类中友元函数的应用

类中友元函数则是为了访问私有变量，如类B需要访问类A中的私有变量，则代码如下：

 

example  one：

 

#include "stdafx.h"
#include
using namespace std;
class B;                               //提前申明类B
class A{
public:
 A(){
 English=19;  
 };
 void between(B &b);        //类B为类A的成员函数
private: 
 double English;
};

class B {
public:
 B(){Eng_score=0;};
 friend void A::between(B &p);       //类A为类B的友元函数，此函数可以访问类A中的私有变量，如English
 private:
 double Eng_score;
};
void A::between(B &p){  
 p.Eng_score=English;    //直接访问English
  cout<<"Eng_score="<<p.Eng_score<<endl; 
 };
int _tmain(int argc, _TCHAR* argv[])
{
 A a;
 B b;
 a.between(b);         //引用类B的对象b
while (1);
 return 0;
}

 

 

 

 example  two：

 

 

 

#include "stdafx.h"

#include

using namespace std;

float RandomRge(float LB, float UB)

{

float x;       x = LB + (UB - LB) / (RAND_MAX)*(float)rand();  return (x);

}

class A{

public:

A(){

English=100;

};

~A(){};

static double average(){

count++;

double ave;

Math=RandomRge(60, 100);

cout<<Math<<endl;

Math+=Math;

ave=Math/count;

return ave;

};

public:

double English;

static double Math;

static int count;

};

double A::Math=0;

int A::count=0;

class B {

public:

friend void friend_a(A &p){      //友元函数

double ave;

ave=p.average();

cout<<"count="<<p.count<<"\t"<<"Math="<<p.Math<<"\t"<<"ave="<<ave<<endl;

};

};

int _tmain(int argc, _TCHAR* argv[])

{

A a[6];

for (int i=0;i<6;i++){

friend_a(a[i]);

}

while (1);

return 0;

}

 

 

example  three：

 

#include "stdafx.h"

#include

using namespace std;

class A;

class B {

private:

int c = 100;

public:

friend void frifun(A &p, B &pp);

};

class A {

public:

A(double r1=0,double i1=0) {

r = r1;

i = i1;

}

friend void frifun(A &p, B &pp) {

cout << p.r <<"\n"<< p.i <<"\n"<<pp.c<< endl;

}

 

public:

double r;

double i;

};

int main()

{

B pp;

A a1(5, 6);

frifun(a1,pp);

while (1);

    return 0;

}