为什么在我的 C++ 代码中抛出这个确切的异常？

Question

I am analyzing part of code that was part of my lectures. I have managed to compile it but I cannot understand: why does my program output "Wyjatek" and 0 instead of "WyjatekNieoznaczony"?

I was pretty sure WyjatekNieoznaczony() should be thrown because a=0 and b=0 as well. Right now i am a little bit confused. Could you help me, please?

class Wyjatek {};
class WyjatekBledny : public Wyjatek {};
class WyjatekNieoznaczony : public Wyjatek {};

double f(double a, double b) {
    if (b == 0) {
        if (a == 0)
            throw WyjatekNieoznaczony();
        else
            throw WyjatekBledny();
    }
    return a / b;
}


double g(double a, double b) throw (int) {
    try {
        return f(a, b);
    }
    catch (WyjatekBledny) {
        cout << "Wyjatek bledny" << endl;
        throw 1;
    }
    catch (Wyjatek) {
        cout << "Wyjatek" << endl;
    }
    catch (WyjatekNieoznaczony) {
        cout << "Wyjatek nieoznaczony" << endl;
        throw;
    }
    return 0;
}

int main()
{
    double a = 0, b = 0;
    try {
        cout << g(a, b) << endl;
    }
    catch (...)
    {
        cout << "Inny wyjatek" << endl;
    }
    system("pause");
    return 0;
}

Answer 1

Yes indeed a WyjatekNieoznaczony is thrown, but at the catch site, catch (Wyjatek) { is a match (due to the inheritance) so it's caught there.

A catch site is more like an if else block in behaviour - with each catch possibility being considered in the order they are written - rather than a switch block where you can put the labels in any order you like.

Note also that it's a good idea to catch exceptions by const reference than by value, else you can suffer the pitfalls of object slicing .

Answer 2

If you enabled (and read) compiler warnings, you would've encountered the following diagnostic:

warning: exception of type 'WyjatekNieoznaczony' will be caught [...] by earlier handler for 'Wyjatek'.

This basically means that WyjatekNieoznaczony , by inheriting from Wyjatek , will be first caught by catch(Wyjatek) clause, since it's convertible. The problem is that due to object slicing , it will lose its Nieoznaczony ness.

What I suggest is to reorder the catch clauses so the possibility of slicing disappears (in this case):

catch (WyjatekBledny) {
    cout << "Wyjatek bledny" << endl;
    throw 1;
}
catch (WyjatekNieoznaczony) {
    cout << "Wyjatek nieoznaczony" << endl;
    throw;
}
catch (Wyjatek) {
    cout << "Wyjatek" << endl;
}