1 /* Copyright (c) 2015-2023. The SimGrid Team. All rights reserved. */
3 /* This program is free software; you can redistribute it and/or modify it
4 * under the terms of the license (GNU LGPL) which comes with this package. */
6 #ifndef XBT_PROMISE_HPP
7 #define XBT_PROMISE_HPP
11 #include <boost/variant.hpp>
14 #include <future> // std::future_error
16 #include <type_traits>
20 namespace simgrid::xbt {
22 /** A value or an exception (or nothing)
24 * This is similar to `optional<expected<T>>`` but it with a Future/Promise
27 * Also the name is not so great.
29 template <class T> class Result {
31 bool is_valid() const { return value_.which() > 0; }
32 void set_exception(std::exception_ptr e) { value_ = std::move(e); }
33 void set_value(T&& value) { value_ = std::move(value); }
34 void set_value(T const& value) { value_ = value; }
36 /** Extract the value from the future
38 * After this, the value is invalid.
42 switch (value_.which()) {
44 T value = std::move(boost::get<T>(value_));
45 value_ = boost::blank();
49 std::exception_ptr exception = std::move(boost::get<std::exception_ptr>(value_));
50 value_ = boost::blank();
51 std::rethrow_exception(std::move(exception));
55 throw std::future_error(std::future_errc::no_state);
60 boost::variant<boost::blank, T, std::exception_ptr> value_;
63 template <> class Result<void> : public Result<std::nullptr_t> {
65 void set_value() { Result<std::nullptr_t>::set_value(nullptr); }
66 void get() { Result<std::nullptr_t>::get(); }
69 template <class T> class Result<T&> : public Result<std::reference_wrapper<T>> {
71 void set_value(T& value) { Result<std::reference_wrapper<T>>::set_value(std::ref(value)); }
72 T& get() { return Result<std::reference_wrapper<T>>::get(); }
75 /** Execute some code and set a promise or result accordingly
80 * promise.set_value(code());
83 * but it takes care of exceptions and works with `void`.
85 * We might need this when working with generic code because
86 * the trivial implementation does not work with `void` (before C++1z).
88 * @param code What we want to do
89 * @param promise Where to want to store the result
91 template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value(code()))
94 promise.set_value(std::forward<F>(code)());
96 promise.set_exception(std::current_exception());
100 template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value())
103 std::forward<F>(code)();
106 promise.set_exception(std::current_exception());
110 /** Set a promise/result from a future/result
114 * <pre>promise.set_value(future);</pre>
116 * but it takes care of exceptions and works with `void`.
118 * We might need this when working with generic code because
119 * the trivial implementation does not work with `void` (before C++1z).
121 * @param promise output (a valid future or a result)
122 * @param future input (a ready/waitable future or a valid result)
124 template <class P, class F> inline void set_promise(P& promise, F&& future)
126 fulfill_promise(promise, [&future] { return std::forward<F>(future).get(); });
128 } // namespace simgrid::xbt