C++ friend 用法总结

1 友元的内容

2 普通的非成员函数友元

//OpeClass.h
#pragma once
class OpeClass
{
	friend int func(const OpeClass xx);
public:
	OpeClass(void);
	OpeClass(int x,int y);
	~OpeClass(void);
private:
	int width;
	int height;
};

//OpeClass.cpp
#include "OpeClass.h"


OpeClass::OpeClass(void)
{
	width = 50;
	height = 50;
}


OpeClass::OpeClass(int x,int y):width(x),height(y)
{
}


OpeClass::~OpeClass(void)
{
}


int func(const OpeClass xx)
{
	return xx.height * xx.width;
}

//main.cpp
#include "OpeClass.h"
#include <iostream>
using namespace std;


void main()
{
	OpeClass XO;
	cout<<func(XO);
	system("pause");
}

3 类作为友元

//A.h
#pragma once
#include <iostream>
using namespace std;
class A
{
	friend class B;
public:
	~A(void);
	static void func()
	{
		cout<<"This is in A"<<endl;
	}
private:
	A(){};
	static const A Test;
};

//A.cpp
#include "A.h"
const A A::Test = A();
A::~A(void)
{
}

//B.h
#pragma once
#include "C.h"
class B
{
public:
	B(void);
	~B(void);
	void func(C& c);
};

//B.cpp
#include "B.h"
#include "A.h"
#include "C.h"
#include <iostream>
using namespace std;


B::B(void)
{
}




B::~B(void)
{
}


void B::func(C& c)
{
	cout<<"This is in B"<<endl;
	A::Test.func();
	c.func(A::Test);
}

//C.h
#pragma once
class A;
class C
{
public:
	C(void);
	~C(void);
	void func(const A& a);
};

//C.cpp
#include "C.h"
#include <iostream>
using namespace std;

C::C(void)
{
}

C::~C(void)
{
}

void C::func(const A& a)
{
	cout<<"This is in C"<<endl;
}

//main.cpp
#include "A.h"
#include "B.h"
#include "C.h"
#include <iostream>
using namespace std;

void main()
{
	B b;
	C c;
	b.func(c);
	system("pause");
}

4 类成员函数作为友元函数

这个稍微有点复杂，因为你要类成员函数作为友元，你在声明友元的时候要用类限定符，所以必须先定义包含友元函数的类，但是在定义友元的函数时候，又必须事先定义原始类。通常的做法先定义包含友元函数的类，再定义原始类，这个顺序不能乱。(如果是友元类，则没有这种这种必须)如下面所示：

//B.h
#pragma once
class A;
class B
{
public:
	B(void);
	~B(void);
	int func(A xx);
};

//A.h
#pragma once
#include "B.h"
class A
{
friend int B::func(A xx);
public:
	A(void):mx(20),my(30){}
	~A(void){}
private:
	int mx;
	int my;
};

//B.cpp
#include "B.h"
#include "A.h"


B::B(void)
{
}


B::~B(void)
{
}

int B::func(A xx)
{
	return xx.mx * xx.my;
}

//main.cpp
#include "A.h"
#include "B.h"
#include <iostream>
using namespace std;
void main()
{
	A a;
	B b;
	cout<<b.func(a)<<endl;
	system("pause");
}

6 友元不能被继承

7 友元不具有传递性

8 相互为友元的类

//A.h
#pragma once
class A
{
friend class B;
public:
	A(void);
	~A(void);
	int funa(B& b);
private:
	int mx;
	int my;
};

//A.cpp
#include "A.h"
#include "B.h"


A::A(void)
{
	mx = 10;
	my = 10;
}


A::~A(void)
{
}


int A::funa(B& b)
{
	return b.mb * b.mc;
}

//B.h
#pragma once
class B
{
	friend class A;
public:
	B(void);
	~B(void);
	int funb(A& a);
private:
	int mb;
	int mc;
};

//B.cpp
#include "B.h"
#include "A.h"


B::B(void)
{
	mb = 20;
	mc = 20;
}

B::~B(void)
{
}

int B::funb(A& a)
{
	return a.mx *a.my;
}

//main.cpp
#include "A.h"
#include "B.h"
#include <iostream>
using namespace std;
void main()
{
	A a;
	B b;
	cout<<a.funa(b)<<endl;
	cout<<b.funb(a)<<endl;
	system("pause");
}

9 如果想要指定两个类都有成员函数作为对方的友元，那么必须第2个类是第一个类的友元

//A.h
#pragma once

// class B is a friend class of A
class A
{
	friend class B;
public:
	A(void):ma(10),mb(20){}
	~A(void){}
	int funa(B& b);
private:
	int	ma;
	int	mb;
};

//B.h
#pragma once
#include "A.h"


// A's function funa is a friend function of B
class B
{
	friend int A::funa(B& b);
public:
	B(void);
	~B(void);
	int funb(A& a);
	int func(A& a);
private:
	int mx;
	int my;
};

//A.cpp
#include "A.h"
#include "B.h"


int A::funa(B& b)
{
	return  b.mx * b.my;
}

//B.cpp
#include "B.h"

B::B(void):mx(12),my(15)
{
}


B::~B(void)
{
}


int B::funb(A& a)
{
	return a.ma + a.mb;
}

int B::func(A& a)
{
	return a.ma * a.mb;
}

//main.cpp
#include "A.h"
#include "B.h"
#include <iostream>
using namespace std;
void main()
{
	A a;
	B b;
	cout<<a.funa(b)<<endl;
	cout<<b.funb(a)<<endl;
	cout<<b.func(a)<<endl;
}