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>
30 namespace folly { namespace wangle {
32 template <typename T> struct isFuture;
33 template <class> class Later;
41 Future(Future const&) = delete;
42 Future& operator=(Future const&) = delete;
45 Future(Future&&) noexcept;
46 Future& operator=(Future&&);
50 /** Return the reference to result. Should not be called if !isReady().
51 Will rethrow the exception if an exception has been
54 This function is not thread safe - the returned Future can only
55 be executed from the thread that the executor runs it in.
56 See below for a thread safe version
58 typename std::add_lvalue_reference<T>::type
60 typename std::add_lvalue_reference<const T>::type
63 /// Returns a Later which will call back on the other side of executor.
65 /// f.via(e).then(a).then(b).launch();
67 /// a and b will execute in the same context (the far side of e)
68 template <typename Executor>
69 Later<T> via(Executor* executor);
71 /** True when the result (or exception) is ready. */
74 /** A reference to the Try of the value */
77 /** When this Future has completed, execute func which is a function that
78 takes a Try<T>&&. A Future for the return type of func is
81 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
83 The Future given to the functor is ready, and the functor may call
84 value(), which may rethrow if this has captured an exception. If func
85 throws, the exception will be captured in the Future that is returned.
87 /* TODO n3428 and other async frameworks have something like then(scheduler,
88 Future), we probably want to support a similar API (instead of
89 via. or rather, via should return a cold future (Later) and we provide
90 then(scheduler, Future) ). */
92 typename std::enable_if<
93 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
94 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
97 /// Variant where func returns a Future<T> instead of a T. e.g.
99 /// Future<string> f2 = f1.then(
100 /// [](Try<T>&&) { return makeFuture<string>("foo"); });
102 typename std::enable_if<
103 isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
104 Future<typename std::result_of<F(Try<T>&&)>::type::value_type> >::type
107 /// Variant where func is an ordinary function (static method, method)
109 /// R doWork(Try<T>&&);
111 /// Future<R> f2 = f1.then(doWork);
116 /// static R doWork(Try<T>&&); }
118 /// Future<R> f2 = f1.then(&Worker::doWork);
119 template <class = T, class R = std::nullptr_t>
120 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
121 inline then(R(*func)(Try<T>&&)) {
122 return then([func](Try<T>&& t) {
123 return (*func)(std::move(t));
127 /// Variant where func returns a Future<R> instead of a R. e.g.
130 /// Future<R> doWork(Try<T>&&); }
132 /// Future<R> f2 = f1.then(&Worker::doWork);
133 template <class = T, class R = std::nullptr_t>
134 typename std::enable_if<isFuture<R>::value, R>::type
135 inline then(R(*func)(Try<T>&&)) {
136 return then([func](Try<T>&& t) {
137 return (*func)(std::move(t));
141 /// Variant where func is an member function
144 /// R doWork(Try<T>&&); }
147 /// Future<R> f2 = f1.then(w, &Worker::doWork);
148 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
149 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
150 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
151 return then([instance, func](Try<T>&& t) {
152 return (instance->*func)(std::move(t));
156 /// Variant where func returns a Future<R> instead of a R. e.g.
159 /// Future<R> doWork(Try<T>&&); }
162 /// Future<R> f2 = f1.then(w, &Worker::doWork);
163 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
164 typename std::enable_if<isFuture<R>::value, R>::type
165 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
166 return then([instance, func](Try<T>&& t) {
167 return (instance->*func)(std::move(t));
171 /// Convenience method for ignoring the value and creating a Future<void>.
172 /// Exceptions still propagate.
175 /// This is not the method you're looking for.
177 /// This needs to be public because it's used by make* and when*, and it's
178 /// not worth listing all those and their fancy template signatures as
179 /// friends. But it's not for public consumption.
181 void setCallback_(F&& func);
184 typedef detail::FutureObject<T>* objPtr;
186 // shared state object
190 Future(objPtr obj) : obj_(obj) {}
192 void throwIfInvalid() const;
194 friend class Promise<T>;
198 Make a completed Future by moving in a value. e.g.
201 auto f = makeFuture(std::move(foo));
205 auto f = makeFuture<string>("foo");
208 Future<typename std::decay<T>::type> makeFuture(T&& t);
210 /** Make a completed void Future. */
211 Future<void> makeFuture();
213 /** Make a completed Future by executing a function. If the function throws
214 we capture the exception, otherwise we capture the result. */
218 typename std::enable_if<
219 !std::is_reference<F>::value, bool>::type sdf = false)
220 -> Future<decltype(func())>;
225 -> Future<decltype(func())>;
227 /// Make a failed Future from an exception_ptr.
228 /// Because the Future's type cannot be inferred you have to specify it, e.g.
230 /// auto f = makeFuture<string>(std::current_exception());
232 Future<T> makeFuture(std::exception_ptr const& e);
234 /** Make a Future from an exception type E that can be passed to
235 std::make_exception_ptr(). */
236 template <class T, class E>
237 typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::type
238 makeFuture(E const& e);
240 /** Make a Future out of a Try */
242 Future<T> makeFuture(Try<T>&& t);
244 /** When all the input Futures complete, the returned Future will complete.
245 Errors do not cause early termination; this Future will always succeed
246 after all its Futures have finished (whether successfully or with an
249 The Futures are moved in, so your copies are invalid. If you need to
250 chain further from these Futures, use the variant with an output iterator.
252 XXX is this still true?
253 This function is thread-safe for Futures running on different threads.
255 The return type for Future<T> input is a Future<std::vector<Try<T>>>
257 template <class InputIterator>
258 Future<std::vector<Try<
259 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
260 whenAll(InputIterator first, InputIterator last);
262 /// This version takes a varying number of Futures instead of an iterator.
263 /// The return type for (Future<T1>, Future<T2>, ...) input
264 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
265 /// The Futures are moved in, so your copies are invalid.
266 template <typename... Fs>
267 typename detail::VariadicContext<
268 typename std::decay<Fs>::type::value_type...>::type
271 /** The result is a pair of the index of the first Future to complete and
272 the Try. If multiple Futures complete at the same time (or are already
273 complete when passed in), the "winner" is chosen non-deterministically.
275 This function is thread-safe for Futures running on different threads.
277 template <class InputIterator>
280 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
281 whenAny(InputIterator first, InputIterator last);
283 /** when n Futures have completed, the Future completes with a vector of
284 the index and Try of those n Futures (the indices refer to the original
285 order, but the result vector will be in an arbitrary order)
289 template <class InputIterator>
290 Future<std::vector<std::pair<
292 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
293 whenN(InputIterator first, InputIterator last, size_t n);
295 /** Wait for the given future to complete on a semaphore. Returns a completed
296 * future containing the result.
298 * NB if the promise for the future would be fulfilled in the same thread that
299 * you call this, it will deadlock.
302 Future<T> waitWithSemaphore(Future<T>&& f);
304 /** Wait for up to `timeout` for the given future to complete. Returns a future
305 * which may or may not be completed depending whether the given future
308 * Note: each call to this starts a (short-lived) thread and allocates memory.
310 template <typename T, class Duration>
311 Future<T> waitWithSemaphore(Future<T>&& f, Duration timeout);
315 #include "Future-inl.h"