Support auto-canceled tasks

libs/util/include/psemek/util/executor.hpp

@@ -17,6 +17,11 @@ namespace psemek::util
 		{
 			std::promise<T> promise;
 			std::atomic<bool> canceled = false;
+			bool const auto_cancel;
+
+			task_state(bool auto_cancel)
+				: auto_cancel{auto_cancel}
+			{}
 		};
 
 		template <typename T, typename F, typename ... Args>
@@ -52,14 +57,38 @@ namespace psemek::util
 		char const * what() const noexcept { return "task canceled"; }
 	};
 
+	struct auto_cancel_tag{};
+	constexpr auto_cancel_tag auto_cancel;
+
 	template <typename T>
 	struct future
 	{
+		future() = default;
+
 		future(std::shared_ptr<detail::task_state<T>> state)
 			: state_(std::move(state))
 			, f_(state_->promise.get_future())
 		{}
 
+		~future()
+		{
+			reset();
+		}
+
+		void reset()
+		{
+			if (state_ && state_->auto_cancel)
+				cancel();
+
+			state_.reset();
+			f_ = std::future<T>();
+		}
+
+		explicit operator bool() const
+		{
+			return static_cast<bool>(state_);
+		}
+
 		bool wait() const
 		{
 			f_.wait();
@@ -80,14 +109,15 @@ namespace psemek::util
 
 		T get()
 		{
-			if (state_->canceled)
+			if (state_ && state_->canceled)
 				throw canceled_task_error{};
 			return f_.get();
 		}
 
 		void cancel()
 		{
-			state_->canceled = true;
+			if (state_)
+				state_->canceled = true;
 		}
 
 		bool ready() const
@@ -143,11 +173,21 @@ namespace psemek::util
 		template <typename F, typename ... Args>
 		auto dispatch(F && f, Args && ... args);
 
+		// Post a callable for execution. Retuns a future.
+		// The task cancels on future destruction.
+		template <typename F, typename ... Args>
+		auto dispatch(auto_cancel_tag, F && f, Args && ... args);
+
 		// Post a callable for execution at a certain time point.
 		// Retuns a future.
 		template <typename TimePoint, typename F, typename ... Args>
 		auto dispatch_at(TimePoint time, F && f, Args && ... args);
 
+		// Post a callable for execution at a certain time point.
+		// Retuns a future. The task cancels on future destruction.
+		template <typename TimePoint, typename F, typename ... Args>
+		auto dispatch_at(TimePoint time, auto_cancel_tag, F && f, Args && ... args);
+
 		virtual ~executor() {}
 	};
 
@@ -155,7 +195,16 @@ namespace psemek::util
 	auto executor::dispatch(F && f, Args && ... args)
 	{
 		using R = decltype(f());
-		auto state = std::make_shared<detail::task_state<R>>();
+		auto state = std::make_shared<detail::task_state<R>>(false);
+		post(detail::wrap_task(state, std::forward<F>(f), std::forward<Args>(args)...));
+		return future<R>(state);
+	}
+
+	template <typename F, typename ... Args>
+	auto executor::dispatch(auto_cancel_tag, F && f, Args && ... args)
+	{
+		using R = decltype(f());
+		auto state = std::make_shared<detail::task_state<R>>(true);
 		post(detail::wrap_task(state, std::forward<F>(f), std::forward<Args>(args)...));
 		return future<R>(state);
 	}
@@ -164,7 +213,16 @@ namespace psemek::util
 	auto executor::dispatch_at(TimePoint time, F && f, Args && ... args)
 	{
 		using R = decltype(f());
-		auto state = std::make_shared<detail::task_state<R>>();
+		auto state = std::make_shared<detail::task_state<R>>(false);
+		post_at(std::chrono::time_point_cast<clock::duration>(time), detail::wrap_task(state, std::forward<F>(f), std::forward<Args>(args)...));
+		return future<R>(state);
+	}
+
+	template <typename TimePoint, typename F, typename ... Args>
+	auto executor::dispatch_at(TimePoint time, auto_cancel_tag, F && f, Args && ... args)
+	{
+		using R = decltype(f());
+		auto state = std::make_shared<detail::task_state<R>>(true);
 		post_at(std::chrono::time_point_cast<clock::duration>(time), detail::wrap_task(state, std::forward<F>(f), std::forward<Args>(args)...));
 		return future<R>(state);
 	}