C++ packaged_task hang in Concurrent Wrapper - c++

Question

I am implementing a concurrent wrapper as introduced by Herb Sutter presented in his talk "C++ and Beyond 2012".
template <typename T>
class ConcurrentWrapper {
private:
std::deque<std::unique_ptr<std::function<void()>>> _tasks;
std::mutex _mutex;
std::condition_variable _cond;
T _object;
std::thread _worker;
std::atomic<bool> _done {false};
public:
template <typename... ArgsT>
ConcurrentWrapper(ArgsT&&... args) :
_object {std::forward<ArgsT>(args)...},
_worker {
[&]() {
typename decltype(_tasks)::value_type task;
while(!_done) {
{
std::unique_lock<std::mutex> lock(_mutex);
while(_tasks.empty()) {
_cond.wait(lock);
}
task = std::move(_tasks.front());
_tasks.pop_front();
}
(*task)();
}
}
} {
}
~ConcurrentWrapper() {
{
std::unique_lock<std::mutex> lock(_mutex);
_tasks.push_back(std::make_unique<std::function<void()>>(
[&](){_done = true;}
));
}
_cond.notify_one();
_worker.join();
}
template <typename F, typename R = std::result_of_t<F(T&)>>
std::future<R> operator()(F&& f) {
std::packaged_task<R(T&)> task(std::forward<F>(f));
auto fu = task.get_future();
{
std::unique_lock<std::mutex> lock(_mutex);
_tasks.push_back(std::make_unique<std::function<void()>>(
[this, task=MoveOnCopy<decltype(task)>(std::move(task))]() {
task.object(this->_object);
}
));
}
_cond.notify_one();
return fu;
}
};
Basically, the idea is to wrap an object and provide thread-safe access in FIFO order using operation (). However, in some runs (not always happen), the following program hanged:
ConcurrentWrapper<std::vector<int>> results;
results(
[&](std::vector<T>& data) {
std::cout << "sorting...\n";
std::sort(data.begin(), data.end());
std::cout << "done ...\n";
EXPECT_EQ(data, golden);
}
).get();
However, the program work correctly without explicitly calling get() method.
results(
[&](std::vector<T>& data) {
std::cout << "sorting...\n";
std::sort(data.begin(), data.end());
std::cout << "done ...\n";
EXPECT_EQ(data, golden);
}
); // Function correctly without calling get
What could the be problem? Did I implement something wrong? I noticed a posted here saying that "a packaged_task needs to be invoked before you call f.get(), otherwise you program will freeze as the future will never become ready." Is this true? If yes, how can I get this problem solved?
I was compiling the code using -std=c++1z -pthread with G++ 6.1