Non-obvious lifetime issue with std::promise and std::future

Question

This question is very similar to a previous one here: race-condition in pthread_once()?

It is essentially the same issue - the lifetime of a std::promise en

Answer 1

Answering my own question, to offer a workable solution. It doesn't use std::promise or std::future, but it achieves the synchronisation which I'm searching for.

Update synchronous_job to use a std::condition_variable and std::mutex instead:

Edit: Updated to include a boolean flag as suggested by Dave S

struct synchronous_job
{
    synchronous_job(std::function job, dispatcher& d)
        : _job(job)
        , _d(d)
        , _done(false)
    {
    }
    void run()
    {
        _d.post(std::bind(&synchronous_job::cb, this));
        std::unique_lock l(_mtx);
        if (!_done)
            _cnd.wait(l);
    }
private:
    void cb()
    {
        _job();
        std::unique_lock l(_mtx);
        _done = true;
        _cnd.notify_all();
    }
    std::function   _job;
    dispatcher&             _d;
    std::condition_variable _cnd;
    std::mutex              _mtx;
    bool                    _done;
};