Re-throwing an exception

Re-throwing means a handler catches an exception, does part of the work (such as logging or cleanup) and then passes the same exception to an outer handler. It is done with throw; (the throw keyword without any operand) inside a catch block. The original exception object is thrown again to the next enclosing try-catch.

Rules:

• throw; is valid only inside a catch handler (or a function called from it); otherwise terminate() is called.
• The exception must be caught by an outer try-catch (the caller), otherwise the program terminates.

Example

#include <iostream>
using namespace std;

void divide(int a, int b) {
    try {
        if (b == 0)
            throw b;                       // throw int
        cout << a / b << endl;
    }
    catch (int) {
        cout << "Inner handler: caught, re-throwing\n";
        throw;                             // re-throw same exception
    }
}

int main() {
    try {
        divide(10, 0);
    }
    catch (int) {
        cout << "Outer handler: division by zero handled\n";
    }
    return 0;
}

Output

Inner handler: caught, re-throwing
Outer handler: division by zero handled

It lets the inner function do local cleanup while the decision about the final action stays with the caller.