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/wangle/Promise.h>
28 #include <folly/wangle/Try.h>
30 namespace folly { namespace wangle {
32 template <typename T> struct isFuture;
40 Future(Future const&) = delete;
41 Future& operator=(Future const&) = delete;
44 Future(Future&&) noexcept;
45 Future& operator=(Future&&);
49 /** Return the reference to result. Should not be called if !isReady().
50 Will rethrow the exception if an exception has been
53 This function is not thread safe - the returned Future can only
54 be executed from the thread that the executor runs it in.
55 See below for a thread safe version
57 typename std::add_lvalue_reference<T>::type
59 typename std::add_lvalue_reference<const T>::type
62 /// Returns an inactive Future which will call back on the other side of
63 /// executor (when it is activated).
65 /// NB remember that Futures activate when they destruct. This is good,
66 /// it means that this will work:
68 /// f.via(e).then(a).then(b);
70 /// a and b will execute in the same context (the far side of e), because
71 /// the Future (temporary variable) created by via(e) does not call back
72 /// until it destructs, which is after then(a) and then(b) have been wired
75 /// But this is still racy:
77 /// f = f.via(e).then(a);
80 /// If you need something like that, use a Later.
81 template <typename Executor>
82 Future<T> via(Executor* executor);
84 /** True when the result (or exception) is ready. */
87 /** A reference to the Try of the value */
90 /** When this Future has completed, execute func which is a function that
91 takes a Try<T>&&. A Future for the return type of func is
94 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
96 The Future given to the functor is ready, and the functor may call
97 value(), which may rethrow if this has captured an exception. If func
98 throws, the exception will be captured in the Future that is returned.
100 /* TODO n3428 and other async frameworks have something like then(scheduler,
101 Future), we might want to support a similar API which could be
102 implemented a little more efficiently than
103 f.via(executor).then(callback) */
105 typename std::enable_if<
106 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
107 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
110 /// Variant where func returns a Future<T> instead of a T. e.g.
112 /// Future<string> f2 = f1.then(
113 /// [](Try<T>&&) { return makeFuture<string>("foo"); });
115 typename std::enable_if<
116 isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
117 Future<typename std::result_of<F(Try<T>&&)>::type::value_type> >::type
120 /// Variant where func is an ordinary function (static method, method)
122 /// R doWork(Try<T>&&);
124 /// Future<R> f2 = f1.then(doWork);
129 /// static R doWork(Try<T>&&); }
131 /// Future<R> f2 = f1.then(&Worker::doWork);
132 template <class = T, class R = std::nullptr_t>
133 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
134 inline then(R(*func)(Try<T>&&)) {
135 return then([func](Try<T>&& t) {
136 return (*func)(std::move(t));
140 /// Variant where func returns a Future<R> instead of a R. e.g.
143 /// Future<R> doWork(Try<T>&&); }
145 /// Future<R> f2 = f1.then(&Worker::doWork);
146 template <class = T, class R = std::nullptr_t>
147 typename std::enable_if<isFuture<R>::value, R>::type
148 inline then(R(*func)(Try<T>&&)) {
149 return then([func](Try<T>&& t) {
150 return (*func)(std::move(t));
154 /// Variant where func is an member function
157 /// R doWork(Try<T>&&); }
160 /// Future<R> f2 = f1.then(w, &Worker::doWork);
161 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
162 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
163 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
164 return then([instance, func](Try<T>&& t) {
165 return (instance->*func)(std::move(t));
169 /// Variant where func returns a Future<R> instead of a R. e.g.
172 /// Future<R> doWork(Try<T>&&); }
175 /// Future<R> f2 = f1.then(w, &Worker::doWork);
176 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
177 typename std::enable_if<isFuture<R>::value, R>::type
178 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
179 return then([instance, func](Try<T>&& t) {
180 return (instance->*func)(std::move(t));
184 /// Convenience method for ignoring the value and creating a Future<void>.
185 /// Exceptions still propagate.
188 /// This is not the method you're looking for.
