Program crash caused by debug watches on uninitialized boost::optional's content: is it a bug in the adapter, in boost::optional, or where?

Question

The repro are simple and shown in the screencast below.

In essence:

write and compile (g++ -std=c++17 -O0 -g ...) a program (e.g. the one below) featuring an empty boost::optional, say b,
put a breakpoint anywhere,
start the program with the debugger, with the adapter vscode-cpptools, and wait for it to reach the breakpoint,
instantiate a watch on *b.

The program will crash with the following self-explanatory error:

debugging: /usr/include/boost/optional/optional.hpp:1271:
boost::optional<T>::reference_type boost::optional<T>::get()
[with T = int; reference_type = int&]: Assertion `this->is_initialized()' failed.

What I don't understand is that the debuggee itself is not invoking UB. Only the watch is, so why is the debuggee crashing? What "actor" is responsible for it?

Considering that the problem is fixed by one or both of

switching from boost::optional to std::optional
switching from vscode-cpptools to CodeLLDB

I'd say that the bug is either in boost::optional or vscode-cpptools; actually, since I don't think either should make assumption on the other, I'd be tempted to say that the bug is in both.

Here's the example program I used in the screencast below:

#include <boost/none.hpp>
#include <boost/optional/optional.hpp>
#include <optional>
#include <iostream>

int main()
{
    auto s = std::optional<int>{};
    auto b = boost::optional<int>{};
    if (s) {
        std::cout << *s << std::endl;
    }
    if (b) {
        std::cout << *b << std::endl;
    }
    std::cout << "done" << std::endl;
}

Here's a screencast showing my observations:

Additionally, I tried with just cli gdb, and I can reproduce the same issue too:

Sorry unrelated comment. What program did you use for screencasting? — Eldinur the Kolibri, Jul 15 '23 at 08:19
Most likely, your code *does* have undefined behavior. [cppreference](https://en.cppreference.com/w/cpp/utility/optional/operator*): "The behavior is undefined if `*this` does not contain a value." — j6t, Jul 15 '23 at 08:24
@j6t, where is my code accessing an empty optional? It isn't. The watch is. — Enlico, Jul 15 '23 at 08:30
If your debugger malfunctions, you may observe anything at all, including debugger crashes, debuggee crashes, or nasal daemons. — n. m. could be an AI, Jul 15 '23 at 09:11
@fabian, you write _you should not use this code in a real program_, but which code? There is no code in the C++ snippet I shared that is trying to dereference the null optional. — Enlico, Jul 15 '23 at 10:24

Program crash caused by debug watches on uninitialized boost::optional's content: is it a bug in the adapter, in boost::optional, or where?

0 Answers0