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&&);
185 template <class F = T>
187 Future(const typename std::enable_if<!std::is_void<F>::value, F>::type& val);
189 template <class F = T>
191 Future(typename std::enable_if<!std::is_void<F>::value, F>::type&& val);
193 template <class F = T,
194 typename std::enable_if<std::is_void<F>::value, int>::type = 0>
199 /** Return the reference to result. Should not be called if !isReady().
200 Will rethrow the exception if an exception has been
203 This function is not thread safe - the returned Future can only
204 be executed from the thread that the executor runs it in.
205 See below for a thread safe version
207 typename std::add_lvalue_reference<T>::type
209 typename std::add_lvalue_reference<const T>::type
212 /// Returns an inactive Future which will call back on the other side of
213 /// executor (when it is activated).
215 /// NB remember that Futures activate when they destruct. This is good,
216 /// it means that this will work:
218 /// f.via(e).then(a).then(b);
220 /// a and b will execute in the same context (the far side of e), because
221 /// the Future (temporary variable) created by via(e) does not call back
222 /// until it destructs, which is after then(a) and then(b) have been wired
225 /// But this is still racy:
227 /// f = f.via(e).then(a);
229 // The ref-qualifier allows for `this` to be moved out so we
230 // don't get access-after-free situations in chaining.
231 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
232 template <typename Executor>
233 Future<T> via(Executor* executor) &&;
235 /// This variant creates a new future, where the ref-qualifier && version
236 /// moves `this` out. This one is less efficient but avoids confusing users
237 /// when "return f.via(x);" fails.
238 template <typename Executor>
239 Future<T> via(Executor* executor) &;
241 /** True when the result (or exception) is ready. */
242 bool isReady() const;
244 /** A reference to the Try of the value */
247 /// Block until the future is fulfilled. Returns the value (moved out), or
248 /// throws the exception. The future must not already have a callback.
251 /// Block until the future is fulfilled, or until timed out. Returns the
252 /// value (moved out), or throws the exception (which might be a TimedOut
256 /// Call e->drive() repeatedly until the future is fulfilled. Examples
257 /// of DrivableExecutor include EventBase and ManualExecutor. Returns the
258 /// value (moved out), or throws the exception.
259 T getVia(DrivableExecutor* e);
261 /** When this Future has completed, execute func which is a function that
271 Func shall return either another Future or a value.
273 A Future for the return type of func is returned.
275 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
277 The Future given to the functor is ready, and the functor may call
278 value(), which may rethrow if this has captured an exception. If func
279 throws, the exception will be captured in the Future that is returned.
281 /* TODO n3428 and other async frameworks have something like then(scheduler,
282 Future), we might want to support a similar API which could be
283 implemented a little more efficiently than
284 f.via(executor).then(callback) */
285 template <typename F, typename R = detail::callableResult<T, F>>
286 typename R::Return then(F func) {
287 typedef typename R::Arg Arguments;
288 return thenImplementation<F, R>(std::move(func), Arguments());
291 /// Variant where func is an member function
294 /// R doWork(Try<T>&&); }
297 /// Future<R> f2 = f1.then(w, &Worker::doWork);
298 template <typename Caller, typename R, typename... Args>
299 Future<typename isFuture<R>::Inner>
300 then(Caller *instance, R(Caller::*func)(Args...));
302 /// Convenience method for ignoring the value and creating a Future<void>.
303 /// Exceptions still propagate.
306 /// Set an error callback for this Future. The callback should take a single
307 /// argument of the type that you want to catch, and should return a value of
308 /// the same type as this Future, or a Future of that type (see overload
309 /// below). For instance,
313 /// throw std::runtime_error("oh no!");
316 /// .onError([] (std::runtime_error& e) {
317 /// LOG(INFO) << "std::runtime_error: " << e.what();
318 /// return -1; // or makeFuture<int>(-1)
321 typename std::enable_if<
322 !detail::Extract<F>::ReturnsFuture::value,
326 /// Overload of onError where the error callback returns a Future<T>
328 typename std::enable_if<
329 detail::Extract<F>::ReturnsFuture::value,
333 /// This is not the method you're looking for.
335 /// This needs to be public because it's used by make* and when*, and it's
336 /// not worth listing all those and their fancy template signatures as
337 /// friends. But it's not for public consumption.
339 void setCallback_(F&& func);
341 /// A Future's callback is executed when all three of these conditions have
342 /// become true: it has a value (set by the Promise), it has a callback (set
343 /// by then), and it is active (active by default).
345 /// Inactive Futures will activate upon destruction.
346 Future<T>& activate() & {
350 Future<T>& deactivate() & {
354 Future<T> activate() && {
356 return std::move(*this);
358 Future<T> deactivate() && {
360 return std::move(*this);
364 return core_->isActive();
368 void raise(E&& exception) {
369 raise(make_exception_wrapper<typename std::remove_reference<E>::type>(
370 std::move(exception)));
373 /// Raise an interrupt. If the promise holder has an interrupt
374 /// handler it will be called and potentially stop asynchronous work from
375 /// being done. This is advisory only - a promise holder may not set an
376 /// interrupt handler, or may do anything including ignore. But, if you know
377 /// your future supports this the most likely result is stopping or
378 /// preventing the asynchronous operation (if in time), and the promise
379 /// holder setting an exception on the future. (That may happen
380 /// asynchronously, of course.)
