2 * Copyright 2014 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
23 #include <type_traits>
26 #include <folly/MoveWrapper.h>
27 #include <folly/futures/Deprecated.h>
28 #include <folly/futures/DrivableExecutor.h>
29 #include <folly/futures/Promise.h>
30 #include <folly/futures/Try.h>
31 #include <folly/futures/FutureException.h>
32 #include <folly/futures/detail/Types.h>
36 template <class> struct Promise;
39 struct isFuture : std::false_type {
44 struct isFuture<Future<T>> : std::true_type {
49 struct isTry : std::false_type {};
52 struct isTry<Try<T>> : std::true_type {};
56 template <class> struct Core;
57 template <class...> struct VariadicContext;
59 template<typename F, typename... Args>
60 using resultOf = decltype(std::declval<F>()(std::declval<Args>()...));
62 template <typename...>
65 template <typename Arg, typename... Args>
66 struct ArgType<Arg, Args...> {
72 typedef void FirstArg;
75 template <bool isTry, typename F, typename... Args>
77 typedef resultOf<F, Args...> Result;
80 template<typename F, typename... Args>
83 typename = detail::resultOf<T, Args...>>
84 static constexpr std::true_type
85 check(std::nullptr_t) { return std::true_type{}; };
88 static constexpr std::false_type
89 check(...) { return std::false_type{}; };
91 typedef decltype(check<F>(nullptr)) type;
92 static constexpr bool value = type::value;
95 template<typename T, typename F>
96 struct callableResult {
97 typedef typename std::conditional<
98 callableWith<F>::value,
99 detail::argResult<false, F>,
100 typename std::conditional<
101 callableWith<F, Try<T>&&>::value,
102 detail::argResult<true, F, Try<T>&&>,
103 typename std::conditional<
104 callableWith<F, Try<T>&>::value,
105 detail::argResult<true, F, Try<T>&>,
106 typename std::conditional<
107 callableWith<F, T&&>::value,
108 detail::argResult<false, F, T&&>,
109 detail::argResult<false, F, T&>>::type>::type>::type>::type Arg;
110 typedef isFuture<typename Arg::Result> ReturnsFuture;
111 typedef Future<typename ReturnsFuture::Inner> Return;
115 struct callableResult<void, F> {
116 typedef typename std::conditional<
117 callableWith<F>::value,
118 detail::argResult<false, F>,
119 typename std::conditional<
120 callableWith<F, Try<void>&&>::value,
121 detail::argResult<true, F, Try<void>&&>,
122 detail::argResult<true, F, Try<void>&>>::type>::type Arg;
123 typedef isFuture<typename Arg::Result> ReturnsFuture;
124 typedef Future<typename ReturnsFuture::Inner> Return;
127 template <typename L>
128 struct Extract : Extract<decltype(&L::operator())> { };
130 template <typename Class, typename R, typename... Args>
131 struct Extract<R(Class::*)(Args...) const> {
132 typedef isFuture<R> ReturnsFuture;
133 typedef Future<typename ReturnsFuture::Inner> Return;
134 typedef typename ReturnsFuture::Inner RawReturn;
135 typedef typename ArgType<Args...>::FirstArg FirstArg;
138 template <typename Class, typename R, typename... Args>
139 struct Extract<R(Class::*)(Args...)> {
140 typedef isFuture<R> ReturnsFuture;
141 typedef Future<typename ReturnsFuture::Inner> Return;
142 typedef typename ReturnsFuture::Inner RawReturn;
143 typedef typename ArgType<Args...>::FirstArg FirstArg;
150 /// This namespace is for utility functions that would usually be static
151 /// members of Future, except they don't make sense there because they don't
152 /// depend on the template type (rather, on the type of their arguments in
153 /// some cases). This is the least-bad naming scheme we could think of. Some
154 /// of the functions herein have really-likely-to-collide names, like "map"
157 /// Returns a Future that will complete after the specified duration. The
158 /// Duration typedef of a `std::chrono` duration type indicates the
159 /// resolution you can expect to be meaningful (milliseconds at the time of
160 /// writing). Normally you wouldn't need to specify a Timekeeper, we will
161 /// use the global futures timekeeper (we run a thread whose job it is to
162 /// keep time for futures timeouts) but we provide the option for power
165 /// The Timekeeper thread will be lazily created the first time it is
166 /// needed. If your program never uses any timeouts or other time-based
167 /// Futures you will pay no Timekeeper thread overhead.
168 Future<void> sleep(Duration, Timekeeper* = nullptr);
174 typedef T value_type;
177 Future(Future const&) = delete;
178 Future& operator=(Future const&) = delete;
181 Future(Future&&) noexcept;
182 Future& operator=(Future&&);
186 /** Return the reference to result. Should not be called if !isReady().
187 Will rethrow the exception if an exception has been
190 This function is not thread safe - the returned Future can only
191 be executed from the thread that the executor runs it in.
192 See below for a thread safe version
194 typename std::add_lvalue_reference<T>::type
196 typename std::add_lvalue_reference<const T>::type
199 /// Returns an inactive Future which will call back on the other side of
200 /// executor (when it is activated).
