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>
25 #include "folly/MoveWrapper.h"
29 namespace folly { namespace wangle {
31 template <typename T> struct isFuture;
39 Future(Future const&) = delete;
40 Future& operator=(Future const&) = delete;
44 Future& operator=(Future&&);
48 /** Return the reference to result. Should not be called if !isReady().
49 Will rethrow the exception if an exception has been
52 This function is not thread safe - the returned Future can only
53 be executed from the thread that the executor runs it in.
54 See below for a thread safe version
56 typename std::add_lvalue_reference<T>::type
58 typename std::add_lvalue_reference<const T>::type
61 template <typename Executor>
62 Future<T> executeWithSameThread(Executor* executor);
65 Thread-safe version of executeWith
67 Since an executor would likely start executing the Future chain
68 right away, it would be a race condition to call:
69 Future.executeWith(...).then(...), as there would be race
70 condition between the then and the running Future.
71 Instead, you may pass in a Promise so that we can set up
72 the rest of the chain in advance, without any racey
73 modifications of the continuation
75 template <typename Executor>
76 void executeWith(Executor* executor, Promise<T>&& cont_promise);
78 /** True when the result (or exception) is ready. */
81 /** A reference to the Try of the value */
84 /** When this Future has completed, execute func which is a function that
85 takes a Try<T>&&. A Future for the return type of func is
88 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
90 The Future given to the functor is ready, and the functor may call
91 value(), which may rethrow if this has captured an exception. If func
92 throws, the exception will be captured in the Future that is returned.
94 /* TODO n3428 and other async frameworks have something like then(scheduler,
95 Future), we probably want to support a similar API (instead of
98 typename std::enable_if<
99 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
100 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
103 /// Variant where func returns a future<T> instead of a T. e.g.
105 /// Future<string> f2 = f1.then(
106 /// [](Try<T>&&) { return makeFuture<string>("foo"); });
108 typename std::enable_if<
109 isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
110 Future<typename std::result_of<F(Try<T>&&)>::type::value_type> >::type
113 /// Convenience method for ignoring the value and creating a Future<void>.
114 /// Exceptions still propagate.
117 /// Use of this method is advanced wizardry.
118 /// XXX should this be protected?
120 void setContinuation(F&& func);
123 typedef detail::FutureObject<T>* objPtr;
125 // shared state object
129 Future(objPtr obj) : obj_(obj) {}
131 void throwIfInvalid() const;
133 friend class Promise<T>;
137 Make a completed Future by moving in a value. e.g.
140 auto f = makeFuture(std::move(foo));
144 auto f = makeFuture<string>("foo");
147 Future<typename std::decay<T>::type> makeFuture(T&& t);
149 /** Make a completed void Future. */
150 Future<void> makeFuture();
152 /** Make a completed Future by executing a function. If the function throws
153 we capture the exception, otherwise we capture the result. */
157 typename std::enable_if<
158 !std::is_reference<F>::value, bool>::type sdf = false)
159 -> Future<decltype(func())>;
164 -> Future<decltype(func())>;
166 /// Make a failed Future from an exception_ptr.
167 /// Because the Future's type cannot be inferred you have to specify it, e.g.
169 /// auto f = makeFuture<string>(std::current_exception());
171 Future<T> makeFuture(std::exception_ptr const& e);
173 /** Make a Future from an exception type E that can be passed to
174 std::make_exception_ptr(). */
175 template <class T, class E>
176 typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::type
177 makeFuture(E const& e);
179 /** When all the input Futures complete, the returned Future will complete.
180 Errors do not cause early termination; this Future will always succeed
181 after all its Futures have finished (whether successfully or with an
184 The Futures are moved in, so your copies are invalid. If you need to
185 chain further from these Futures, use the variant with an output iterator.
187 XXX is this still true?
188 This function is thread-safe for Futures running on different threads.
190 The return type for Future<T> input is a Future<std::vector<Try<T>>>
192 template <class InputIterator>
193 Future<std::vector<Try<
194 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
195 whenAll(InputIterator first, InputIterator last);
197 /// This version takes a varying number of Futures instead of an iterator.
198 /// The return type for (Future<T1>, Future<T2>, ...) input
199 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
200 /// The Futures are moved in, so your copies are invalid.
201 template <typename... Fs>
202 typename detail::VariadicContext<
203 typename std::decay<Fs>::type::value_type...>::type
206 /** The result is a pair of the index of the first Future to complete and
207 the Try. If multiple Futures complete at the same time (or are already
208 complete when passed in), the "winner" is chosen non-deterministically.
210 This function is thread-safe for Futures running on different threads.
212 template <class InputIterator>
215 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
216 whenAny(InputIterator first, InputIterator last);
218 /** when n Futures have completed, the Future completes with a vector of
219 the index and Try of those n Futures (the indices refer to the original
220 order, but the result vector will be in an arbitrary order)
224 template <class InputIterator>
225 Future<std::vector<std::pair<
227 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
228 whenN(InputIterator first, InputIterator last, size_t n);
232 #include "Future-inl.h"