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include/asio/experimental/promise.hpp

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+//
+// experimental/promise.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2021-2023 Klemens D. Morgenstern
+//                         (klemens dot morgenstern at gmx dot net)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_EXPERIMENTAL_PROMISE_HPP
+#define ASIO_EXPERIMENTAL_PROMISE_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include "asio/detail/type_traits.hpp"
+#include "asio/any_io_executor.hpp"
+#include "asio/associated_cancellation_slot.hpp"
+#include "asio/associated_executor.hpp"
+#include "asio/bind_executor.hpp"
+#include "asio/cancellation_signal.hpp"
+#include "asio/dispatch.hpp"
+#include "asio/experimental/impl/promise.hpp"
+#include "asio/post.hpp"
+
+#include <algorithm>
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace experimental {
+
+template <typename T>
+struct is_promise : std::false_type {};
+
+template <typename ... Ts>
+struct is_promise<promise<Ts...>> : std::true_type {};
+
+template <typename T>
+constexpr bool is_promise_v = is_promise<T>::value;
+
+template <typename ... Ts>
+struct promise_value_type
+{
+  using type = std::tuple<Ts...>;
+};
+
+template <typename T>
+struct promise_value_type<T>
+{
+  using type = T;
+};
+
+template <>
+struct promise_value_type<>
+{
+  using type = std::tuple<>;
+};
+
+#if defined(GENERATING_DOCUMENTATION)
+/// A disposable handle for an eager operation.
+/**
+ * @tparam Signature The signature of the operation.
+ *
+ * @tparam Executor The executor to be used by the promise (taken from the
+ * operation).
+ *
+ * @tparam Allocator The allocator used for the promise. Can be set through
+ * use_allocator.
+ *
+ * A promise can be used to initiate an asynchronous option that can be
+ * completed later. If the promise gets destroyed before completion, the
+ * operation gets a cancel signal and the result is ignored.
+ *
+ * A promise fulfills the requirements of async_operation.
+ *
+ * @par Examples
+ * Reading and writing from one coroutine.
+ * @code
+ * awaitable<void> read_write_some(asio::ip::tcp::socket & sock,
+ *     asio::mutable_buffer read_buf, asio::const_buffer to_write)
+ * {
+ *   auto p = asio::async_read(read_buf,
+ *       asio::experimental::use_promise);
+ *   co_await asio::async_write_some(to_write);
+ *   co_await p;
+ * }
+ * @endcode
+ */
+template<typename Signature = void(),
+    typename Executor = asio::any_io_executor,
+    typename Allocator = std::allocator<void>>
+struct promise
+#else
+template <typename ... Ts, typename Executor, typename Allocator>
+struct promise<void(Ts...), Executor,  Allocator>
+#endif // defined(GENERATING_DOCUMENTATION)
+{
+  /// The value that's returned by the promise.
+  using value_type = typename promise_value_type<Ts...>::type;
+
+  /// Cancel the promise. Usually done through the destructor.
+  void cancel(cancellation_type level = cancellation_type::all)
+  {
+    if (impl_ && !impl_->done)
+    {
+      asio::dispatch(impl_->executor,
+          [level, impl = impl_]{ impl->cancel.emit(level); });
+    }
+  }
+
+  /// Check if the promise is completed already.
+  bool completed() const noexcept
+  {
+    return impl_ && impl_->done;
+  }
+
+  /// Wait for the promise to become ready.
+  template <ASIO_COMPLETION_TOKEN_FOR(void(Ts...)) CompletionToken>
+  inline ASIO_INITFN_AUTO_RESULT_TYPE(CompletionToken, void(Ts...))
+  operator()(CompletionToken&& token)
+  {
+    assert(impl_);
+
+    return async_initiate<CompletionToken, void(Ts...)>(
+        initiate_async_wait{impl_}, token);
+  }
+
+  promise() = delete;
+  promise(const promise& ) = delete;
+  promise(promise&& ) noexcept = default;
+
+  /// Destruct the promise and cancel the operation.
+  /**
+   * It is safe to destruct a promise of a promise that didn't complete.
+   */
+  ~promise() { cancel(); }
+
+private:
+#if !defined(GENERATING_DOCUMENTATION)
+  template <typename, typename, typename> friend struct promise;
+  friend struct detail::promise_handler<void(Ts...), Executor, Allocator>;
+#endif // !defined(GENERATING_DOCUMENTATION)
+
+  std::shared_ptr<detail::promise_impl<
+    void(Ts...), Executor, Allocator>> impl_;
+
+  promise(
+      std::shared_ptr<detail::promise_impl<
+        void(Ts...), Executor, Allocator>> impl)
+    : impl_(impl)
+  {
+  }
+
+  struct initiate_async_wait
+  {
+    std::shared_ptr<detail::promise_impl<
+      void(Ts...), Executor, Allocator>> self_;
+
+    template <typename WaitHandler>
+    void operator()(WaitHandler&& handler) const
+    {
+      const auto alloc = get_associated_allocator(
+          handler, self_->get_allocator());
+
+      auto cancel = get_associated_cancellation_slot(handler);
+
+      if (self_->done)
+      {
+        auto exec = asio::get_associated_executor(
+            handler, self_->get_executor());
+
+        asio::post(exec,
+            [self = std::move(self_),
+              handler = std::forward<WaitHandler>(handler)]() mutable
+            {
+              self->apply(std::move(handler));
+            });
+      }
+      else
+      {
+        if (cancel.is_connected())
+        {
+          struct cancel_handler
+          {
+            std::weak_ptr<detail::promise_impl<
+              void(Ts...), Executor, Allocator>> self;
+
+            cancel_handler(
+                std::weak_ptr<detail::promise_impl<
+                  void(Ts...), Executor, Allocator>> self)
+              : self(std::move(self))
+            {
+            }
+
+            void operator()(cancellation_type level) const
+            {
+              if (auto p = self.lock())
+              {
+                p->cancel.emit(level);
+                p->cancel_();
+              }
+            }
+          };
+          cancel.template emplace<cancel_handler>(self_);
+        }
+
+        self_->set_completion(alloc, std::forward<WaitHandler>(handler));
+      }
+    }
+  };
+};
+
+} // namespace experimental
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_EXPERIMENTAL_PROMISE_HPP