流读取操作符cin遇到文件结束符返回0

#include <iostream>
using std::cout;
using std::cin;
using std::endl;

int main()
{
    int grade,hightestGrade=-1;
    cout<<"enter grade(enter end-of-file to end):";
    while(cin>>grade){
        if (grade>hightestGrade)
            hightestGrade=grade;
        
        cout<<"enter grade (enter end-of-file to end): ";
    }
    cout<<"Hightest grade is: "<<hightestGrade<<endl;
    return 0;
}