个对象}

Main.cpp

#include <iostream>
#include "boy.h"int main(void) {Boy boy1("lucifer", 16, 10000, 10);Boy boy2, boy3;std::cout << boy1.description() << std::endl;std::cout << boy2.description() << std::endl;std::cout << boy3.description() << std::endl;boy3 = boy2 = boy1;std::cout << boy2.description() << std::endl;std::cout << boy3.description() << std::endl;system("pause");return 0;
}

重载运算符> < ==

Boy.h 的方法加入

public:		bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);
private:int power() const; //综合能力值

Boy.cpp

bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}

下标运算符重载[ ]

Boy.h

#pragma once
#include <string>class Boy
{
public:Boy(const char* name=NULL, int age=0, int salary=0, int darkHorse=0);~Boy();Boy& operator=(const Boy& boy);bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);int operator[](std::string index);int operator[](int index);std::string description(void);
private:char* name;int age;int salary;int darkHorse; //黑马值，潜力系数unsigned int id; // 编号static int LAST_ID;int power() const; //综合能力值
};

Boy.cpp

#include "boy.h"
#include <string.h>
#include <sstream>int Boy::LAST_ID = 0;  //初始值是0Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name)+1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;
}Boy::~Boy()
{if (name) {delete name;}
}Boy& Boy::operator=(const Boy& boy)
{if (name) {delete name;  //释放原来的内存}name = new char[strlen(boy.name) + 1]; //分配新的内存strcpy_s(name, strlen(boy.name)+1, boy.name);this->age = boy.age;this->salary = boy.salary;this->darkHorse = boy.darkHorse;//this->id = boy.id;  //根据需求来确定是否要拷贝idreturn *this;
}bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}int Boy::operator[](std::string index)
{if (index == "age") {return age;}else if (index == "salary") {return salary;}else if (index == "darkHorse") {return darkHorse;}else if (index == "power") {return power();}else {return -1;}
}int Boy::operator[](int index)
{if (index == 0) {return age;}else if (index == 1) {return salary;}else if (index == 2) {return darkHorse;}else if (index == 3) {return power();}else {return -1;}
}std::string Boy::description(void)
{std::stringstream ret;ret << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return ret.str();
}int Boy::power() const
{// 薪资* 黑马系数 + (100 - 年龄） * 1000int value = salary * darkHorse + (100 - age) * 100;return value;
}

main.cpp

#include <iostream>
#include "boy.h"int main(void) {Boy boy1("Rock", 38, 58000, 5);Boy boy2("Jack", 25, 50000, 10);std::cout << "age:" << boy1["age"] << std::endl;std::cout << "salary:" << boy1["salary"] << std::endl;std::cout << "darkHorse:" << boy1["darkHorse"] << std::endl;std::cout << "power:" << boy1["power"] << std::endl;std::cout << "[0]:" << boy1[0] << std::endl;std::cout << "[1]:" << boy1[1] << std::endl;std::cout << "[2]:" << boy1[2] << std::endl;std::cout << "[3]:" << boy1[3] << std::endl;system("pause");return 0;
}

输入输出的重载<< >>

为了更方便的实现复杂对象的输入和输出。

方式1（使用成员函数）

不推荐，该方式没有实际意义

Boy.h

#pragma once
#include <string>
#include <iostream>using namespace std;class Boy
{
public:Boy(const char* name = NULL, int age = 0, int salary = 0, int darkHorse = 0);~Boy();Boy& operator=(const Boy& boy);bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);int operator[](std::string index);int operator[](int index);ostream& operator<<(ostream& os) const;std::string description(void);private:char* name;int age;int salary;int darkHorse; //黑马值，潜力系数unsigned int id; // 编号static int LAST_ID;int power() const; //综合能力值
};

