6. friend와 static 그리고 const

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출처: 윤성우 열혈강의 C++ 예제코드

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6.1. const와 관련해서 아직 못다한 이야기

• const를 이용한 overloading

#include <iostream>
using namespace std;

class SoSimple
{
private:
	int num;
public:
	SoSimple(int n) : num(n)
	{ }
	SoSimple& AddNum(int n)
	{
		num+=n;
		return *this;
	}
	void SimpleFunc () 
	{
		cout<<"SimpleFunc: "<<num<<endl;
	}
	void SimpleFunc () const
	{
		cout<<"const SimpleFunc: "<<num<<endl;
	}
};

void YourFunc(const SoSimple &obj)
{
	obj.SimpleFunc();
}

int main(void)
{
	SoSimple obj1(2);
	const SoSimple obj2(7);

	obj1.SimpleFunc();
	obj2.SimpleFunc();

	YourFunc(obj1);
	YourFunc(obj2);
	return 0;
}

SimpleFunc: 2
const SimpleFunc: 7
const SimpleFunc: 2
const SimpleFunc: 7

6.2. 클래스와 함수에 대한 friend 선언

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• 클래스에 friend 선언하기(클래스의 private 멤버를 외부에서 접근)

#include <iostream>
#include <cstring>
using namespace std;

class Girl;

class Boy
{
private:
	int height;
	friend class Girl;
public:
	Boy(int len) : height(len)
	{ }
	void ShowYourFriendInfo(Girl &frn);
};

class Girl
{
private:
	char phNum[20];
public:
	Girl(char * num)
	{
		strcpy(phNum, num);
	}
	void ShowYourFriendInfo(Boy &frn);
	friend class Boy;
};

void Boy::ShowYourFriendInfo(Girl &frn)
{
	cout<<"Her phone number: "<<frn.phNum<<endl;
}

void Girl::ShowYourFriendInfo(Boy &frn)
{
	cout<<"His height: "<<frn.height<<endl;
}

int main(void)
{
	Boy boy(170);
	Girl girl("010-1234-5678");

	boy.ShowYourFriendInfo(girl);
	girl.ShowYourFriendInfo(boy);
	return 0;
}

• 함수에 friend 선언하기(함수가 선언된 클래스의 private 멤버를 외부에서 접근)

#include <iostream>
using namespace std;

class Point;

class PointOP
{
private:
	int opcnt;
public:
	PointOP() : opcnt(0)
	{  }

	Point PointAdd(const Point&, const Point&);
	Point PointSub(const Point&, const Point&);
	~PointOP()
	{
		cout<<"Operation times: "<<opcnt<<endl;
	}
};

class Point
{
private:
	int x; 
	int y;    
public:
	Point(const int &xpos, const int &ypos) : x(xpos), y(ypos)
	{  }
	friend Point PointOP::PointAdd(const Point&, const Point&);
	friend Point PointOP::PointSub(const Point&, const Point&);
	friend void ShowPointPos(const Point&);
};

Point PointOP::PointAdd(const Point& pnt1, const Point& pnt2)
{
	opcnt++;
	return Point(pnt1.x+pnt2.x, pnt1.y+pnt2.y);
}

Point PointOP::PointSub(const Point& pnt1, const Point& pnt2)
{
	opcnt++;
	return Point(pnt1.x-pnt2.x, pnt1.y-pnt2.y);
}

int main(void)
{
	Point pos1(1, 2);
	Point pos2(2, 4);
	PointOP op;

	ShowPointPos(op.PointAdd(pos1, pos2));
	ShowPointPos(op.PointSub(pos2, pos1));
	return 0;
}

void ShowPointPos(const Point& pos)
{
	cout<<"x: "<<pos.x<<", ";
	cout<<"y: "<<pos.y<<endl;
}

6.3. C++에서의 static

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• static 멤버변수 사용하기(private)

#include <iostream>
using namespace std;

class SoSimple
{
private:
	static int simObjCnt;
public:
	SoSimple()
	{
		simObjCnt++;
		cout<<simObjCnt<<"번째 SoSimple 객체"<<endl;
	}
};
int SoSimple::simObjCnt=0;

class SoComplex
{
private:
	static int cmxObjCnt;
public:
	SoComplex()
	{
		cmxObjCnt++;
		cout<<cmxObjCnt<<"번째 SoComplex 객체"<<endl;
	}
	SoComplex(SoComplex &copy)
	{
		cmxObjCnt++;
		cout<<cmxObjCnt<<"번째 SoComplex 객체"<<endl;
	}
};
int SoComplex::cmxObjCnt=0;

int main(void)
{
	SoSimple sim1;
	SoSimple sim2;

	SoComplex cmx1;
	SoComplex cmx2=cmx1;
	SoComplex();
	return 0;
}

• static 멤버변수 사용하기(public)

#include <iostream>
using namespace std;

class SoSimple
{
public:
	static int simObjCnt;
public:
	SoSimple()
	{
		simObjCnt++;
	}
};
int SoSimple::simObjCnt=0;

int main(void)
{
	cout<<SoSimple::simObjCnt<<"번째 SoSimple 객체"<<endl;
	SoSimple sim1;
	SoSimple sim2;

	cout<<SoSimple::simObjCnt<<"번째 SoSimple 객체"<<endl;
	cout<<sim1.simObjCnt<<"번째 SoSimple 객체"<<endl;
	cout<<sim2.simObjCnt<<"번째 SoSimple 객체"<<endl;
	return 0;
}

• static 상수 사용하기

#include <iostream>
using namespace std;

class CountryArea
{
public:
	const static int RUSSIA			=1707540;
	const static int CANADA			=998467;
	const static int CHINA			=957290;
	const static int SOUTH_KOREA	=9922;
};

int main(void)
{
	cout<<"러시아 면적: "<<CountryArea::RUSSIA<<"㎢"<<endl;
	cout<<"캐나다 면적: "<<CountryArea::CANADA<<"㎢"<<endl;
	cout<<"중국 면적: "<<CountryArea::CHINA<<"㎢"<<endl;
	cout<<"한국 면적: "<<CountryArea::SOUTH_KOREA<<"㎢"<<endl;
	return 0;
}

• mutable 키워드를 이용하여 const함수에서 변수값 변경하기

#include <iostream>
using namespace std;

class SoSimple
{
private:
	int num1;
	mutable int num2;
public:
	SoSimple(int n1, int n2)
		: num1(n1), num2(n2)
	{  }
	void ShowSimpleData() const
	{
		cout<<num1<<", "<<num2<<endl;
	}
	void CopyToNum2() const
	{
		num2=num1;
	}
};

int main(void)
{
	SoSimple sm(1, 2);
	sm.ShowSimpleData();
	sm.CopyToNum2();
	sm.ShowSimpleData();
	return 0;
}