二元運算子過載：

全域性函式：

#include <iostream>
using namespace std;

class Complex{//複數類
private:
    int a;
    int b;
    friend Complex operator + (Complex &, Complex &);//注意宣告友元函式
public:
    Complex(int a = 0, int b = 0){
        this->a = a;
        this->b = b;
    }
    void print(){
        cout << "a: " << a  << " " << "b: " << b << endl;
    }
};

Complex operator + (Complex &a1, Complex &a2){
    Complex tmp(a1.a + a2.a, a1.b + a2.b);
    return tmp;
}

int main()
{
    Complex c1(1, 2), c2(3, 4), c3;
    c3 = c1 + c2;
    c3.print();
    return 0;
}
 

#include <iostream>
using namespace std;

class Complex{//複數類
private:
    int a;
    int b;
public:
    Complex(int a = 0, int b = 0){
        this->a = a;
        this->b = b;
    }
    void print(){
        cout << "a: " << a  << " " << "b: " << b << endl;
    }
    Complex operator - (Complex &a2){
        Complex tmp(this->a - a2.a, this->b - a2.b);
        return tmp;
    }
};

int main()
{
    Complex c1(1, 2), c2(3, 4), c3;
    c3 = c1 - c2;
    c3.print();
    c3 = c2 - c1;
    c3.print();
    return 0;
}
 

類中和全域性函式寫在一塊兒了

#include <iostream>
using namespace std;

class Complex{//複數類
private:
    int a;
    int b;
    friend Complex operator ++ (Complex &);
    friend Complex operator ++ (Complex &, int);
public:
    Complex(int a = 0, int b = 0){
        this->a = a;
        this->b = b;
    }
    void print(){
        cout << "a: " << a  << " " << "b: " << b << endl;
    }
    Complex operator -- (){ //前置--操作符，成員函式方法實現
        this->a --;
        this->b --;
        return (*this);
    }
    Complex operator --(int){ //後置--操作符，成員函式方法實現
        Complex tmp = (*this);
        this->a --;
        this->b --;
        return tmp;
    }
};

Complex operator ++ (Complex &A){ //前置++操作符，用全域性函式實現
    A.a ++;
    A.b ++;
    return A;
}

Complex operator ++ (Complex &A, int){//後置++操作符，全域性函式實現     加一個佔位符跟前置加以區分
    Complex tmp = A;
    A.a ++;
    A.b ++;
    return tmp;
}

int main()
{
    Complex c1(1, 2);

    //前置++操作符，用全域性函式實現
    ++c1;          //Complex c = operator ++ (c1);
    c1.print();

    //前置--操作符，成員函式方法實現
    --c1;          //Complex c = c1.operator -- (&c1)
    c1.print();

    //後置++操作符，全域性函式實現
    c1++;          //Complex c = operator ++ (c1, 0);
    c1.print();
    
    //後置--操作符，成員函式方法實現
    c1--;          //Complex c = c1.operator -- (0);
    c1.print();

    return 0;
}