1 /* Copyright (c) 2015-2022. The SimGrid Team.
2 * All rights reserved. */
4 /* This program is free software; you can redistribute it and/or modify it
5 * under the terms of the license (GNU LGPL) which comes with this package. */
7 #ifndef XBT_PROMISE_HPP
8 #define XBT_PROMISE_HPP
12 #include <boost/variant.hpp>
15 #include <future> // std::future_error
17 #include <type_traits>
24 /** A value or an exception (or nothing)
26 * This is similar to `optional<expected<T>>`` but it with a Future/Promise
29 * Also the name is not so great.
31 template <class T> class Result {
33 bool is_valid() const { return value_.which() > 0; }
34 void set_exception(std::exception_ptr e) { value_ = std::move(e); }
35 void set_value(T&& value) { value_ = std::move(value); }
36 void set_value(T const& value) { value_ = value; }
38 /** Extract the value from the future
40 * After this, the value is invalid.
44 switch (value_.which()) {
46 T value = std::move(boost::get<T>(value_));
47 value_ = boost::blank();
51 std::exception_ptr exception = std::move(boost::get<std::exception_ptr>(value_));
52 value_ = boost::blank();
53 std::rethrow_exception(std::move(exception));
57 throw std::future_error(std::future_errc::no_state);
62 boost::variant<boost::blank, T, std::exception_ptr> value_;
65 template <> class Result<void> : public Result<std::nullptr_t> {
67 void set_value() { Result<std::nullptr_t>::set_value(nullptr); }
68 void get() { Result<std::nullptr_t>::get(); }
71 template <class T> class Result<T&> : public Result<std::reference_wrapper<T>> {
73 void set_value(T& value) { Result<std::reference_wrapper<T>>::set_value(std::ref(value)); }
74 T& get() { return Result<std::reference_wrapper<T>>::get(); }
77 /** Execute some code and set a promise or result accordingly
82 * promise.set_value(code());
85 * but it takes care of exceptions and works with `void`.
87 * We might need this when working with generic code because
88 * the trivial implementation does not work with `void` (before C++1z).
90 * @param code What we want to do
91 * @param promise Where to want to store the result
93 template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value(code()))
96 promise.set_value(std::forward<F>(code)());
98 promise.set_exception(std::current_exception());
102 template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value())
105 std::forward<F>(code)();
108 promise.set_exception(std::current_exception());
112 /** Set a promise/result from a future/result
116 * <pre>promise.set_value(future);</pre>
118 * but it takes care of exceptions and works with `void`.
120 * We might need this when working with generic code because
121 * the trivial implementation does not work with `void` (before C++1z).
123 * @param promise output (a valid future or a result)
124 * @param future input (a ready/waitable future or a valid result)
126 template <class P, class F> inline void set_promise(P& promise, F&& future)
128 fulfill_promise(promise, [&future] { return std::forward<F>(future).get(); });