boy.cpp

#include "boy.h"
#include <string.h>
#include <sstream>int Boy::LAST_ID = 0;  //初始值是0Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;
}Boy::~Boy()
{if (name) {delete name;}
}Boy& Boy::operator=(const Boy& boy)
{if (name) {delete name;  //释放原来的内存}name = new char[strlen(boy.name) + 1]; //分配新的内存strcpy_s(name, strlen(boy.name) + 1, boy.name);this->age = boy.age;this->salary = boy.salary;this->darkHorse = boy.darkHorse;//this->id = boy.id;  //根据需求来确定是否要拷贝idreturn *this;
}bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}int Boy::operator[](std::string index)
{if (index == "age") {return age;}else if (index == "salary") {return salary;}else if (index == "darkHorse") {return darkHorse;}else if (index == "power") {return power();}else {return -1;}
}int Boy::operator[](int index)
{if (index == 0) {return age;}else if (index == 1) {return salary;}else if (index == 2) {return darkHorse;}else if (index == 3) {return power();}else {return -1;}
}ostream& Boy::operator<<(ostream& os) const
{os << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return os;
}std::string Boy::description(void)
{std::stringstream ret;ret << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return ret.str();
}int Boy::power() const
{// 薪资* 黑马系数 + (100 - 年龄） * 1000int value = salary * darkHorse + (100 - age) * 100;return value;
}

mian.cpp

#include <iostream>
#include "boy.h"int main(void) {Boy boy1("Rock", 38, 58000, 5);Boy boy2("Jack", 25, 50000, 10);// 调用： boy1.operator<<(cout);boy1 << cout;// 先调用 boy1.operator<<(cout)// 再调用 boy2.operator<<(cout)boy2 << (boy1 << cout);system("pause");return 0;
}

使用取来不方便

方式二（使用友元函数）

Boy.h

#pragma once
#include <string>
#include <iostream>#define AGE_KEY			"age"
#define SALARY_KEY		"salary"
#define DARK_HORSE_KEY  "darkHorse"
#define POWER_KEY		"power"typedef enum {AGE,SALARY,DARK_HORSE,POWER
}BOY_KEY_TYPE;using namespace std;class Boy
{
public:Boy(const char* name = NULL, int age = 0, int salary = 0, int darkHorse = 0);~Boy();Boy& operator=(const Boy& boy);bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);// 下标运算符的重载int operator[](std::string index);int operator[](int index);// 该方式不适合//ostream& operator<<(ostream& os) const;friend ostream& operator<<(ostream& os, const Boy& boy);friend istream& operator>>(istream& is, Boy& boy);std::string description(void);
private:char* name;int age;int salary;int darkHorse; //黑马值，潜力系数unsigned int id; // 编号static int LAST_ID;int power() const; //综合能力值
};

Boy.cpp

#include "boy.h"
#include <string.h>
#include <sstream>int Boy::LAST_ID = 0;  //初始值是0Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;
}Boy::~Boy()
{if (name) {delete name;}
}Boy& Boy::operator=(const Boy& boy)
{if (name) {delete name;  //释放原来的内存}name = new char[strlen(boy.name) + 1]; //分配新的内存strcpy_s(name, strlen(boy.name) + 1, boy.name);this->age = boy.age;this->salary = boy.salary;this->darkHorse = boy.darkHorse;//this->id = boy.id;  //根据需求来确定是否要拷贝idreturn *this;
}bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}int Boy::operator[](std::string index)
{if (index == AGE_KEY) {return age;}else if (index == SALARY_KEY) {return salary;}else if (index == DARK_HORSE_KEY) {return darkHorse;}else if (index == POWER_KEY) {return power();}else {return -1;}
}int Boy::operator[](int index)
{if (index == 0) {return age;}else if (index == 1) {return salary;}else if (index == 2) {return darkHorse;}else if (index == 3) {return power();}else {return -1;}
}//ostream& Boy::operator<<(ostream& os) const
//{
//	os << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"
//		<< salary << "\t黑马系数:" << darkHorse;
//	return os;
//}std::string Boy::description(void)
{std::stringstream ret;ret << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return ret.str();
}int Boy::power() const
{// 薪资* 黑马系数 + (100 - 年龄） * 1000int value = salary * darkHorse + (100 - age) * 100;return value;
}

