--- /dev/null
+/* Copyright (c) 2015-2021. The SimGrid Team.
+ * All rights reserved. */
+
+/* This program is free software; you can redistribute it and/or modify it
+ * under the terms of the license (GNU LGPL) which comes with this package. */
+
+#ifndef XBT_PROMISE_HPP
+#define XBT_PROMISE_HPP
+
+#include <cstddef>
+
+#include <boost/variant.hpp>
+#include <exception>
+#include <functional>
+#include <future> // std::future_error
+#include <stdexcept>
+#include <type_traits>
+#include <utility>
+#include <xbt/ex.h>
+
+namespace simgrid {
+namespace xbt {
+
+/** A value or an exception (or nothing)
+ *
+ * This is similar to `optional<expected<T>>`` but it with a Future/Promise
+ * like API.
+ *
+ * Also the name is not so great.
+ **/
+template <class T> class Result {
+public:
+ bool is_valid() const { return value_.which() > 0; }
+ void set_exception(std::exception_ptr e) { value_ = std::move(e); }
+ void set_value(T&& value) { value_ = std::move(value); }
+ void set_value(T const& value) { value_ = value; }
+
+ /** Extract the value from the future
+ *
+ * After this, the value is invalid.
+ **/
+ T get()
+ {
+ switch (value_.which()) {
+ case 1: {
+ T value = std::move(boost::get<T>(value_));
+ value_ = boost::blank();
+ return value;
+ }
+ case 2: {
+ std::exception_ptr exception = std::move(boost::get<std::exception_ptr>(value_));
+ value_ = boost::blank();
+ std::rethrow_exception(std::move(exception));
+ break;
+ }
+ default:
+ throw std::future_error(std::future_errc::no_state);
+ }
+ }
+
+private:
+ boost::variant<boost::blank, T, std::exception_ptr> value_;
+};
+
+template <> class Result<void> : public Result<std::nullptr_t> {
+public:
+ void set_value() { Result<std::nullptr_t>::set_value(nullptr); }
+ void get() { Result<std::nullptr_t>::get(); }
+};
+
+template <class T> class Result<T&> : public Result<std::reference_wrapper<T>> {
+public:
+ void set_value(T& value) { Result<std::reference_wrapper<T>>::set_value(std::ref(value)); }
+ T& get() { return Result<std::reference_wrapper<T>>::get(); }
+};
+
+/** Execute some code and set a promise or result accordingly
+ *
+ * Roughly this does:
+ *
+ * <pre>
+ * promise.set_value(code());
+ * </pre>
+ *
+ * but it takes care of exceptions and works with `void`.
+ *
+ * We might need this when working with generic code because
+ * the trivial implementation does not work with `void` (before C++1z).
+ *
+ * @param code What we want to do
+ * @param promise Where to want to store the result
+ */
+template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value(code()))
+{
+ try {
+ promise.set_value(std::forward<F>(code)());
+ } catch (...) {
+ promise.set_exception(std::current_exception());
+ }
+}
+
+template <class R, class F> auto fulfill_promise(R& promise, F&& code) -> decltype(promise.set_value())
+{
+ try {
+ std::forward<F>(code)();
+ promise.set_value();
+ } catch (...) {
+ promise.set_exception(std::current_exception());
+ }
+}
+
+/** Set a promise/result from a future/result
+ *
+ * Roughly this does:
+ *
+ * <pre>promise.set_value(future);</pre>
+ *
+ * but it takes care of exceptions and works with `void`.
+ *
+ * We might need this when working with generic code because
+ * the trivial implementation does not work with `void` (before C++1z).
+ *
+ * @param promise output (a valid future or a result)
+ * @param future input (a ready/waitable future or a valid result)
+ */
+template <class P, class F> inline void set_promise(P& promise, F&& future)
+{
+ fulfill_promise(promise, [&future] { return std::forward<F>(future).get(); });
+}
+}
+}
+
+#endif
