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include/asio/experimental/use_coro.hpp

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+//
+// experimental/use_coro.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_USE_CORO_HPP
+#define ASIO_EXPERIMENTAL_USE_CORO_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include <memory>
+#include "asio/deferred.hpp"
+#include "asio/detail/source_location.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+
+class any_io_executor;
+
+namespace experimental {
+
+/// A @ref completion_token that creates another coro for the task completion.
+/**
+ * The @c use_coro_t class, with its value @c use_coro, is used to represent an
+ * operation that can be awaited by the current resumable coroutine. This
+ * completion token may be passed as a handler to an asynchronous operation.
+ * For example:
+ *
+ * @code coro<void> my_coroutine(tcp::socket my_socket)
+ * {
+ *   std::size_t n = co_await my_socket.async_read_some(buffer, use_coro);
+ *   ...
+ * } @endcode
+ *
+ * When used with co_await, the initiating function (@c async_read_some in the
+ * above example) suspends the current coroutine. The coroutine is resumed when
+ * the asynchronous operation completes, and the result of the operation is
+ * returned.
+ *
+ * Note that this token is not the most efficient (use the default completion
+ * token @c asio::deferred for that) but does provide type erasure, as it
+ * will always return a @c coro.
+ */
+template <typename Allocator = std::allocator<void>>
+struct use_coro_t
+{
+
+  /// The allocator type. The allocator is used when constructing the
+  /// @c std::promise object for a given asynchronous operation.
+  typedef Allocator allocator_type;
+
+  /// Default constructor.
+  constexpr use_coro_t(
+      allocator_type allocator = allocator_type{}
+#if defined(ASIO_ENABLE_HANDLER_TRACKING)
+# if defined(ASIO_HAS_SOURCE_LOCATION)
+      , asio::detail::source_location location =
+        asio::detail::source_location::current()
+# endif // defined(ASIO_HAS_SOURCE_LOCATION)
+#endif // defined(ASIO_ENABLE_HANDLER_TRACKING)
+    )
+    : allocator_(allocator)
+#if defined(ASIO_ENABLE_HANDLER_TRACKING)
+# if defined(ASIO_HAS_SOURCE_LOCATION)
+    , file_name_(location.file_name()),
+      line_(location.line()),
+      function_name_(location.function_name())
+# else // defined(ASIO_HAS_SOURCE_LOCATION)
+    , file_name_(0),
+      line_(0),
+      function_name_(0)
+# endif // defined(ASIO_HAS_SOURCE_LOCATION)
+#endif // defined(ASIO_ENABLE_HANDLER_TRACKING)
+  {
+  }
+
+  /// Specify an alternate allocator.
+  template <typename OtherAllocator>
+  use_coro_t<OtherAllocator> rebind(const OtherAllocator& allocator) const
+  {
+    return use_future_t<OtherAllocator>(allocator);
+  }
+
+  /// Obtain allocator.
+  allocator_type get_allocator() const
+  {
+    return allocator_;
+  }
+
+  /// Constructor used to specify file name, line, and function name.
+  constexpr use_coro_t(const char* file_name,
+      int line, const char* function_name,
+      allocator_type allocator = allocator_type{}) :
+#if defined(ASIO_ENABLE_HANDLER_TRACKING)
+      file_name_(file_name),
+      line_(line),
+      function_name_(function_name),
+#endif // defined(ASIO_ENABLE_HANDLER_TRACKING)
+      allocator_(allocator)
+  {
+#if !defined(ASIO_ENABLE_HANDLER_TRACKING)
+    (void)file_name;
+    (void)line;
+    (void)function_name;
+#endif // !defined(ASIO_ENABLE_HANDLER_TRACKING)
+  }
+
+  /// Adapts an executor to add the @c use_coro_t completion token as the
+  /// default.
+  template <typename InnerExecutor>
+  struct executor_with_default : InnerExecutor
+  {
+    /// Specify @c use_coro_t as the default completion token type.
+    typedef use_coro_t default_completion_token_type;
+
+    /// Construct the adapted executor from the inner executor type.
+    template <typename InnerExecutor1>
+    executor_with_default(const InnerExecutor1& ex,
+        constraint_t<
+          conditional_t<
+            !is_same<InnerExecutor1, executor_with_default>::value,
+            is_convertible<InnerExecutor1, InnerExecutor>,
+            false_type
+          >::value
+        > = 0) noexcept
+      : InnerExecutor(ex)
+    {
+    }
+  };
+
+  /// Type alias to adapt an I/O object to use @c use_coro_t as its
+  /// default completion token type.
+  template <typename T>
+  using as_default_on_t = typename T::template rebind_executor<
+      executor_with_default<typename T::executor_type>>::other;
+
+  /// Function helper to adapt an I/O object to use @c use_coro_t as its
+  /// default completion token type.
+  template <typename T>
+  static typename decay_t<T>::template rebind_executor<
+      executor_with_default<typename decay_t<T>::executor_type>
+    >::other
+  as_default_on(T&& object)
+  {
+    return typename decay_t<T>::template rebind_executor<
+        executor_with_default<typename decay_t<T>::executor_type>
+      >::other(static_cast<T&&>(object));
+  }
+
+#if defined(ASIO_ENABLE_HANDLER_TRACKING)
+  const char* file_name_;
+  int line_;
+  const char* function_name_;
+#endif // defined(ASIO_ENABLE_HANDLER_TRACKING)
+
+private:
+  Allocator allocator_;
+};
+
+/// A @ref completion_token object that represents the currently executing
+/// resumable coroutine.
+/**
+ * See the documentation for asio::use_coro_t for a usage example.
+ */
+#if defined(GENERATING_DOCUMENTATION)
+ASIO_INLINE_VARIABLE constexpr use_coro_t<> use_coro;
+#else
+ASIO_INLINE_VARIABLE constexpr use_coro_t<> use_coro(0, 0, 0);
+#endif
+
+} // namespace experimental
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#include "asio/experimental/impl/use_coro.hpp"
+#include "asio/experimental/coro.hpp"
+
+#endif // ASIO_EXPERIMENTAL_USE_CORO_HPP