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. value() will not block
79 when this returns true. */
82 /** Wait until the result (or exception) is ready. Once this returns,
83 value() will not block, and isReady() will return true.
85 XXX This implementation is simplistic and inefficient, but it does work
86 and a fully intelligent implementation is coming down the pipe.
91 std::this_thread::yield();
97 /** When this Future has completed, execute func which is a function that
98 takes a Try<T>&&. A Future for the return type of func is
101 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
103 The functor given may call value() without blocking, which may rethrow if
104 this has captured an exception. If func throws, the exception will be
105 captured in the Future that is returned.
107 /* n3428 has then(scheduler&, F&&), we might want to reorganize to use
108 similar API. or maybe not */
110 typename std::enable_if<
111 !isFuture<typename std::result_of<F(Try<T>&&)>::type>::value,
112 Future<typename std::result_of<F(Try<T>&&)>::type> >::type
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::value_type> >::type
121 /** Use this method on the Future when we don't really care about the
122 returned value and want to convert the Future<T> to a Future<void>
128 void setContinuation(F&& func);
131 /* Eventually this may not be a shared_ptr, but something similar without
132 expensive thread-safety. */
133 typedef detail::FutureObject<T>* objPtr;
135 // shared state object
139 Future(objPtr obj) : obj_(obj) {}
141 void throwIfInvalid() const;
143 friend class Promise<T>;
146 /** Make a completed Future by moving in a value. e.g.
147 auto f = makeFuture(string("foo"));
150 Future<typename std::decay<T>::type> makeFuture(T&& t);
152 /** Make a completed void Future. */
153 Future<void> makeFuture();
155 /** Make a completed Future by executing a function. If the function throws
156 we capture the exception, otherwise we capture the result. */
160 typename std::enable_if<
161 !std::is_reference<F>::value, bool>::type sdf = false)
162 -> Future<decltype(func())>;
167 -> Future<decltype(func())>;
169 /** Make a completed (error) Future from an exception_ptr. Because the type
170 can't be inferred you have to give it, e.g.
172 auto f = makeFuture<string>(std::current_exception());
175 Future<T> makeFuture(std::exception_ptr const& e);
177 /** Make a Future from an exception type E that can be passed to
178 std::make_exception_ptr(). */
179 template <class T, class E>
180 typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::type
181 makeFuture(E const& e);
183 /** When all the input Futures complete, the returned Future will complete.
184 Errors do not cause early termination; this Future will always succeed
185 after all its Futures have finished (whether successfully or with an
188 The Futures are moved in, so your copies are invalid. If you need to
189 chain further from these Futures, use the variant with an output iterator.
191 This function is thread-safe for Futures running on different threads.
193 The return type for Future<T> input is a Future<vector<Try<T>>>
195 template <class InputIterator>
196 Future<std::vector<Try<
197 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
198 whenAll(InputIterator first, InputIterator last);
200 /** This version takes a varying number of Futures instead of an iterator.
201 The return type for (Future<T1>, Future<T2>, ...) input
202 is a Future<tuple<Try<T1>, Try<T2>, ...>>.
204 template <typename... Fs>
205 typename detail::VariadicContext<typename Fs::value_type...>::type
208 /** The result is a pair of the index of the first Future to complete and
209 the Try. If multiple Futures complete at the same time (or are already
210 complete when passed in), the "winner" is chosen non-deterministically.
212 This function is thread-safe for Futures running on different threads.
214 template <class InputIterator>
217 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
218 whenAny(InputIterator first, InputIterator last);
220 /** when n Futures have completed, the Future completes with a vector of
221 the index and Try of those n Futures (the indices refer to the original
222 order, but the result vector will be in an arbitrary order)
226 template <class InputIterator>
227 Future<std::vector<std::pair<
229 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
230 whenN(InputIterator first, InputIterator last, size_t n);
234 #include "Future-inl.h"
240 I haven't included a Future<T&> specialization because I don't forsee us
241 using it, however it is not difficult to add when needed. Refer to
242 Future<void> for guidance. std::Future and boost::Future code would also be
245 I think that this might be a good candidate for folly, once it has baked for