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Correction for previous #dailybiteofcpp.
Unfortunately, I have misinterpreted a corner case behaviour of std::async, and it managed to survive in the example.
The packaged task will not be run automatically when the future is destroyed. You still need to call .wait() or .get() manually.
Thanks to Pavel Pokutnev on LinkedIn for pointing it out.
Compiler Explorer link: https://compiler-explorer.com/z/4nPjMxxfj
Sorry for the confusion.
@simontoth so not a replacement for RAII or custom deleter anyway. Couldn't really think of a good usecase there. Thanks for the update.
@paiusco yeah, the actual use case is primarily this: https://compiler-explorer.com/z/jMWc7EYe1
compiler-explorer.comCompiler Explorer - C++
int main() {
using namespace std::literals;
// main thread
std::osyncstream(std::cout) << "Main thread: " << std::this_thread::get_id() << "\n";
auto t1 = std::chrono::steady_clock::now();
// Spawn a task to run asynchronously (in a thread)
auto result = std::async(std::launch::async,[](){
std::osyncstream(std::cout) << "Running in a thread: " << std::this_thread::get_id() << "\n";
std::this_thread::sleep_for(200ms);
std::osyncstream(std::cout) << "Finished thread: " << std::this_thread::get_id() << "\n";
});
auto t2 = std::chrono::steady_clock::now();
std::osyncstream(std::cout) << "Launching took: " << (t2-t1) << "\n";
// Wait for the task to finish
result.wait();
auto t3 = std::chrono::steady_clock::now();
std::cout << "Asynchronous call now finished: " << (t3-t1) << "\n";
// std::async returns a std::future
auto future = std::async(std::launch::async, [](){
// Slow operation
return 42;
});
// Block until the result is available and then retrieve it
auto value = future.get();
// value == 42
std::cout << "value == " << value << "\n";
// The deferred policy will execute the packaged task on the same thread
// however, only when we call get()/wait().
auto thread_info = [](){
std::osyncstream(std::cout) << "Running in a thread: " << std::this_thread::get_id() << "\n";
};
{
auto deferred = std::async(std::launch::deferred, thread_info);
// Run the task on this thread.
deferred.wait();
}
{
auto deferred = std::async(std::launch::deferred, thread_info);
// However, nothing prevents us from moving the future to a different thread
std::jthread([handle=std::move(deferred)](){
handle.wait(); // Run the deferred task in this new thread
});
}
}