恢复final_suspend会导致错误，但不会破坏协程

Question

James McNellis in his presentation “Introduction to C++ Coroutines" ( https://youtu.be/ZTqHjjm86Bw?t=1898 ) says the following:

A coroutine is destroyed when:

• final_suspend is resumed,
• coroutine_handle<>::destroy() is called,

whichever happens first.

In my tests I see (VS 2015, VS 2017 RC), that resuming coroutine that is suspended on final_suspend causes an error instead:

Unhandled exception at 0x010B9EDD in Awaits2017.exe: RangeChecks instrumentation code detected an out of range array access. occurred

Any ideas what might be going on here?

#include <experimental/resumable>

using namespace std;
using namespace std::experimental;

struct Coro
{
    coroutine_handle<> m_coro;
    Coro(coroutine_handle<> coro) : m_coro(coro) {}

    struct promise_type
    {
        Coro get_return_object()
        {
            return Coro(coroutine_handle<promise_type>::from_promise(*this));
        }

        auto initial_suspend() { return false; }
        auto final_suspend() { return true; }
        void return_void() {}
    };
};

Coro simple()
{
    co_return;
}

int main()
{
    Coro c = simple();
    c.m_coro.resume(); // runtime error here
}

Answer 1

Rule of thumb:

• If final_suspend returns true , you should call coroutine_handle<>::destroy() , instead of resume() .

• If final_suspend returns false , you should not call destroy() as well, the coroutine will cleanup itself.

Note that the coroutine included in VS 2015 is not what James McNellis shown in the video (the proposal has many revisions), and the description:

final_suspend is resumed

may be confusing. It does not really mean resume() is called.