Handling Errors with Exceptions

Things go wrong: a file is missing, the user types letters instead of a number, someone withdraws more money than they have. C++ lets a function throw an exception to say “something went wrong”, and other code can catch it and deal with it.

Tip: The browser editor on this site uses a small WebAssembly compiler that has exceptions switched off, so examples with try and throw won't run there. The outputs below come from g++ on a normal computer. Try them on your own machine!

try, throw and catch

#include <iostream>
#include <stdexcept>
using namespace std;

double divide(double a, double b) {
    if (b == 0) {
        throw runtime_error("cannot divide by zero");
    }
    return a / b;
}

int main() {
    try {
        cout << divide(10, 2) << "\n";
        cout << divide(5, 0) << "\n";       // throws: the rest of try is skipped
        cout << "This line never runs\n";
    } catch (const exception& e) {
        cout << "Error: " << e.what() << "\n";
    }
    cout << "The program keeps going\n";
    return 0;
}

Output (on your own computer with g++ – exceptions don't run in the browser editor)

5
Error: cannot divide by zero
The program keeps going

• throw stops the function right away and sends the exception up to whoever called it.
• try { ... } wraps code that might throw.
• catch (const exception& e) catches it. e.what() gives the message.

Think of it like this: Throwing an exception is like pulling the emergency cord on a train. Everything stops, and the person responsible (the catch) comes to sort it out.

Standard exception types

<stdexcept> has ready-made types: runtime_error, invalid_argument, out_of_range and more. Some library functions throw them too, like stoi and vector::at:

#include <iostream>
#include <stdexcept>
#include <string>
#include <vector>
using namespace std;

int main() {
    try {
        int n = stoi("hello");
        cout << n << "\n";
    } catch (const invalid_argument& e) {
        cout << "That is not a number\n";
    }

    vector<int> v = {1, 2, 3};
    try {
        cout << v.at(10) << "\n";
    } catch (const out_of_range& e) {
        cout << "Index 10 is out of range\n";
    }
    return 0;
}

Output (on your own computer with g++ – exceptions don't run in the browser editor)

That is not a number
Index 10 is out of range

Without exceptions

Many C++ programs (games, small devices, and this site's editor) don't use exceptions at all. Instead a function returns a value that says whether it worked. Both styles are common:

#include <iostream>
using namespace std;

bool safeDivide(int a, int b, int& result) {
    if (b == 0) {
        return false;            // tell the caller it failed
    }
    result = a / b;
    return true;
}

int main() {
    int r;
    if (safeDivide(20, 4, r)) {
        cout << "Result: " << r << "\n";
    }
    if (!safeDivide(1, 0, r)) {
        cout << "Division failed\n";
    }
    return 0;
}

Output

Result: 5
Division failed

Remember: Use exceptions for things that are truly unusual. For checks you expect to fail often (like validating user input), a simple if is clearer.