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;
40 Future(Future const&) = delete;
41 Future& operator=(Future const&) = delete;
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 a future which will call back on the other side of executor.
64 /// f.via(e).then(a); // safe
66 /// f.via(e).then(a).then(b); // faux pas
68 /// a will definitely execute in the intended thread, but b may execute
69 /// either in that thread, or in the current thread. If you need to
70 /// guarantee where b executes, use a Later.
71 template <typename Executor>
72 Future<T> via(Executor* executor);
74 /// Deprecated alias for via
75 template <typename Executor>
76 Future<T> executeWithSameThread(Executor* executor) {
81 Thread-safe version of executeWith
83 Since an executor would likely start executing the Future chain
84 right away, it would be a race condition to call:
85 Future.executeWith(...).then(...), as there would be race
86 condition between the then and the running Future.
87 Instead, you may pass in a Promise so that we can set up
88 the rest of the chain in advance, without any racey
89 modifications of the continuation
91 Deprecated. Use a Later.
93 template <typename Executor>
94 void executeWith(Executor* executor, Promise<T>&& cont_promise);
96 /** True when the result (or exception) is ready. */
99 /** A reference to the Try of the value */
102 /** When this Future has completed, execute func which is a function that
103 takes a Try<T>&&. A Future for the return type of func is
106 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
108 The Future given to the functor is ready, and the functor may call
109 value(), which may rethrow if this has captured an exception. If func
110 throws, the exception will be captured in the Future that is returned.
112 /* TODO n3428 and other async frameworks have something like then(scheduler,
113 Future), we probably want to support a similar API (instead of
116 typename std::enable_if<
117 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
118 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
121 /// Variant where func returns a Future<T> instead of a T. e.g.
123 /// Future<string> f2 = f1.then(
124 /// [](Try<T>&&) { return makeFuture<string>("foo"); });
126 typename std::enable_if<
127 isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
128 Future<typename std::result_of<F(Try<T>&&)>::type::value_type> >::type
131 /// Variant where func is an ordinary function (static method, method)
133 /// R doWork(Try<T>&&);
135 /// Future<R> f2 = f1.then(doWork);
140 /// static R doWork(Try<T>&&); }
142 /// Future<R> f2 = f1.then(&Worker::doWork);
143 template <class = T, class R = std::nullptr_t>
144 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
145 inline then(R(*func)(Try<T>&&)) {
146 return then([func](Try<T>&& t) {
147 return (*func)(std::move(t));
151 /// Variant where func returns a Future<R> instead of a R. e.g.
154 /// Future<R> doWork(Try<T>&&); }
156 /// Future<R> f2 = f1.then(&Worker::doWork);
157 template <class = T, class R = std::nullptr_t>
158 typename std::enable_if<isFuture<R>::value, R>::type
159 inline then(R(*func)(Try<T>&&)) {
160 return then([func](Try<T>&& t) {
161 return (*func)(std::move(t));
165 /// Variant where func is an member function
168 /// R doWork(Try<T>&&); }
171 /// Future<R> f2 = f1.then(w, &Worker::doWork);
172 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
173 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
174 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
175 return then([instance, func](Try<T>&& t) {
176 return (instance->*func)(std::move(t));
180 /// Variant where func returns a Future<R> instead of a R. e.g.
183 /// Future<R> doWork(Try<T>&&); }
186 /// Future<R> f2 = f1.then(w, &Worker::doWork);
187 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
188 typename std::enable_if<isFuture<R>::value, R>::type
189 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
190 return then([instance, func](Try<T>&& t) {
191 return (instance->*func)(std::move(t));
195 /// Convenience method for ignoring the value and creating a Future<void>.
196 /// Exceptions still propagate.
199 /// Use of this method is advanced wizardry.
200 /// XXX should this be protected?
202 void setContinuation(F&& func);
205 typedef detail::FutureObject<T>* objPtr;
207 // shared state object
211 Future(objPtr obj) : obj_(obj) {}
213 void throwIfInvalid() const;
215 friend class Promise<T>;
219 Make a completed Future by moving in a value. e.g.
222 auto f = makeFuture(std::move(foo));
226 auto f = makeFuture<string>("foo");
229 Future<typename std::decay<T>::type> makeFuture(T&& t);
231 /** Make a completed void Future. */
232 Future<void> makeFuture();
234 /** Make a completed Future by executing a function. If the function throws
235 we capture the exception, otherwise we capture the result. */
239 typename std::enable_if<
240 !std::is_reference<F>::value, bool>::type sdf = false)
241 -> Future<decltype(func())>;
246 -> Future<decltype(func())>;
248 /// Make a failed Future from an exception_ptr.
249 /// Because the Future's type cannot be inferred you have to specify it, e.g.
251 /// auto f = makeFuture<string>(std::current_exception());
253 Future<T> makeFuture(std::exception_ptr const& e);
255 /** Make a Future from an exception type E that can be passed to
256 std::make_exception_ptr(). */
257 template <class T, class E>
258 typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::type
259 makeFuture(E const& e);
261 /** Make a Future out of a Try */
263 Future<T> makeFuture(Try<T>&& t);
265 /** When all the input Futures complete, the returned Future will complete.
266 Errors do not cause early termination; this Future will always succeed
267 after all its Futures have finished (whether successfully or with an
270 The Futures are moved in, so your copies are invalid. If you need to
271 chain further from these Futures, use the variant with an output iterator.
273 XXX is this still true?
274 This function is thread-safe for Futures running on different threads.
276 The return type for Future<T> input is a Future<std::vector<Try<T>>>
278 template <class InputIterator>
279 Future<std::vector<Try<
280 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
281 whenAll(InputIterator first, InputIterator last);
283 /// This version takes a varying number of Futures instead of an iterator.
284 /// The return type for (Future<T1>, Future<T2>, ...) input
285 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
286 /// The Futures are moved in, so your copies are invalid.
287 template <typename... Fs>
288 typename detail::VariadicContext<
289 typename std::decay<Fs>::type::value_type...>::type
292 /** The result is a pair of the index of the first Future to complete and
293 the Try. If multiple Futures complete at the same time (or are already
294 complete when passed in), the "winner" is chosen non-deterministically.
296 This function is thread-safe for Futures running on different threads.
298 template <class InputIterator>
301 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
302 whenAny(InputIterator first, InputIterator last);
304 /** when n Futures have completed, the Future completes with a vector of
305 the index and Try of those n Futures (the indices refer to the original
306 order, but the result vector will be in an arbitrary order)
310 template <class InputIterator>
311 Future<std::vector<std::pair<
313 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
314 whenN(InputIterator first, InputIterator last, size_t n);
316 /** Wait for the given future to complete on a semaphore. Returns a completed
317 * future containing the result.
319 * NB if the promise for the future would be fulfilled in the same thread that
320 * you call this, it will deadlock.
323 Future<T> waitWithSemaphore(Future<T>&& f);
325 /** Wait for up to `timeout` for the given future to complete. Returns a future
326 * which may or may not be completed depending whether the given future
329 * Note: each call to this starts a (short-lived) thread and allocates memory.
331 template <typename T, class Duration>
332 Future<T> waitWithSemaphore(Future<T>&& f, Duration timeout);
336 #include "Future-inl.h"