[C++] Friend 함수 (프렌드 함수)

#include <iostream>
using namespace std;
 
class A {
    int a;
    int b;
public:
    A(int a = 100000) {
        this->a = a;
    }
    void setA(int a) { this->a = a; }
    int getA() { return this->a = a; }
    int operator+ (int data) {
        return this->a + data;
    }
    A& operator= (const A &aa) {
        cout << "대입" << endl;
        a = aa.a;
        return *this;
    }
    /*int operator + (int data) {
        return this->a + data;
    }*///연산자 함수를 사용할 때는 본래의 의미를 잃지 말아야 한다
    //friend int operator + (const A &aa, int data);
};
//int operator + (a &aa, int data) {
//    return aa.a + data;
//}
void main() {
    //cout << 3 + 4 << endl;
    A aa(10);
 
    //cout << aa.operator+ (4) << endl; //aa. operator + (4)     operator+(aa.4)
    A bb;
    A cc;
 
    cout << aa.getA() << endl;
    cout << bb.getA() << endl;
 
    cc = bb = aa;
    //cc.operator=(bb.operator= (aa);)
 
    cout << aa.getA() << endl;
    cout << bb.getA() << endl;
    cout << cc.getA() << endl;
 
}