190 /// This needs to be public because it's used by make* and when*, and it's
191 /// not worth listing all those and their fancy template signatures as
192 /// friends. But it's not for public consumption.
194 void setCallback_(F&& func);
196 /// A Future's callback is executed when all three of these conditions have
197 /// become true: it has a value (set by the Promise), it has a callback (set
198 /// by then), and it is active (active by default).
200 /// Inactive Futures will activate upon destruction.
208 return core_->isActive();
212 typedef detail::Core<T>* corePtr;
214 // shared core state object
218 Future(corePtr obj) : core_(obj) {}
222 void throwIfInvalid() const;
224 friend class Promise<T>;
228 Make a completed Future by moving in a value. e.g.
231 auto f = makeFuture(std::move(foo));
235 auto f = makeFuture<string>("foo");
238 Future<typename std::decay<T>::type> makeFuture(T&& t);
240 /** Make a completed void Future. */
241 Future<void> makeFuture();
243 /** Make a completed Future by executing a function. If the function throws
244 we capture the exception, otherwise we capture the result. */
248 typename std::enable_if<
249 !std::is_reference<F>::value, bool>::type sdf = false)
250 -> Future<decltype(func())>;
255 -> Future<decltype(func())>;
257 /// Make a failed Future from an exception_ptr.
258 /// Because the Future's type cannot be inferred you have to specify it, e.g.
260 /// auto f = makeFuture<string>(std::current_exception());
262 Future<T> makeFuture(std::exception_ptr const& e);
264 /** Make a Future from an exception type E that can be passed to
265 std::make_exception_ptr(). */
266 template <class T, class E>
267 typename std::enable_if<std::is_base_of<std::exception, E>::value,
269 makeFuture(E const& e);
271 /** Make a Future out of a Try */
273 Future<T> makeFuture(Try<T>&& t);
275 /** When all the input Futures complete, the returned Future will complete.
276 Errors do not cause early termination; this Future will always succeed
277 after all its Futures have finished (whether successfully or with an
280 The Futures are moved in, so your copies are invalid. If you need to
281 chain further from these Futures, use the variant with an output iterator.
283 XXX is this still true?
284 This function is thread-safe for Futures running on different threads.
286 The return type for Future<T> input is a Future<std::vector<Try<T>>>
288 template <class InputIterator>
289 Future<std::vector<Try<
290 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
291 whenAll(InputIterator first, InputIterator last);
293 /// This version takes a varying number of Futures instead of an iterator.
294 /// The return type for (Future<T1>, Future<T2>, ...) input
295 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
296 /// The Futures are moved in, so your copies are invalid.
297 template <typename... Fs>
298 typename detail::VariadicContext<
299 typename std::decay<Fs>::type::value_type...>::type
302 /** The result is a pair of the index of the first Future to complete and
303 the Try. If multiple Futures complete at the same time (or are already
304 complete when passed in), the "winner" is chosen non-deterministically.
306 This function is thread-safe for Futures running on different threads.
308 template <class InputIterator>
311 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
312 whenAny(InputIterator first, InputIterator last);
314 /** when n Futures have completed, the Future completes with a vector of
315 the index and Try of those n Futures (the indices refer to the original
316 order, but the result vector will be in an arbitrary order)
320 template <class InputIterator>
321 Future<std::vector<std::pair<
323 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
324 whenN(InputIterator first, InputIterator last, size_t n);
326 /** Wait for the given future to complete on a semaphore. Returns a completed
327 * future containing the result.
329 * NB if the promise for the future would be fulfilled in the same thread that
330 * you call this, it will deadlock.
333 Future<T> waitWithSemaphore(Future<T>&& f);
335 /** Wait for up to `timeout` for the given future to complete. Returns a future
336 * which may or may not be completed depending whether the given future
339 * Note: each call to this starts a (short-lived) thread and allocates memory.
341 template <typename T, class Duration>
342 Future<T> waitWithSemaphore(Future<T>&& f, Duration timeout);
346 #include <folly/wangle/Future-inl.h>