Throwing an error to main from a function called by another function

Question

Let's say I have a function ( mnHw ) that calculates the mean of a vector (a vector of homework grades). This function throws a domain_error("Student did no homework") exception when the vector is empty. When I call mnHw from main , things are simple when I want to print out the error:

        try
        {
            cout  <<  mnHw(student.homework);
        }
        catch (domain_error e)
        {
            cout << e.what();
        }

However, things are complicated if I only want to store the mean homework grade, instead of vector of all grades, for the student. In that case I call mnHw within the function that reads in the student information ( readStudent ), and put up a flag (-1) when no homework is entered:

        try
        {
            student.finalGrade=mnHw(hwGrades);
        }
        catch (domain_error e)
        {
            student.finalGrade = -1;
        }

Then, within main , I can do the following to recover the error:

    if (allStudents[i].finalGrade == -1) 
        cout << "Student did no homework";
    else  
        cout << allStudents[i].finalGrade;

Intuitively, though, this flag method seems less elegant than having the actual error message pass directly to main, at least for those cases when I want to display it from main (which is the case here). Is there some trick or technique I am missing that would directly give main access to e.what() from mnHw ?

What is good practice?

Note I have full functions for each of these cases that I could post, but they seemed too long. Please let me know if I should have included them to make myself more clear. Also, please feel free to correct any other mistakes I'm making. I am just now learning C++ the right way, and want to be corrected as much as possible. <flame-retardant vest>

Answer 1

You can re-throw the exception catched (const domain_error* e){ throw e;} . Or better, you can create based on domain_error create another exception ie, student_final_grade_exception and throw it.

EDIT:

Something like:

 try
        {
            student.finalGrade=mnHw(hwGrades);
        }
        catch (domain_error& e)
        {
              student_final_grade_exception mean_excep;
              mean_excep.addInfo("Error in calculation means...");
              mean_excep.addInfo(e.what()); 
              throw mean_excep;
        }

And then, you prepare your main to catch student_final_grade_exception . Of course, there is a litle bit more work to create a new exception type, but it can make possible to get more info and let the functions to only what they suppose to do as were told.