c++ - How are exceptions allocated on the stack caught beyond their scope?

Question

In the following code, the stack-based variable 'ex' is thrown and caught in a function beyond the scope in which ex was declared. This seems a bit strange to me, since (AFAIK) stack-based variables cannot be used outside the scope in which they were declared (the stack is unwound).

void f() {
    SomeKindOfException ex(...);
    throw ex;
}

void g() {
    try {
        f();
    } catch (SomeKindOfException& ex) {
        //Handling code...
    }
}

I've added a print statement to SomeKindOfException's destructor and it shows that ex is destructed once it goes out of scope in f() but then it's caught in g() and destructed again once it goes out of scope there as well.

Any help?

Answer 1

The exception object is copied to a special location to survive the stack unwinding. The reason you see two destructions is because when you exit f() the original exception is destroyed and when you exit g() the copy is destroyed.