如何使boost_to_unique_lock + condition_variable协同工作？

Question

I use a std::list to hold some string, one thread writes to the list and some other threads read from it(get the first and erase from the list). Here's the code:

std::list<string> list_;

boost::condition_variable cond;
boost::shared_mutex mtx;

int get_size() {
    boost::shared_lock<boost::shared_mutex> lock(mtx);
    return list_.size();
}

// add a string to the list
// invoked by only one thread
void add_one(const string& p) {
    {
        boost::upgrade_lock<boost::shared_mutex> lock(mtx);
        boost::upgrade_to_unique_lock<boost::shared_mutex> uniquelock(lock);

        list_.push_back(p);
    }
    cond.notify_one();
}

// get the first string and remove it from the list
// invoked by many threads
string pick_one() {
    string ret;
    {
        boost::upgrade_lock<boost::shared_mutex> lock(mtx);
        boost::upgrade_to_unique_lock<boost::shared_mutex> uniquelock(lock);

        if(!list_.size()) { // if empty, wait for notify
            cond.wait(uniquelock, [&]{ return list_.size() > 0; }); // compile error
        } 
        ret = list_.front();
        list_.pop_front();
    }
    return ret;
}

There's compile error at line cond.wait(uniquelock, ...

What's the correct way of using read/write lock with condition_variable?

Answer 1

You cannot do this - upgrade_to_unique_lock is a RAII helper to obtain exclusive access on the lock for its lifetime. See docs here .

boost::upgrade_to_unique_lock allows for a temporary upgrade of an boost::upgrade_lock to exclusive ownership. When constructed with a reference to an instance of boost::upgrade_lock, if that instance has upgrade ownership on some Lockable object, that ownership is upgraded to exclusive ownership. When the boost::upgrade_to_unique_lock instance is destroyed, the ownership of the Lockable is downgraded back to upgrade ownership.

If you wish to use shared_mutex with a Boost condition variable you must use boost::condition_variable_any as described here . The difference between the two conditional variables (albeit in reference to std , not Boost ) is covered here .