如何向此 C++ 代码添加错误消息？

Question

我如何为此添加错误消息？例如：您不小心按下了成绩中的一个字母而不是数字，然后会显示一条错误消息 ＃包括 使用命名空间标准；

int main()
{
        double assignment1, assignment2, seatwork1, seatwork2, quiz1, quiz2, majorexam, grade, major, assignment, seatwork, quiz;
        string name;

        cout<<"Enter Student name: ";
        cin>>name;
        
        cout<<"Enter grade for assignment 1: ";
        cin>>assignment1;
        cout<<"Enter grade for assignment 2: ";
        cin>>assignment2;
        cout<<"\nEnter grade for seatwork 1: ";
        cin>>seatwork1;
        cout<<"Enter grade for seatwork 2: ";
        cin>>seatwork2;
        cout<<"\nEnter grade for quiz 1: ";
        cin>>quiz1;
        cout<<"Enter grade for quiz 2: ";
        cin>>quiz2;
        cout<<"\nEnter grade for major exam: ";
        cin>>majorexam;
       
        
        float aveAssignment=(assignment1+assignment2)/2;
        assignment = (aveAssignment)*.1;
        float aveSeatwork=(seatwork1+seatwork2)/2;
        seatwork = (aveSeatwork)*.2;
        float aveQuiz=(quiz1+quiz2)/2;
        quiz = (aveQuiz)*.3;
        major = majorexam*.4;
        grade = (assignment+seatwork+quiz+major);
        cout<<"Final Grade is: "<<grade;
        return 0;
}