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 /** True when the result (or exception) is ready. */
77 /** A reference to the Try of the value */
80 /** When this Future has completed, execute func which is a function that
81 takes a Try<T>&&. A Future for the return type of func is
84 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
86 The Future given to the functor is ready, and the functor may call
87 value(), which may rethrow if this has captured an exception. If func
88 throws, the exception will be captured in the Future that is returned.
90 /* TODO n3428 and other async frameworks have something like then(scheduler,
91 Future), we probably want to support a similar API (instead of
92 via. or rather, via should return a cold future (Later) and we provide
93 then(scheduler, Future) ). */
95 typename std::enable_if<
96 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
97 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
100 /// Variant where func returns a Future<T> instead of a T. e.g.
102 /// Future<string> f2 = f1.then(
103 /// [](Try<T>&&) { return makeFuture<string>("foo"); });
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::value_type> >::type
110 /// Variant where func is an ordinary function (static method, method)
112 /// R doWork(Try<T>&&);
114 /// Future<R> f2 = f1.then(doWork);
119 /// static R doWork(Try<T>&&); }
121 /// Future<R> f2 = f1.then(&Worker::doWork);
122 template <class = T, class R = std::nullptr_t>
123 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
124 inline then(R(*func)(Try<T>&&)) {
125 return then([func](Try<T>&& t) {
126 return (*func)(std::move(t));
130 /// Variant where func returns a Future<R> instead of a R. e.g.
133 /// Future<R> doWork(Try<T>&&); }
135 /// Future<R> f2 = f1.then(&Worker::doWork);
136 template <class = T, class R = std::nullptr_t>
137 typename std::enable_if<isFuture<R>::value, R>::type
138 inline then(R(*func)(Try<T>&&)) {
139 return then([func](Try<T>&& t) {
140 return (*func)(std::move(t));
144 /// Variant where func is an member function
147 /// R doWork(Try<T>&&); }
150 /// Future<R> f2 = f1.then(w, &Worker::doWork);
151 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
152 typename std::enable_if<!isFuture<R>::value, Future<R>>::type
153 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
154 return then([instance, func](Try<T>&& t) {
155 return (instance->*func)(std::move(t));
159 /// Variant where func returns a Future<R> instead of a R. e.g.
162 /// Future<R> doWork(Try<T>&&); }
165 /// Future<R> f2 = f1.then(w, &Worker::doWork);
166 template <class = T, class R = std::nullptr_t, class Caller = std::nullptr_t>
167 typename std::enable_if<isFuture<R>::value, R>::type
168 inline then(Caller *instance, R(Caller::*func)(Try<T>&&)) {
169 return then([instance, func](Try<T>&& t) {
170 return (instance->*func)(std::move(t));
174 /// Convenience method for ignoring the value and creating a Future<void>.
175 /// Exceptions still propagate.
178 /// This is not the method you're looking for.
180 /// This needs to be public because it's used by make* and when*, and it's
181 /// not worth listing all those and their fancy template signatures as
182 /// friends. But it's not for public consumption.
184 void setContinuation(F&& func);
187 typedef detail::FutureObject<T>* objPtr;
189 // shared state object
193 Future(objPtr obj) : obj_(obj) {}
195 void throwIfInvalid() const;
197 friend class Promise<T>;
201 Make a completed Future by moving in a value. e.g.
204 auto f = makeFuture(std::move(foo));
208 auto f = makeFuture<string>("foo");
211 Future<typename std::decay<T>::type> makeFuture(T&& t);
213 /** Make a completed void Future. */
214 Future<void> makeFuture();
216 /** Make a completed Future by executing a function. If the function throws
217 we capture the exception, otherwise we capture the result. */
221 typename std::enable_if<
222 !std::is_reference<F>::value, bool>::type sdf = false)
223 -> Future<decltype(func())>;
228 -> Future<decltype(func())>;
230 /// Make a failed Future from an exception_ptr.
231 /// Because the Future's type cannot be inferred you have to specify it, e.g.
233 /// auto f = makeFuture<string>(std::current_exception());
235 Future<T> makeFuture(std::exception_ptr const& e);
237 /** Make a Future from an exception type E that can be passed to
238 std::make_exception_ptr(). */
239 template <class T, class E>
240 typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::type
241 makeFuture(E const& e);
243 /** Make a Future out of a Try */
245 Future<T> makeFuture(Try<T>&& t);
247 /** When all the input Futures complete, the returned Future will complete.
248 Errors do not cause early termination; this Future will always succeed
249 after all its Futures have finished (whether successfully or with an
252 The Futures are moved in, so your copies are invalid. If you need to
253 chain further from these Futures, use the variant with an output iterator.
255 XXX is this still true?
256 This function is thread-safe for Futures running on different threads.
258 The return type for Future<T> input is a Future<std::vector<Try<T>>>
260 template <class InputIterator>
261 Future<std::vector<Try<
262 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
263 whenAll(InputIterator first, InputIterator last);
265 /// This version takes a varying number of Futures instead of an iterator.
266 /// The return type for (Future<T1>, Future<T2>, ...) input
267 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
268 /// The Futures are moved in, so your copies are invalid.
269 template <typename... Fs>
270 typename detail::VariadicContext<
271 typename std::decay<Fs>::type::value_type...>::type
274 /** The result is a pair of the index of the first Future to complete and
275 the Try. If multiple Futures complete at the same time (or are already
276 complete when passed in), the "winner" is chosen non-deterministically.
278 This function is thread-safe for Futures running on different threads.
280 template <class InputIterator>
283 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
284 whenAny(InputIterator first, InputIterator last);
286 /** when n Futures have completed, the Future completes with a vector of
287 the index and Try of those n Futures (the indices refer to the original
288 order, but the result vector will be in an arbitrary order)
292 template <class InputIterator>
293 Future<std::vector<std::pair<
295 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
296 whenN(InputIterator first, InputIterator last, size_t n);
298 /** Wait for the given future to complete on a semaphore. Returns a completed
299 * future containing the result.
301 * NB if the promise for the future would be fulfilled in the same thread that
302 * you call this, it will deadlock.
305 Future<T> waitWithSemaphore(Future<T>&& f);
307 /** Wait for up to `timeout` for the given future to complete. Returns a future
308 * which may or may not be completed depending whether the given future
311 * Note: each call to this starts a (short-lived) thread and allocates memory.
313 template <typename T, class Duration>
314 Future<T> waitWithSemaphore(Future<T>&& f, Duration timeout);
318 #include "Future-inl.h"