Main.cpp

#include <iostream>
#include "Boy.h"using namespace std;ostream& operator<<(ostream& os, const Boy& boy) {os << "ID:" << boy.id << "\t姓名:" << boy.name << "\t年龄:" << boy.age << "\t薪资:"<< boy.salary << "\t黑马系数:" << boy.darkHorse;return os;
}istream& operator>>(istream& is, Boy& boy)
{string name2;is >> name2 >> boy.age >> boy.salary >> boy.darkHorse;boy.name = (char*)malloc((name2.length()+1) * sizeof(char));strcpy_s(boy.name, name2.length() + 1, name2.c_str());return is;
}int main(void) {Boy boy1("Rock", 38, 58000, 5);Boy boy2("Jack", 25, 50000, 10);cout << boy1 << endl;cin >> boy1;cout << boy1;system("pause");return 0;
}

重载-普通类型 =>类类型

调用对应的只有一个参数【参数的类型就是这个普通类型】的构造函数

需求： Boy boy1 = 10000; // 薪资 构造函数Boy(int);

Boy boy2 = "Rock" // 姓名 构造函数Boy(char *)

Boy.h

Boy(int salary);Boy(const char*);

Boy.cpp

Boy::Boy(int salary)
{const char* defaultName = "Unknow";this->name = new char[strlen(defaultName) + 1];strcpy_s(this->name, strlen(defaultName) + 1, defaultName);this->age =0;this->salary = salary;this->darkHorse = 0;this->id = ++LAST_ID;
}Boy::Boy(const char* name)
{this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = 0;this->salary = 0;this->darkHorse = 0;this->id = ++LAST_ID;
}

重载类类型=> 普通类型

调用特殊的运算符重载函数，类型转换函数，不需要写返回类型

类型转换函数：operator 普通类型 ( )

需求：

Boy boy1(“Rock”, 28, 10000, 5);int power = boy1; // power();char *name = boy1; // “Rock”

Boy.h

#pragma once
#include <string>
#include <iostream>#define AGE_KEY			"age"
#define SALARY_KEY		"salary"
#define DARK_HORSE_KEY  "darkHorse"
#define POWER_KEY		"power"typedef enum {AGE,SALARY,DARK_HORSE,POWER
}BOY_KEY_TYPE;using namespace std;class Boy
{
public:Boy(const char* name , int age, int  , int darkHorse);Boy(int salary);Boy(const char*);~Boy();Boy& operator=(const Boy& boy);bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);// 下标运算符的重载int operator[](std::string index);int operator[](int index);//类型运算符重载 不需要返回类型operator char* ()const;operator int()const;// 该方式不适合//ostream& operator<<(ostream& os) const;friend ostream& operator<<(ostream& os, const Boy& boy);friend istream& operator>>(istream& is, Boy& boy);std::string description(void);
private:char* name;int age;int salary;int darkHorse; //黑马值，潜力系数unsigned int id; // 编号static int LAST_ID;int power() const; //综合能力值
};

