C++系列-运算符+重载

运算符重载含义

• C++ 允许在同一作用域中的某个运算符指定多个定义，称为运算符重载。
• 通过运算符重载，可以使得已有的运算符具有其它的功能，适应于不同的操作数类型，经常运算符重载应用于自定义数据类型。

重载的方式

• 当成员函数和普通函数均可实现重载时，优先选用成员函数重载。
• 内置数据类型的表达式的运算符不能重载。比如3+2， 这个+不能变化其含义。
• 不要乱用重载，需要做到见名知意。

成员函数实现重载

code:
	#include 
	using namespace std;
	class Horse
	{
	public:
		int age = 3;
		string color = "white";
		// 用普通的成员函数实现自定义类型相加
		Horse horse_plus(Horse& h1)		// 定义成员函数，实现Horse类型相加，返回Horse类型
		{
			Horse result;
			result.age = this->age + h1.age;
			result.color = this->color + h1.color;
			return result;
		}
		// 通过成员函数重载+，只是系统给自定义了名称operator+，使用+运算符和调用operator+及horse_plus函数是一样的
		Horse operator+ (Horse& h1)
		{
			Horse result;
			result.age = this->age + h1.age;
			result.color = this->color + h1.color;
			return result;
		}
	};
	
	int main()
	{
		Horse h1;
		Horse h2;
		h1.age = 10;
		h2.age = 20;
		h1.color = "brown";
		h2.color = "white";
		cout << "horse_plus, age:" << h1.horse_plus(h2).age << endl;		// 使用了成员函数实现加法
		cout << "horse_plus, color: " << h1.horse_plus(h2).color << endl;	// 使用了成员函数实现加法
		cout << "+, age: " << (h1 + h2).age << endl;		// + 运算符重载
		cout << "+, color: " << (h1 + h2).color << endl;	// + 运算符重载
		cout << "operator+, age: " << h1.operator+(h2).age << endl;		// + 运算符重载调用的本质其实是成员函数调用
		cout << "operator+, color : " << h1.operator+(h2).color << endl;// + 运算符重载调用的本质其实是成员函数调用
		system("pause");
		return 0;
	}
result:
	horse_plus, age:30
	horse_plus, color: brownwhite
	+, age: 30
	+, color: brownwhite
	operator+, age: 30
	operator+, color : brownwhite

全局函数实现重载

code:
	#include 
	using namespace std;
	class Horse
	{
	public:
		int age = 3;
		string color = "white";
	};
	// 通过全局函数重载+，系统给自定义了名称operator+，使用+运算符和调用operator+是一样的
	Horse operator+ (Horse& h1, Horse& h2)		// 全局函数重载，参数要传两个
	{
		Horse result;
		result.age = h1.age + h2.age;
		result.color = h1.color + h2.color;
		return result;
	}
	
	int main()
	{
		Horse h1;
		Horse h2;
		h1.age = 10;
		h2.age = 20;
		h1.color = "brown";
		h2.color = "white";
		// h1 + h2相当于是operator+(h1, h2)
		cout << "+, age: " << (h1 + h2).age << endl;			
		cout << "+, color: " << (h1 + h2).color << endl;
		cout << "operator+, age: " << operator+(h1, h2).age << endl;
		cout << "operator+, color : " << operator+(h1, h2).color << endl;
		system("pause");
		return 0;
	}
result:
	+, age: 30
	+, color: brownwhite
	operator+, age: 30
	operator+, color : brownwhite

运算符重载也可以实现函数重载

code:
	#include 
	using namespace std;
	class Horse
	{
	public:
		int age = 3;
		string color = "white";
		Horse horse_plus(Horse& h1)
		{
			Horse result;
			result.age = this->age + h1.age;
			result.color = this->color + h1.color;
			return result;
		}
		// 通过成员函数重载+，只是系统给自定义了名称operator+，和调用operator+及horse_plus函数是一样的。
		Horse operator+ (Horse& h1)
		{
			Horse result;
			result.age = this->age + h1.age;
			result.color = this->color + h1.color;
			return result;
		}
		// 运算符重载也可以实现函数重载
		Horse operator+ (int num)
		{
			Horse result;
			result.age = this->age + num;
			return result;
		}
	};
	
	int main()
	{
		Horse h1;
		Horse h2;
		h1.age = 10;
		h2.age = 20;
		h1.color = "brown";
		h2.color = "white";
		cout << "horse_plus, age:" << h1.horse_plus(h2).age << endl;
		cout << "horse_plus, color: " << h1.horse_plus(h2).color << endl;
		cout << "+, age: " << (h1 + h2).age << endl;					// 运算符重载实现的函数重载
		cout << "+, color: " << (h1 + h2).color << endl;				// 运算符重载实现的函数重载
		cout << "operator+, age: " << h1.operator+(h2).age << endl;		// 运算符重载调用的本质是函数调用
		cout << "operator+, color : " << h1.operator+(h2).color << endl;// 运算符重载调用的本质是函数调用
		cout << "operator+2, age: " << (h1 + 100).age << endl;			// 运算符重载实现的函数重载
		system("pause");
		return 0;
	}
result:
	horse_plus, age:30
	horse_plus, color: brownwhite
	+, age: 30
	+, color: brownwhite
	operator+, age: 30
	operator+, color : brownwhite
	operator+2, age: 110

运算符实现连续运算

在int，float等类型的加法运算中，是可以实现连加的，那么自定义类型的对象的连加如何实现？

• 返回值必须是和运算符本身得到的结果的含义一致。如int+int，返回int，object+object，返回object。
• 返回值必须是对象的引用，这样连续操作时是对原对象进行操作，才能在原基础上练加。
• 当形参类型不同时，如果要实现操作数交换顺序，则需要进行运算符的函数重载。

code:
	#include 
	using namespace std;
	class Horse
	{
	public:
		int age = 3;
		string color = "white";
		Horse& operator+ (Horse& h1)		// 运算符+重载，返回引用
		{
			Horse result;
			this->age = this->age + h1.age;
			this->color = this->color + h1.color;
			return *this;					// this是对象本身的指针，*this是对象本身，返回值:自定义类型& 是对象本身的引用
		}
	};
	
	int main()
	{
		Horse h1;
		Horse h2;
		Horse h3;
		h1.age = 10;
		h2.age = 20;
		h3.age = 30;
		h1.color = "black";
		h2.color = "white";
		h3.color = "brown";
		cout << "+, age: " << (h1 + h2 + h3).age << endl;
		cout << "h1.color: " << h1.color << endl;
		cout << "+, color: " << (h1 + h2 + h3).color << endl;
		system("pause");
		return 0;
result:
	+, age: 60
	h1.color: blackwhitebrown
	+, color: blackwhitebrownwhitebrown
}

code:
	#include 
	using namespace std;
	class Horse
	{
	public:
		int age = 3;
		string color = "white";
	};
	
	Horse& operator+ (Horse& h1, Horse& h2)
	{
		Horse result;
		h1.age = h1.age + h2.age;
		h1.color = h1.color + h2.color;
		return h1;
	}
	
	int main()
	{
		Horse h1;
		Horse h2;
		Horse h3;
		h1.age = 10;
		h2.age = 20;
		h3.age = 30;
		h1.color = "black";
		h2.color = "white";
		h3.color = "brown";
		cout << "+, age: " << (h1 + h2 + h3).age << endl;
		cout << "h1.color: " << h1.color << endl;
		cout << "+, color: " << (h1 + h2 + h3).color << endl;
		system("pause");
		return 0;
	}
result:
	+, age: 60
	h1.color: blackwhitebrown
	+, color: blackwhitebrownwhitebrown