c++ - Catching exception: divide by zero

Question

The following code does not catch an exception, when I try to divide by 0. Do I need to throw an exception, or does the computer automatically throw one at runtime?

int i = 0;

cin >> i;  // what if someone enters zero?

try {
    i = 5/i;
}
catch (std::logic_error e) {

    cerr << e.what();
}

Answer 1

You need to check it yourself and throw an exception. Integer divide by zero is not an exception in standard C++.

Neither is floating point divide by zero but at least that has specific means for dealing with it.

The exceptions listed in the ISO standard are:

namespace std {
    class logic_error;
        class domain_error;
        class invalid_argument;
        class length_error;
        class out_of_range;
    class runtime_error;
        class range_error;
        class overflow_error;
        class underflow_error;
}

and you could argue quite cogently that either overflow_error (the infinity generated by IEEE754 floating point could be considered overflow) or domain_error (it is a problem with the input value) would be ideal for indicating a divide by zero.

However, section 5.6 (of C++11, though I don't think this has changed from the previous iteration) specifically states:

If the second operand of / or % is zero, the behavior is undefined.

So, it could throw those (or any other) exceptions. It could also format your hard disk and laugh derisively :-)

---

If you wanted to implement such a beast, you could use something like intDivEx in the following program (using the overflow variant):

#include <iostream>
#include <stdexcept>

// Integer division/remainder, catching divide by zero.

inline int intDivEx (int numerator, int denominator) {
    if (denominator == 0)
        throw std::overflow_error("Divide by zero exception");
    return numerator / denominator;
}

inline int intModEx (int numerator, int denominator) {
    if (denominator == 0)
        throw std::overflow_error("Divide by zero exception");
    return numerator % denominator;
}

int main (void) {
    int i = 42;

    try { i = intDivEx (10, 0); }
    catch (std::overflow_error &e) {
        std::cout << e.what() << " -> ";
    }
    std::cout << i << std::endl;

    try { i = intDivEx (10, 2); }
    catch (std::overflow_error &e) {
        std::cout << e.what() << " -> ";
    }
    std::cout << i << std::endl;

    return 0;
}

This outputs:

Divide by zero exception -> 42
5

and you can see it throws and catches the exception (leaving the return variable untouched) for the divide by zero case.

---

The % equivalent is almost exactly the same: