std::set 上的异常安全事务

Question

std::set<T> e;

void f(/* Some parameters. */) {
    /* Some non-throwing code. */

    try {
        e.erase(a);
        e.insert(b);
        e.erase(c);
    } catch(...) {
        // Undo failed operations.
    }
}

I want f to have a strong exception guarantee, regardless of type T. Let's say, erasing a and inserting b succeedes, but erasing c throws. Then I have to undo the first two operations, but that would involve inserting and erasing from e in the catch block, but these operations can also throw. Is there any way to perform this rollback? I'm using C++17.

Answer 1

So if something throws on destruction, the strong exception guarantee is almost always pooched.

When running stack unwinding due to exceptions, if a destructor throws your program calls std terminate for pretty much the same reason.

When adding, you can just duplicate the entire collection, then add to the duplicate, then swap collections. Then you can destroy the duplicate; but if that destruction throws, there really isn't a sane option. You where stating this data is no longer needed, and are being informed that this operation failed. What does that even mean? How do you undo "this data isn't needed"?

Require lookup < (or == and hash ) to be exception-free. Do the same for destruction. Then .erase is also nothrow. Don't borrow trouble and posit insane types; most insane types (they can exist) can be wrapped up in a sanity-wrapper and handle exceptions elsewhen.