202 /// NB remember that Futures activate when they destruct. This is good,
203 /// it means that this will work:
205 /// f.via(e).then(a).then(b);
207 /// a and b will execute in the same context (the far side of e), because
208 /// the Future (temporary variable) created by via(e) does not call back
209 /// until it destructs, which is after then(a) and then(b) have been wired
212 /// But this is still racy:
214 /// f = f.via(e).then(a);
216 // The ref-qualifier allows for `this` to be moved out so we
217 // don't get access-after-free situations in chaining.
218 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
219 template <typename Executor>
220 Future<T> via(Executor* executor) &&;
222 /// This variant creates a new future, where the ref-qualifier && version
223 /// moves `this` out. This one is less efficient but avoids confusing users
224 /// when "return f.via(x);" fails.
225 template <typename Executor>
226 Future<T> via(Executor* executor) &;
228 /** True when the result (or exception) is ready. */
229 bool isReady() const;
231 /** A reference to the Try of the value */
234 /// Block until the future is fulfilled. Returns the value (moved out), or
235 /// throws the exception. The future must not already have a callback.
238 /// Block until the future is fulfilled, or until timed out. Returns the
239 /// value (moved out), or throws the exception (which might be a TimedOut
243 /// Call e->drive() repeatedly until the future is fulfilled. Examples
244 /// of DrivableExecutor include EventBase and ManualExecutor. Returns the
245 /// value (moved out), or throws the exception.
246 T getVia(DrivableExecutor* e);
248 /** When this Future has completed, execute func which is a function that
258 Func shall return either another Future or a value.
260 A Future for the return type of func is returned.
262 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
264 The Future given to the functor is ready, and the functor may call
265 value(), which may rethrow if this has captured an exception. If func
266 throws, the exception will be captured in the Future that is returned.
268 /* TODO n3428 and other async frameworks have something like then(scheduler,
269 Future), we might want to support a similar API which could be
270 implemented a little more efficiently than
271 f.via(executor).then(callback) */
272 template <typename F, typename R = detail::callableResult<T, F>>
273 typename R::Return then(F func) {
274 typedef typename R::Arg Arguments;
275 return thenImplementation<F, R>(std::move(func), Arguments());
278 /// Variant where func is an member function
281 /// R doWork(Try<T>&&); }
284 /// Future<R> f2 = f1.then(w, &Worker::doWork);
285 template <typename Caller, typename R, typename... Args>
286 Future<typename isFuture<R>::Inner>
287 then(Caller *instance, R(Caller::*func)(Args...));
289 /// Convenience method for ignoring the value and creating a Future<void>.
290 /// Exceptions still propagate.
293 /// Set an error callback for this Future. The callback should take a single
294 /// argument of the type that you want to catch, and should return a value of
295 /// the same type as this Future, or a Future of that type (see overload
296 /// below). For instance,
300 /// throw std::runtime_error("oh no!");
303 /// .onError([] (std::runtime_error& e) {
304 /// LOG(INFO) << "std::runtime_error: " << e.what();
305 /// return -1; // or makeFuture<int>(-1)
308 typename std::enable_if<
309 !detail::Extract<F>::ReturnsFuture::value,
313 /// Overload of onError where the error callback returns a Future<T>
315 typename std::enable_if<
316 detail::Extract<F>::ReturnsFuture::value,
320 /// This is not the method you're looking for.
322 /// This needs to be public because it's used by make* and when*, and it's
323 /// not worth listing all those and their fancy template signatures as
324 /// friends. But it's not for public consumption.
326 void setCallback_(F&& func);
328 /// A Future's callback is executed when all three of these conditions have
329 /// become true: it has a value (set by the Promise), it has a callback (set
330 /// by then), and it is active (active by default).
332 /// Inactive Futures will activate upon destruction.
333 Future<T>& activate() & {
337 Future<T>& deactivate() & {
341 Future<T> activate() && {
343 return std::move(*this);
345 Future<T> deactivate() && {
347 return std::move(*this);
351 return core_->isActive();
355 void raise(E&& exception) {
356 raise(make_exception_wrapper<typename std::remove_reference<E>::type>(
357 std::move(exception)));
360 /// Raise an interrupt. If the promise holder has an interrupt
361 /// handler it will be called and potentially stop asynchronous work from
362 /// being done. This is advisory only - a promise holder may not set an
363 /// interrupt handler, or may do anything including ignore. But, if you know
364 /// your future supports this the most likely result is stopping or
365 /// preventing the asynchronous operation (if in time), and the promise
366 /// holder setting an exception on the future. (That may happen
367 /// asynchronously, of course.)
368 void raise(exception_wrapper interrupt);
371 raise(FutureCancellation());
374 /// Throw TimedOut if this Future does not complete within the given
375 /// duration from now. The optional Timeekeeper is as with futures::sleep().
376 Future<T> within(Duration, Timekeeper* = nullptr);
378 /// Throw the given exception if this Future does not complete within the
379 /// given duration from now. The optional Timeekeeper is as with
380 /// futures::sleep().
