类中静态数据成员的作用

#include<iostream>
#include<string>
using namespace std;


class Student
{
public:
    Student(string pname = "null", int page = 0);
    void PrintNum();
    ~Student();

private:
    string name;
    int age;
    static int num;

};
int Student::num = 0;

Student::Student(string pname, int page):name(pname),age(page)
{
    num++;
}
void Student::PrintNum() {
    cout << "num of student:" << num << endl;;
}

Student::~Student()
{
}

int main() {
    Student p1("jibisheng",25);
//    cout << "num of student:\n";
    p1.PrintNum();
    Student p2("zhangjie",24);
//    cout << "\nnum of student:\n";
    p1.PrintNum();
    Student p3("jijie", 24);
//    cout << "\nnum of student:\n";
    p1.PrintNum();
    Student p4("zhangsheng", 24);
//    cout << "\nnum of student:\n";
    p4.PrintNum();
    Student p5;
//    cout << "\nnum of student:\n";
    p5.PrintNum();
    return 0;
}