Boy.cpp

#include "boy.h"
#include <string.h>
#include <sstream>int Boy::LAST_ID = 0;  //初始值是0Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;
}Boy::Boy(int salary)
{const char* defaultName = "Unknow";this->name = new char[strlen(defaultName) + 1];strcpy_s(this->name, strlen(defaultName) + 1, defaultName);this->age =0;this->salary = salary;this->darkHorse = 0;this->id = ++LAST_ID;
}Boy::Boy(const char* name)
{this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = 0;this->salary = 0;this->darkHorse = 0;this->id = ++LAST_ID;
}Boy::~Boy()
{if (name) {delete name;}
}Boy& Boy::operator=(const Boy& boy)
{if (name) {delete name;  //释放原来的内存}name = new char[strlen(boy.name) + 1]; //分配新的内存strcpy_s(name, strlen(boy.name) + 1, boy.name);this->age = boy.age;this->salary = boy.salary;this->darkHorse = boy.darkHorse;//this->id = boy.id;  //根据需求来确定是否要拷贝idreturn *this;
}bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}int Boy::operator[](std::string index)
{if (index == AGE_KEY) {return age;}else if (index == SALARY_KEY) {return salary;}else if (index == DARK_HORSE_KEY) {return darkHorse;}else if (index == POWER_KEY) {return power();}else {return -1;}
}int Boy::operator[](int index)
{if (index == 0) {return age;}else if (index == 1) {return salary;}else if (index == 2) {return darkHorse;}else if (index == 3) {return power();}else {return -1;}
}Boy::operator char* () const
{return name;
}Boy::operator int() const
{return power();
}//ostream& Boy::operator<<(ostream& os) const
//{
//	os << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"
//		<< salary << "\t黑马系数:" << darkHorse;
//	return os;
//}std::string Boy::description(void)
{std::stringstream ret;ret << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return ret.str();
}int Boy::power() const
{// 薪资* 黑马系数 + (100 - 年龄） * 1000int value = salary * darkHorse + (100 - age) * 100;return value;
}

main.cpp

#include <iostream>
#include "Boy.h"using namespace std;ostream& operator<<(ostream& os, const Boy& boy) {os << "ID:" << boy.id << "\t姓名:" << boy.name << "\t年龄:" << boy.age << "\t薪资:"<< boy.salary << "\t黑马系数:" << boy.darkHorse;return os;
}istream& operator>>(istream& is, Boy& boy)
{string name2;is >> name2 >> boy.age >> boy.salary >> boy.darkHorse;boy.name = (char*)malloc((name2.length() + 1) * sizeof(char));strcpy_s(boy.name, name2.length() + 1, name2.c_str());return is;
}int main(void) {Boy boy1("Rock", 38, 58000, 5);int power = boy1;char* name = boy1;system("pause");return 0;
}

类类型之间的转换 类类型A=> 类类型B

调用对应的只有一个参数【参数的类型就是类类型A】的构造函数

也可以使用类型转换函数，但是使用对应的构造函数更合适

实例：

把Boy类型，转换为Man类型

Boy.h

#pragma once
#include <string>
#include <iostream>
#include <ostream>
#include <istream>
#include <fstream>
#define AGE_KEY			"age"
#define SALARY_KEY		"salary"
#define DARK_HORSE_KEY  "darkHorse"
#define POWER_KEY		"power"typedef enum {AGE,SALARY,DARK_HORSE,POWER
}BOY_KEY_TYPE;using namespace std;
class Man;
class Boy
{
public:Boy(const char* name , int age, int  , int darkHorse);Boy(int salary);Boy(const char*);~Boy();char* getname()const;Boy& operator=(const Boy& boy);bool operator>(const Boy& boy);bool operator<(const Boy& boy);bool operator==(const Boy& boy);// 下标运算符的重载int operator[](std::string index)const;int operator[](int index)const;//类型运算符重载 不需要返回类型operator char* ()const;operator int()const;// 该方式不适合//ostream& operator<<(ostream& os) const;friend ostream& operator<<(ostream& os, const Boy& boy);friend istream& operator>>(istream& is, Boy& boy);std::string description(void);
private:char* name;int age;int salary;int darkHorse; //黑马值，潜力系数unsigned int id; // 编号static int LAST_ID;int power() const; //综合能力值
};//istream& operator>>(istream& is, Boy& boy);
//ostream& operator<<(ostream& os, const Boy& boy);

