4.21

#include <iostream>#include <string>using namespace std;class student{private:float num;string name;float score;static int count;static float sum;float ave;public:student(float num1,string name1,float score1);static void show(student &stu);static void show_all(student &Stu);};void student::show(student &stu0){cout<<"学号"<<stu0.num<<endl;cout<<"姓名"<<stu0.name<<endl;cout<<"成绩"<<stu0.score<<endl;}void student::show_all(student &Stu){    cout<<"总成绩"<<sum<<endl;cout<<"总人数"<<count<<endl;cout<<"平均成绩"<<Stu.ave<<endl;}student::student(float num1,string name1,float score1){num=num1;name=name1;score=score1;sum=sum+score;count=count+1;ave=sum/count;}int student::count=0;float student::sum=0.0;int main(){student stu[3]={        student(1,"zhang",96),student(2,"si",89),student(3,"xin",69)};    student::show(stu[0]);student::show(stu[1]);student::show(stu[2]);    stu[2].show_all(stu[2]);return 0;}