#include<iostream>
using namespace std;
#include<string>
class Student
{
private:
int number;
string xingming;
float score;
public:
     friend Lesson;
    Student(int n,string x,float s ):number(n),xingming(x),score(s){}
    void display()
    {
        cout<<xingming<<endl;
       cout<<number<<endl;
       cout<<score<<endl;
    }
};
class Lesson
{
private:
int id, s;
string name,teacher;
public:
    Lesson(int i,string n,string t,int sd):id(i),name(n),teacher(t),s(sd){}
    void AddStudent(int i);
    float GetPassRate(int s);
    void ShowInfo();
};
void Lesson::AddStudent(int i)
{
    Student stud[3]={
      Student(1002,"wangfang",98),
      Student(1003,"lingping",72),
      Student(1004,"zhangmin",59.3f)};
      stud[i].display();
   
}
float Lesson::GetPassRate(int s)
{
    int n=0;
    Student stud[3]={
      Student(1002,"wangfang",98),
      Student(1003,"lingping",72),
      Student(1004,"zhangmin",59)};
      for(int m=0;m<3;m++)
        if(stud[m].score>=60.0)
         n++;
    return(n/s);
}
void Lesson::ShowInfo()
{
    cout<<id<<endl;
    cout<<name<<endl;
    cout<<teacher<<endl;
    cout<<s<<endl;
}
void main()
{
   
      Lesson less(1,"c++","chen",3);
      int i;
      for(i=0;i<3;i++)
      Lesson::AddStudent(i);
      Lesson::GetPassRate(3);
      less.ShowInfo();
}

#include<iostream>
using namespace std;
#include<string>
class Lesson; // 加上这个
class Student
{
private:
int number;
string xingming;
float score;
public:
     friend Lesson;
    Student(int n,string x,float s ):number(n),xingming(x),score(s){}
    void display()
    {
        cout<<xingming<<endl;
       cout<<number<<endl;
       cout<<score<<endl;
    }
};
class Lesson
{
private:
int id, s;
string name,teacher;
public:
    Lesson(int i,string n,string t,int sd):id(i),name(n),teacher(t),s(sd){}
    void AddStudent(int i);
    float GetPassRate(int s);
    void ShowInfo();
};
void Lesson::AddStudent(int i)
{
    Student stud[3]={
      Student(1002,"wangfang",98),
      Student(1003,"lingping",72),
      Student(1004,"zhangmin",59.3f)};
      stud[i].display();
   
}
float Lesson::GetPassRate(int s)
{
    int n=0;
    Student stud[3]={
      Student(1002,"wangfang",98),
      Student(1003,"lingping",72),
      Student(1004,"zhangmin",59)};
      for(int m=0;m<3;m++)
        if(stud[m].score>=60.0)
         n++;
    return(n/s);
}
void Lesson::ShowInfo()
{
    cout<<id<<endl;
    cout<<name<<endl;
    cout<<teacher<<endl;
    cout<<s<<endl;
}
void main()
{
   
      Lesson less(1,"c++","chen",3);
      int i;
      for(i=0;i<3;i++)
      less.AddStudent(i); // Lesson::AddStudent(i);
      less.GetPassRate(3); // Lesson::GetPassRate(i);
      less.ShowInfo();
}

student类前应有前向引用声明，

可是我想把那个求及格率的结果输出，在那个函数里面我后来加了cout，但是为啥最后的结果没有呢？