8.12 친구 함수와 클래스 friend

#include <iostream>

using namespace std;

class A
{
	private:
	int m_value = 1;
	
	friend void doSomething(A& a);
};

void doSomething(A& a)
{
	cout << a.m_value << endl;
}

int main()
{
	A a;
	doSomething(a);

	return 0;
}

// 1

 

private이지만 friend이기때문에 접근 가능하다.

 

방법 1.

#include <iostream>

using namespace std;

class B; // forward declaration

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;
}

// 1 2

 

방법 2.

#include <iostream>

using namespace std;

class B; // forward declaration

class A
{
	private:
	int m_value = 1;
	
	friend class B;
};

class B
{
	private:
	int m_value = 2;
	
	public:
	void doSomething(A& a)
	{
		cout << a.m_value << endl;
	}
};

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

	return 0;
}

// 1

 

#특정함수만을 friend시키는 방법.

class A
{
	private:
	int m_value = 1;
	
	friend void B::doSomething(A& a);
};

 

첫번째 문제, B안에 doSomething이 있다는걸 A가 알 방법이 없다.

 

해결방안.

#include <iostream>

using namespace std;

class A; // forward declaration

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

class A
{
	private:
	int m_value = 1;
	
	friend void B::doSomething(A& a);
};

void B::doSomething(A& a)
{
	cout << a.m_value << endl;
}

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

	return 0;
}