一、重载为一般函数
格式：返回类型 operator 运算符(参数列表)

struct Complex{//定义一个复数结构：包括实部与虚部两部分 
	double real;//实部 
	double imag;//虚部 
};
Complex operator+(Complex c1,Complex c2){//对加法运算的重载：将运算符'+' 定义为两复数相加 
	Complex c;
	c.real=c1.real+c2.real;
	c.imag=c1.imag+c2.imag;
	return c;
}
ostream&operator<<(ostream&out,Complex c){
	out<<"("<<c.real<<"+"<<c.imag<<"i)";
	return out;
}
int main()
{
	Complex c1={1,2},c2={3,4},c3;//结构体变量的声明
	c3=c1+c2; 
	cout<<c1<<"+"<<c2<<"="<<c3<<endl;
}

运行结果

二、重载为友元函数
需要访问类中的私有成员，则要将该重载函数在类中声明为友元类。
格式：friend 类型 重载函数名(参数);

#include<iostream>
using namespace std;
class Complex{//定义一个复数类包括实部与虚部两部分 
    private:
	   double real,imag;
	public:
		Complex(double r=0,double i=0):real(r),imag(i){
		}
		friend Complex operator+(Complex c1,Complex c2);
		friend ostream&operator<<(ostream&out,Complex c);
};
Complex operator+(Complex c1,Complex c2){//对加法运算的重载：将运算符'+' 定义为两复数相加,需要对复数类的私有属性进行访问 
	Complex c;
	c.real=c1.real+c2.real;
	c.imag=c1.imag+c2.imag;
	return c;
}
ostream&operator<<(ostream&out,Complex c){//需要对复数类的私有属性进行访问,声明成友元类 
	out<<"("<<c.real<<"+"<<c.imag<<"i)";
	return out;
}
int main()
{
	Complex c1(1,2),c2(3,4),c3;
	c3=c1+c2; 
	cout<<c1<<"+"<<c2<<"="<<c3<<endl;
}

三、重载为成员函数
直接声明为成员函数

#include<iostream>
using namespace std;
class Complex{//定义一个复数类包括实部与虚部两部分 
    private:
	   double real,imag;
	public:
		Complex(double r=0,double i=0):real(r),imag(i){
		}
		Complex operator+(Complex c);//直接在类中声明重载运算符 
		friend ostream&operator<<(ostream&out,Complex c);
};
Complex Complex::operator+(Complex c){//定义重载运算符 
	return Complex (real+c.real,imag+c.imag);
}
ostream&operator<<(ostream&out,Complex c){//需要对复数类的私有属性进行访问,声明成友元类 
	out<<"("<<c.real<<"+"<<c.imag<<"i)";
	return out;
}
int main()
{
	Complex c1(1,2),c2(3,4),c3;
	c3=c1+c2; 
	cout<<c1<<"+"<<c2<<"="<<c3<<endl;
}