381 void raise(exception_wrapper interrupt);
384 raise(FutureCancellation());
387 /// Throw TimedOut if this Future does not complete within the given
388 /// duration from now. The optional Timeekeeper is as with futures::sleep().
389 Future<T> within(Duration, Timekeeper* = nullptr);
391 /// Throw the given exception if this Future does not complete within the
392 /// given duration from now. The optional Timeekeeper is as with
393 /// futures::sleep().
395 Future<T> within(Duration, E exception, Timekeeper* = nullptr);
397 /// Delay the completion of this Future for at least this duration from
398 /// now. The optional Timekeeper is as with futures::sleep().
399 Future<T> delayed(Duration, Timekeeper* = nullptr);
401 /// Block until this Future is complete. Returns a new Future containing the
405 /// Block until this Future is complete or until the given Duration passes.
406 /// Returns a new Future which either contains the result or is incomplete,
407 /// depending on whether the Duration passed.
408 Future<T> wait(Duration);
410 /// Call e->drive() repeatedly until the future is fulfilled. Examples
411 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
412 /// reference to this Future so that you can chain calls if desired.
413 /// value (moved out), or throws the exception.
414 Future<T>& waitVia(DrivableExecutor* e) &;
416 /// Overload of waitVia() for rvalue Futures
417 Future<T> waitVia(DrivableExecutor* e) &&;
420 typedef detail::Core<T>* corePtr;
422 // shared core state object
426 Future(corePtr obj) : core_(obj) {}
430 void throwIfInvalid() const;
432 friend class Promise<T>;
434 // Variant: returns a value
435 // e.g. f.then([](Try<T> t){ return t.value(); });
436 template <typename F, typename R, bool isTry, typename... Args>
437 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
438 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
440 // Variant: returns a Future
441 // e.g. f.then([](Try<T> t){ return makeFuture<T>(t); });
442 template <typename F, typename R, bool isTry, typename... Args>
443 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
444 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
448 Make a completed Future by moving in a value. e.g.
451 auto f = makeFuture(std::move(foo));
455 auto f = makeFuture<string>("foo");
458 Future<typename std::decay<T>::type> makeFuture(T&& t);
460 /** Make a completed void Future. */
461 Future<void> makeFuture();
463 /** Make a completed Future by executing a function. If the function throws
464 we capture the exception, otherwise we capture the result. */
468 typename std::enable_if<
469 !std::is_reference<F>::value, bool>::type sdf = false)
470 -> Future<decltype(func())>;
475 -> Future<decltype(func())>;
477 /// Make a failed Future from an exception_ptr.
478 /// Because the Future's type cannot be inferred you have to specify it, e.g.
480 /// auto f = makeFuture<string>(std::current_exception());
482 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
484 /// Make a failed Future from an exception_wrapper.
486 Future<T> makeFuture(exception_wrapper ew);
488 /** Make a Future from an exception type E that can be passed to
489 std::make_exception_ptr(). */
490 template <class T, class E>
491 typename std::enable_if<std::is_base_of<std::exception, E>::value,
493 makeFuture(E const& e);
495 /** Make a Future out of a Try */
497 Future<T> makeFuture(Try<T>&& t);
500 * Return a new Future that will call back on the given Executor.
501 * This is just syntactic sugar for makeFuture().via(executor)
503 * @param executor the Executor to call back on
505 * @returns a void Future that will call back on the given executor
507 template <typename Executor>
508 Future<void> via(Executor* executor);
510 /** When all the input Futures complete, the returned Future will complete.
511 Errors do not cause early termination; this Future will always succeed
512 after all its Futures have finished (whether successfully or with an
515 The Futures are moved in, so your copies are invalid. If you need to
516 chain further from these Futures, use the variant with an output iterator.
518 XXX is this still true?
519 This function is thread-safe for Futures running on different threads.
521 The return type for Future<T> input is a Future<std::vector<Try<T>>>
523 template <class InputIterator>
524 Future<std::vector<Try<
525 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
526 whenAll(InputIterator first, InputIterator last);
528 /// This version takes a varying number of Futures instead of an iterator.
529 /// The return type for (Future<T1>, Future<T2>, ...) input
530 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
531 /// The Futures are moved in, so your copies are invalid.
532 template <typename... Fs>
533 typename detail::VariadicContext<
534 typename std::decay<Fs>::type::value_type...>::type
537 /** The result is a pair of the index of the first Future to complete and
538 the Try. If multiple Futures complete at the same time (or are already
539 complete when passed in), the "winner" is chosen non-deterministically.
541 This function is thread-safe for Futures running on different threads.
543 template <class InputIterator>
546 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
547 whenAny(InputIterator first, InputIterator last);
549 /** when n Futures have completed, the Future completes with a vector of
550 the index and Try of those n Futures (the indices refer to the original
551 order, but the result vector will be in an arbitrary order)
555 template <class InputIterator>
556 Future<std::vector<std::pair<
558 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
559 whenN(InputIterator first, InputIterator last, size_t n);
563 #include <folly/futures/Future-inl.h>