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include/asio/experimental/as_single.hpp

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+//
+// experimental/as_single.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2025 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// 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_AS_SINGLE_HPP
+#define ASIO_EXPERIMENTAL_AS_SINGLE_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/detail/push_options.hpp"
+
+namespace asio {
+namespace experimental {
+
+/// A @ref completion_token adapter used to specify that the completion handler
+/// arguments should be combined into a single argument.
+/**
+ * The as_single_t class is used to indicate that any arguments to the
+ * completion handler should be combined and passed as a single argument.
+ * If there is already one argument, that argument is passed as-is. If
+ * there is more than argument, the arguments are first moved into a
+ * @c std::tuple and that tuple is then passed to the completion handler.
+ */
+template <typename CompletionToken>
+class as_single_t
+{
+public:
+  /// Tag type used to prevent the "default" constructor from being used for
+  /// conversions.
+  struct default_constructor_tag {};
+
+  /// Default constructor.
+  /**
+   * This constructor is only valid if the underlying completion token is
+   * default constructible and move constructible. The underlying completion
+   * token is itself defaulted as an argument to allow it to capture a source
+   * location.
+   */
+  constexpr as_single_t(
+      default_constructor_tag = default_constructor_tag(),
+      CompletionToken token = CompletionToken())
+    : token_(static_cast<CompletionToken&&>(token))
+  {
+  }
+
+  /// Constructor.
+  template <typename T>
+  constexpr explicit as_single_t(
+      T&& completion_token)
+    : token_(static_cast<T&&>(completion_token))
+  {
+  }
+
+  /// Adapts an executor to add the @c as_single_t completion token as the
+  /// default.
+  template <typename InnerExecutor>
+  struct executor_with_default : InnerExecutor
+  {
+    /// Specify @c as_single_t as the default completion token type.
+    typedef as_single_t default_completion_token_type;
+
+    /// Construct the adapted executor from the inner executor type.
+    executor_with_default(const InnerExecutor& ex) noexcept
+      : InnerExecutor(ex)
+    {
+    }
+
+    /// Convert the specified executor to the inner executor type, then use
+    /// that to construct the adapted executor.
+    template <typename OtherExecutor>
+    executor_with_default(const OtherExecutor& ex,
+        constraint_t<
+          is_convertible<OtherExecutor, InnerExecutor>::value
+        > = 0) noexcept
+      : InnerExecutor(ex)
+    {
+    }
+  };
+
+  /// Type alias to adapt an I/O object to use @c as_single_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 as_single_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));
+  }
+
+//private:
+  CompletionToken token_;
+};
+
+/// Adapt a @ref completion_token to specify that the completion handler
+/// arguments should be combined into a single argument.
+template <typename CompletionToken>
+ASIO_NODISCARD inline
+constexpr as_single_t<decay_t<CompletionToken>>
+as_single(CompletionToken&& completion_token)
+{
+  return as_single_t<decay_t<CompletionToken>>(
+      static_cast<CompletionToken&&>(completion_token));
+}
+
+} // namespace experimental
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#include "asio/experimental/impl/as_single.hpp"
+
+#endif // ASIO_EXPERIMENTAL_AS_SINGLE_HPP