[C++/8.12] friend function and class

1. forward declaration 전방선언

#include<iostream>
using namespace std;

class B; // forward declaration (전방 선언)

class A
{
private:
    int m_value = 1;

    friend void doSomething(A &a, B &b);
};

class B
{
private:
    int m_value = 2;
    friend void doSomething(A &a, B &b);
};

void doSomething(A &a, B &b)
{
    cout << a.m_value << " " << b.m_value << endl;

}

int main()
{
    A a;
    B b;
    doSomething(a, b);
    return 0;
}

2. member function 선선언 및 body 후 선언.

#include<iostream>
using namespace std;

class A; // forward declaration (전방 선언)

class B
{
private:
    int m_value = 2;
public:
    // 선 선언
    void doSomething(A &a); 
};

class A
{
private:
    int m_value = 1;

    friend void B::doSomething(A &a);
};
// body 후 선언
void B::doSomething(A &a)
{
    cout << a.m_value << endl;
}

int main()
{
    A a;
    B b;
    b.doSomething(a);

    return 0;
}