Boy.cpp

#include "Boy.h"
#include <string.h>
#include <sstream>int Boy::LAST_ID = 0;  //初始值是0
ostream& operator<<(ostream& os, const Boy& boy) {os << "ID:" << boy.id << "\t姓名:" << boy.name << "\t年龄:" << boy.age << "\t薪资:"<< boy.salary << "\t黑马系数:" << boy.darkHorse;return os;
}istream& operator>>(istream& is, Boy& boy)
{string name2;is >> name2 >> boy.age >> boy.salary >> boy.darkHorse;boy.name = (char*)malloc((name2.length() + 1) * sizeof(char));strcpy_s(boy.name, name2.length() + 1, name2.c_str());return is;
}Boy::Boy(const char* name, int age, int salary, int darkHorse)
{if (!name) {name = "未命名";}this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = age;this->salary = salary;this->darkHorse = darkHorse;this->id = ++LAST_ID;
}Boy::Boy(int salary)
{const char* defaultName = "Unknow";this->name = new char[strlen(defaultName) + 1];strcpy_s(this->name, strlen(defaultName) + 1, defaultName);this->age =0;this->salary = salary;this->darkHorse = 0;this->id = ++LAST_ID;
}Boy::Boy(const char* name)
{this->name = new char[strlen(name) + 1];strcpy_s(this->name, strlen(name) + 1, name);this->age = 0;this->salary = 0;this->darkHorse = 0;this->id = ++LAST_ID;
}Boy::~Boy()
{if (name) {delete name;}
}char* Boy::getname() const
{return name;
}Boy& Boy::operator=(const Boy& boy) 
{if (name) {delete name;  //释放原来的内存}name = new char[strlen(boy.name) + 1]; //分配新的内存strcpy_s(name, strlen(boy.name) + 1, boy.name);this->age = boy.age;this->salary = boy.salary;this->darkHorse = boy.darkHorse;//this->id = boy.id;  //根据需求来确定是否要拷贝idreturn *this;
}bool Boy::operator>(const Boy& boy)
{// 设置比较规则：// 薪资 * 黑马系数 + (100-年龄）*100if (power() > boy.power()) {return true;}else {return false;}
}bool Boy::operator<(const Boy& boy)
{if (power() < boy.power()) {return true;}else {return false;}
}bool Boy::operator==(const Boy& boy)
{if (power() == boy.power()) {return true;}else {return false;}
}int Boy::operator[](std::string index)const
{if (index == AGE_KEY) {return age;}else if (index == SALARY_KEY) {return salary;}else if (index == DARK_HORSE_KEY) {return darkHorse;}else if (index == POWER_KEY) {return power();}else {return -1;}
}int Boy::operator[](int index)const
{if (index == 0) {return age;}else if (index == 1) {return salary;}else if (index == 2) {return darkHorse;}else if (index == 3) {return power();}else {return -1;}
}Boy::operator char* () const
{return name;
}Boy::operator int() const
{return power();
}//ostream& Boy::operator<<(ostream& os) const
//{
//	os << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"
//		<< salary << "\t黑马系数:" << darkHorse;
//	return os;
//}std::string Boy::description(void)
{std::stringstream ret;ret << "ID:" << id << "\t姓名:" << name << "\t年龄:" << age << "\t薪资:"<< salary << "\t黑马系数:" << darkHorse;return ret.str();
}int Boy::power() const
{// 薪资* 黑马系数 + (100 - 年龄） * 1000int value = salary * darkHorse + (100 - age) * 100;return value;
}

Man.h

#pragma once
#include <iostream>
#include <ostream>
#include <istream>
#include <fstream>
class Boy;using namespace std;class Man
{
public:Man(const char* name, int age, int salary);Man(const Boy& boy);~Man();friend ostream&operator<<(ostream& os, const Man& man);friend istream&operator>>(istream& is, Man& man);private:int age;int salary;char* name;
};
//ostream& operator<<(ostream& os, const Man& man);
//istream& operator<<(istream& is, const Man& man);