382 Future<T> within(Duration, E exception, Timekeeper* = nullptr);
384 /// Delay the completion of this Future for at least this duration from
385 /// now. The optional Timekeeper is as with futures::sleep().
386 Future<T> delayed(Duration, Timekeeper* = nullptr);
388 /// Block until this Future is complete. Returns a new Future containing the
392 /// Block until this Future is complete or until the given Duration passes.
393 /// Returns a new Future which either contains the result or is incomplete,
394 /// depending on whether the Duration passed.
395 Future<T> wait(Duration);
397 /// Call e->drive() repeatedly until the future is fulfilled. Examples
398 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
399 /// reference to this Future so that you can chain calls if desired.
400 /// value (moved out), or throws the exception.
401 Future<T>& waitVia(DrivableExecutor* e) &;
403 /// Overload of waitVia() for rvalue Futures
404 Future<T> waitVia(DrivableExecutor* e) &&;
407 typedef detail::Core<T>* corePtr;
409 // shared core state object
413 Future(corePtr obj) : core_(obj) {}
417 void throwIfInvalid() const;
419 friend class Promise<T>;
421 // Variant: returns a value
422 // e.g. f.then([](Try<T> t){ return t.value(); });
423 template <typename F, typename R, bool isTry, typename... Args>
424 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
425 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
427 // Variant: returns a Future
428 // e.g. f.then([](Try<T> t){ return makeFuture<T>(t); });
429 template <typename F, typename R, bool isTry, typename... Args>
430 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
431 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
435 Make a completed Future by moving in a value. e.g.
438 auto f = makeFuture(std::move(foo));
442 auto f = makeFuture<string>("foo");
445 Future<typename std::decay<T>::type> makeFuture(T&& t);
447 /** Make a completed void Future. */
448 Future<void> makeFuture();
450 /** Make a completed Future by executing a function. If the function throws
451 we capture the exception, otherwise we capture the result. */
455 typename std::enable_if<
456 !std::is_reference<F>::value, bool>::type sdf = false)
457 -> Future<decltype(func())>;
462 -> Future<decltype(func())>;
464 /// Make a failed Future from an exception_ptr.
465 /// Because the Future's type cannot be inferred you have to specify it, e.g.
467 /// auto f = makeFuture<string>(std::current_exception());
469 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
471 /// Make a failed Future from an exception_wrapper.
473 Future<T> makeFuture(exception_wrapper ew);
475 /** Make a Future from an exception type E that can be passed to
476 std::make_exception_ptr(). */
477 template <class T, class E>
478 typename std::enable_if<std::is_base_of<std::exception, E>::value,
480 makeFuture(E const& e);
482 /** Make a Future out of a Try */
484 Future<T> makeFuture(Try<T>&& t);
487 * Return a new Future that will call back on the given Executor.
488 * This is just syntactic sugar for makeFuture().via(executor)
490 * @param executor the Executor to call back on
492 * @returns a void Future that will call back on the given executor
494 template <typename Executor>
495 Future<void> via(Executor* executor);
497 /** When all the input Futures complete, the returned Future will complete.
498 Errors do not cause early termination; this Future will always succeed
499 after all its Futures have finished (whether successfully or with an
502 The Futures are moved in, so your copies are invalid. If you need to
503 chain further from these Futures, use the variant with an output iterator.
505 XXX is this still true?
506 This function is thread-safe for Futures running on different threads.
508 The return type for Future<T> input is a Future<std::vector<Try<T>>>
510 template <class InputIterator>
511 Future<std::vector<Try<
512 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
513 whenAll(InputIterator first, InputIterator last);
515 /// This version takes a varying number of Futures instead of an iterator.
516 /// The return type for (Future<T1>, Future<T2>, ...) input
517 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
518 /// The Futures are moved in, so your copies are invalid.
519 template <typename... Fs>
520 typename detail::VariadicContext<
521 typename std::decay<Fs>::type::value_type...>::type
524 /** The result is a pair of the index of the first Future to complete and
525 the Try. If multiple Futures complete at the same time (or are already
526 complete when passed in), the "winner" is chosen non-deterministically.
528 This function is thread-safe for Futures running on different threads.
530 template <class InputIterator>
533 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
534 whenAny(InputIterator first, InputIterator last);
536 /** when n Futures have completed, the Future completes with a vector of
537 the index and Try of those n Futures (the indices refer to the original
538 order, but the result vector will be in an arbitrary order)
542 template <class InputIterator>
543 Future<std::vector<std::pair<
545 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
546 whenN(InputIterator first, InputIterator last, size_t n);
548 /** Wait for the given future to complete on a semaphore. Returns a completed
549 * future containing the result.
551 * NB if the promise for the future would be fulfilled in the same thread that
552 * you call this, it will deadlock.
555 Future<T> waitWithSemaphore(Future<T>&& f);
557 /** Wait for up to `timeout` for the given future to complete. Returns a future
558 * which may or may not be completed depending whether the given future
561 * Note: each call to this starts a (short-lived) thread and allocates memory.
563 template <typename T, class Dur>
564 Future<T> waitWithSemaphore(Future<T>&& f, Dur timeout);
568 #include <folly/futures/Future-inl.h>