C++ std::async run on main thread - c++

Question

IS there a way of running a function back on the main thread ?
So if I called a function via Async that downloaded a file and then parsed the data. It would then call a callback function which would run on my main UI thread and update the UI ?
I know threads are equal in the default C++ implementation so would I have to create a shared pointer to my main thread. How would I do this and pass the Async function not only the shared pointer to the main thread but also a pointer to the function I want to rrun on it and then run it on that main thread ?

Answer 1

I have been reading C++ Concurrency in Action and chapter four (AKA "The Chapter I Just Finished") describes a solution.
The Short Version
Have a shared std::deque<std::packaged_task<void()>> (or a similar sort of message/task queue). Your std::async-launched functions can push tasks to the queue, and your GUI thread can process them during its loop.
There Isn't Really a Long Version, but Here Is an Example
Shared Data
std::deque<std::packaged_task<void()>> tasks;
std::mutex tasks_mutex;
std::atomic<bool> gui_running;
The std::async Function
void one_off()
{
std::packaged_task<void()> task(FUNCTION TO RUN ON GUI THREAD); //!!
std::future<void> result = task.get_future();
{
std::lock_guard<std::mutex> lock(tasks_mutex);
tasks.push_back(std::move(task));
}
// wait on result
result.get();
}
The GUI Thread
void gui_thread()
{
while (gui_running) {
// process messages
{
std::unique_lock<std::mutex> lock(tasks_mutex);
while (!tasks.empty()) {
auto task(std::move(tasks.front()));
tasks.pop_front();
// unlock during the task
lock.unlock();
task();
lock.lock();
}
}
// "do gui work"
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
}
}
Notes:
I am (always) learning, so there is a decent chance that my code is not great. The concept is at least sound though.
The destructor of the return value from std::async (a std::future<>) will block until the operation launched with std::async completes (see std::async ), so waiting on the result of a task (as I do in my example) in one_off might not be a brilliant idea.
You may want to (I would, at least) create your own threadsafe MessageQueue type to improve code readability/maintainability/blah blah blah.
I swear there was one more thing I wanted to point out, but it escapes me right now.
Full Example
#include <atomic>
#include <chrono>
#include <deque>
#include <iostream>
#include <mutex>
#include <future>
#include <thread>
// shared stuff:
std::deque<std::packaged_task<void()>> tasks;
std::mutex tasks_mutex;
std::atomic<bool> gui_running;
void message()
{
std::cout << std::this_thread::get_id() << std::endl;
}
void one_off()
{
std::packaged_task<void()> task(message);
std::future<void> result = task.get_future();
{
std::lock_guard<std::mutex> lock(tasks_mutex);
tasks.push_back(std::move(task));
}
// wait on result
result.get();
}
void gui_thread()
{
std::cout << "gui thread: "; message();
while (gui_running) {
// process messages
{
std::unique_lock<std::mutex> lock(tasks_mutex);
while (!tasks.empty()) {
auto task(std::move(tasks.front()));
tasks.pop_front();
// unlock during the task
lock.unlock();
task();
lock.lock();
}
}
// "do gui work"
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
}
}
int main()
{
gui_running = true;
std::cout << "main thread: "; message();
std::thread gt(gui_thread);
for (unsigned i = 0; i < 5; ++i) {
// note:
// these will be launched sequentially because result's
// destructor will block until one_off completes
auto result = std::async(std::launch::async, one_off);
// maybe do something with result if it is not void
}
// the for loop will not complete until all the tasks have been
// processed by gui_thread
// ...
// cleanup
gui_running = false;
gt.join();
}
Dat Output
$ ./messages
main thread: 140299226687296
gui thread: 140299210073856
140299210073856
140299210073856
140299210073856
140299210073856
140299210073856

Answer 2

Are you looking for std::launch::deferred ? Passing this parameter to std::async makes the task executed on the calling thread when the get() function is called for the first time.