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>

"this flag method seems less elegant than having the actual error message pass directly to main" - not quite. Personally, I hate functions that are supposed to do computations, but they are printing to stdout/stderr. It hurts my sense of balance. :P If a function is doing computation, then let it do its task then return either a valid value, or an out-of-range value in order to signal an error. Then its user/client/caller (think of your code as if it was going to be a library!) can do whatever it wants to when it encounters an error. — , Feb 01 '14 at 18:19
Why not let your funcion throw the exception and do the catch on `main`? — wesley.mesquita, Feb 01 '14 at 18:25
@wesley.mesquita: that's what the first version does (the first block of code in my original post is from main). But the question is, how do I do this in main when the exception-throwing function is called by another function? Note this is part of my self-study of problem 4.6 of **Accelerated C++**. I think the main point of this question is to figure out this puzzle. My "solution" is to use a flag, but I can't help but think there should be a better way. — eric, Feb 01 '14 at 18:37

wesley.mesquita · Answer 1 · 2014-02-01T18:47:18.070

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 i.e., student_final_grade_exceptionand 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.

edited Feb 01 '14 at 18:47

answered Feb 01 '14 at 18:41

wesley.mesquita

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Thanks I'll try this. Any recommendations for good sources with quick explanation of how to make a new exceptio type? – eric Feb 02 '14 at 13:28
1

@neuronet Take a look [here](http://stackoverflow.com/questions/8152720/correct-way-to-inherit-from-stdexception). I used to inherit from `std::exception`, but seems to be a better pratice to it from `std::runtime_error` or `std::logic_error`. – wesley.mesquita Feb 02 '14 at 15:10
I wonder if there is a worked out example for novices...that is pretty esoteric for someone on chapter 4 of an intro book? Maybe I should just cut my losses now and come answer this with a fully annotated example from head to toe once I have done classes....Though this link seems helpful: http://stackoverflow.com/questions/16182781/creating-exceptions-c – eric Feb 12 '14 at 23:57

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

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