Man.cpp

#include <iostream>
#include <fstream>
#include <ostream>
#include <istream>
#include "Boy.h"
#include "Man.h"
using namespace std;Man::Man(const char* name, int age, int salary)
{if (!name) {name = "未命名";}this->name = new char[strlen(name)+1];strcpy_s(this->name, strlen(name) + 1, name);this->age = age;this->salary = salary;
}Man::Man(const Boy& boy)
{int len = strlen((char*)boy) + 1;this->name = new char[len];strcpy_s(name, len, (char*)boy);age = boy[AGE];salary = boy[SALARY];}Man::~Man()
{delete name;
}ostream& operator<<(ostream& os, const Man& man)
{os <<"[男人]"<<"\t姓名:"<< man.name <<"\t年龄:" << man.age << "\t薪资:"<< man.salary;return os;
}istream& operator>>(istream& is, Man& man)
{// TODO: 在此处插入 return 语句string name2;is >> name2 >> man.salary;man.name = (char*)malloc((name2.length() + 1) * sizeof(char));strcpy_s(man.name, name2.length() + 1, name2.c_str());return is;
}	//istream& operator>>(istream& is,const Man& man)
//{
//	string name2;
//	//is >> name2 >>man.salary ;
//	is >> name2 >> man.salary;
//	man.name = (char*)malloc((name2.length() + 1) * sizeof(char));
//	strcpy_s(man.name, name2.length() + 1, name2.c_str());
//	return is;
//}

Main.cpp

#include <iostream>
#include "Boy.h"
#include "Man.h"using namespace std;int main(void) {Boy boy("Rock", 38, 58000, 5);Man man = boy;cout << boy << endl;cout << man << endl;system("pause");return 0;
}

注意类型转换中的const const只能调用const方法 （operator函数）

常见错误总结-

const异常导致的BUG

小结：

const对象，只能调用对应的const方法

所以：

类的成员函数，如果已经确定不会修改任何数据成员，

那么，最好把这个成员函数，定义为const函数（在函数体的前面，参数列表的后面添加const）

main.cpp

#include <iostream>
#include "Human.h"
using namespace std;
int main()
{const Human lucifer("lucifer", 16, 10000);cout << lucifer[0] << endl;return 0;
}Human.cpp#include "Human.h"
#include <string.h>Human::Human(const char* name, int age, int salary) {int len = strlen(name) + 1;this->name = new char[len];strcpy_s(this->name, len, name);this->age = age;this->salary = salary;}Human::~Human()
{if (name) {delete name;}
}int Human::operator[](std::string index)const
{/*if (index == NAME) {}*/if (index == AGE_KEY) {return age;}else if (index == SALARY_KEY) {return salary;}else {return -1;}return 0;
}int Human::operator[](int index)
{if (index == AGE) {return age;}else if(index == SALARY){return salary;}else {return -1;}}

Human.h

#pragma once
#include <string>#define AGE_KEY			"age"
#define SALARY_KEY		"salary"
#define DARK_HORSE_KEY  "darkHorse"
#define POWER_KEY		"power"typedef enum {AGE,SALARY,POWER
}BOY_KEY_TYPE;class Human
{
public:Human(const char* name, int age,int salary);~Human();int operator[](std::string index)const;int operator[](int index);
private:char* name;int age;int salary;
};

如果此时调用main函数，那么此时的执行结果是

 

报错原因 const对象只能调用cosnt方法

operator= 的参数问题

赋值运算符的重载，应该使用这种方式：

Boy& operator=(const Boy &boy);

就是：参数要使用引用！

如果定义成：

Boy& operator=(const Boy *boy);

将会没有效果，编译器不会识别为赋值运算符的重载，

也就是：boy2 = boy1时不会调用这个函数

如果定义：

Boy& operator=(const Boy boy);

有效果，但是在调用时，会执行参数的传递：

比如：boy2 = boy1;

就会执行： boy2.operator=(boy1);

就会执行: const Boy boy = boy1;

就会执行： Boy类的赋值构造函数

有两个影响：

1） 浪费性能

2） 如果没有自定义的拷贝构造函数，而且这个类又有指针成员时，就会调用自动生成的拷贝构造函数，导致浅拷贝

如果析构函数中，对这个指针指向的内存做了释放，那就导致数据损坏或崩溃！

小结：

1）赋值·运算符的重载，一定要使用引用参数

2）如果一个类有指针成员，而且使用了动态内存分配，那么一定要定义自己的拷贝构造函数【要使用深拷贝】，避免调用自动生成的拷贝构造函数

因为自动生成的拷贝构造函数，是浅拷贝！