Is it safe to throw / catch on stack unwind?

Question

Q: Is it safe to throw and catch an exception on stack unwind, or does the application call terminate on the second throw?

minimal example:

void some_function()
{
    try
    {
        // do stuff here that can throw
        throw std::runtime_error("blah");
    } catch(const std::exception& re)
    {
        try // this code could be in some function called from here
        {
            // do something with re here that throws a logical_error
            throw std::logical_error("blah blah"); // does this call terminate?
        } catch(const std::logical_error& le)
        {
        }
    }
}

I got curious after reading this question.

Note: I know you can/should catch(...) in a destructor, but does it make sense in general to have a try/catch in a catch block - maybe in some function called on the exception (re in my example)?

That's not really during a stack unwinding though, right? AFAIK, once the `catch` block is entered, the stack has already been unwound. — Pubby, Mar 19 '13 at 19:19
No need to close, I could be wrong. My comment was more of a guess. — Pubby, Mar 19 '13 at 19:21

score 4 · Accepted Answer · edited May 23 '17 at 12:28

4

That's not really during stack unwinding. Once a catch block is entered, the stack has already been unwound.

And yes, that code is legal. See this question: Nested try...catch inside C++ exception handler?

edited May 23 '17 at 12:28

Community

1
1

answered Mar 19 '13 at 19:24

Pubby

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Conversely, throwing in a destructor **could** be "on stack unwind". And there's well-defined behavior specifically for that scenario. – Drew Dormann Mar 19 '13 at 19:24
1

+1. Yes, when it enters into `catch` block, there is no exception in the flight, which means the `catch` block can throw; the behavior is well-defined. In fact, rethrow (which is allowed from `catch` only) is one kind of throw. – Nawaz Mar 19 '13 at 19:26

Drew Dormann · Answer 2 · 2023-02-28T01:37:52.650

3

Pubby's answer best answers the scenario you're describing.

As an addendum, while a stack is unwinding, the only user code that's executed is destructors (and the code those destructors call).

If you do throw in a destructor during this scenario (and don't catch the exception in the same destructor), the standard specifies that std::terminate() will be called.

edited Feb 28 '23 at 01:37

answered Mar 19 '13 at 19:29

Drew Dormann

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Not quite. If you throw an exception **that propagates out of the destructor** in this scenario the standard specifies that `std::terminate()` will be called. `try { throw 3; } catch(int) { }` inside a destructor won't lead to `std::terminate()`. – Pete Becker Mar 19 '13 at 21:00

Is it safe to throw / catch on stack unwind?

2 Answers2