用友元函数做C++
定义学生类Student，包括学号、姓名、三平均、门课的成绩成绩。设计一个友元函数grade()，输出按平均成绩对应的等级：大于等于90为优
80~89为良
70~79为中
60~69为及格
小于60为不及格*/
以下程序是我写的，但不知道怎样用友元把这个程序弄好，麻烦帮我修改下，谢谢
#include "iostream.h"
#include "string.h"
class student
{
public:
student(char name[20],char num[20],int yu,int shu,int ying);
void changename(char name[20],char num[20],int yu,int shu,int ying);
void ave(int a,int b,int c);
friend void grade(int pp);
private:
char qname[20];
char qnum[20];
int qyu;
int qshu;
int qying;
int ping;
};
student::student(char name[20],char num[20],int yu,int shu,int ying)
{
strcpy(qname,name);
strcpy( qnum,num);
qyu=yu;
qshu=shu;
qying=ying;
ping=(yu+shu+ying)/3;
}
void student::changename(char name[20],char num[20],int yu,int shu,int ying)
{
strcpy(qname,name);
strcpy( qnum,num);
qyu=yu;
qshu=shu;
qying=ying;
ping=(yu+shu+ying)/3;
}
void student::ave(int a,int b,int c)
{
double w;
w=(a+b+c)/3;
cout<<"平均成绩为"<<w<<endl;
}
void grade(int pp)
{
if (pp>=90)
{
cout<<"优"<<endl;
}
if (pp>=80&&pp<=89)
{
cout<<"良"<<endl;
}
if (pp>=70&&pp<=79)
{
cout<<"中"<<endl;
}
if (pp>=60&&pp<=69)
{
cout<<"及格"<<endl;
}
if (pp<60)
{
cout<<"不及格"<<endl;
}
}
void main()
{
student a("mm","0000",99,99,99);
a.ave(99,99,99);
cout<<grade(a.ave)<<endl;
char name[20];
char num[20];
int yu,shu,ying;
cout<<"请输入姓名"<<endl;
cin>>name;
cout<<"请输入学号"<<endl;
cin>>num;
cout<<"请输入语文成绩"<<endl;
cin>>yu;
cout<<"请输入数学成绩"<<endl;
cin>>shu;
cout<<"请输入英语成绩"<<endl;
cin>>ying;
a.changename(name,num,yu,shu,ying);
a.ave(yu,shu,ying);
}
