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include/asio/any_completion_handler.hpp

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+//
+// any_completion_handler.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_ANY_COMPLETION_HANDLER_HPP
+#define ASIO_ANY_COMPLETION_HANDLER_HPP
+
+#include "asio/detail/config.hpp"
+#include <cstring>
+#include <functional>
+#include <memory>
+#include <utility>
+#include "asio/any_completion_executor.hpp"
+#include "asio/any_io_executor.hpp"
+#include "asio/associated_allocator.hpp"
+#include "asio/associated_cancellation_slot.hpp"
+#include "asio/associated_executor.hpp"
+#include "asio/associated_immediate_executor.hpp"
+#include "asio/cancellation_state.hpp"
+#include "asio/recycling_allocator.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+class any_completion_handler_impl_base
+{
+public:
+  template <typename S>
+  explicit any_completion_handler_impl_base(S&& slot)
+    : cancel_state_(static_cast<S&&>(slot), enable_total_cancellation())
+  {
+  }
+
+  cancellation_slot get_cancellation_slot() const noexcept
+  {
+    return cancel_state_.slot();
+  }
+
+private:
+  cancellation_state cancel_state_;
+};
+
+template <typename Handler>
+class any_completion_handler_impl :
+  public any_completion_handler_impl_base
+{
+public:
+  template <typename S, typename H>
+  any_completion_handler_impl(S&& slot, H&& h)
+    : any_completion_handler_impl_base(static_cast<S&&>(slot)),
+      handler_(static_cast<H&&>(h))
+  {
+  }
+
+  struct uninit_deleter
+  {
+    typename std::allocator_traits<
+      associated_allocator_t<Handler,
+        asio::recycling_allocator<void>>>::template
+          rebind_alloc<any_completion_handler_impl> alloc;
+
+    void operator()(any_completion_handler_impl* ptr)
+    {
+      std::allocator_traits<decltype(alloc)>::deallocate(alloc, ptr, 1);
+    }
+  };
+
+  struct deleter
+  {
+    typename std::allocator_traits<
+      associated_allocator_t<Handler,
+        asio::recycling_allocator<void>>>::template
+          rebind_alloc<any_completion_handler_impl> alloc;
+
+    void operator()(any_completion_handler_impl* ptr)
+    {
+      std::allocator_traits<decltype(alloc)>::destroy(alloc, ptr);
+      std::allocator_traits<decltype(alloc)>::deallocate(alloc, ptr, 1);
+    }
+  };
+
+  template <typename S, typename H>
+  static any_completion_handler_impl* create(S&& slot, H&& h)
+  {
+    uninit_deleter d{
+        (get_associated_allocator)(h,
+          asio::recycling_allocator<void>())};
+
+    std::unique_ptr<any_completion_handler_impl, uninit_deleter> uninit_ptr(
+        std::allocator_traits<decltype(d.alloc)>::allocate(d.alloc, 1), d);
+
+    any_completion_handler_impl* ptr =
+      new (uninit_ptr.get()) any_completion_handler_impl(
+        static_cast<S&&>(slot), static_cast<H&&>(h));
+
+    uninit_ptr.release();
+    return ptr;
+  }
+
+  void destroy()
+  {
+    deleter d{
+        (get_associated_allocator)(handler_,
+          asio::recycling_allocator<void>())};
+
+    d(this);
+  }
+
+  any_completion_executor executor(
+      const any_completion_executor& candidate) const noexcept
+  {
+    return any_completion_executor(std::nothrow,
+        (get_associated_executor)(handler_, candidate));
+  }
+
+  any_completion_executor immediate_executor(
+      const any_io_executor& candidate) const noexcept
+  {
+    return any_completion_executor(std::nothrow,
+        (get_associated_immediate_executor)(handler_, candidate));
+  }
+
+  void* allocate(std::size_t size, std::size_t align_size) const
+  {
+    typename std::allocator_traits<
+      associated_allocator_t<Handler,
+        asio::recycling_allocator<void>>>::template
+          rebind_alloc<unsigned char> alloc(
+            (get_associated_allocator)(handler_,
+              asio::recycling_allocator<void>()));
+
+    std::size_t space = size + align_size - 1;
+    unsigned char* base =
+      std::allocator_traits<decltype(alloc)>::allocate(
+        alloc, space + sizeof(std::ptrdiff_t));
+
+    void* p = base;
+    if (detail::align(align_size, size, p, space))
+    {
+      std::ptrdiff_t off = static_cast<unsigned char*>(p) - base;
+      std::memcpy(static_cast<unsigned char*>(p) + size, &off, sizeof(off));
+      return p;
+    }
+
+    std::bad_alloc ex;
+    asio::detail::throw_exception(ex);
+    return nullptr;
+  }
+
+  void deallocate(void* p, std::size_t size, std::size_t align) const
+  {
+    if (p)
+    {
+      typename std::allocator_traits<
+        associated_allocator_t<Handler,
+          asio::recycling_allocator<void>>>::template
+            rebind_alloc<unsigned char> alloc(
+              (get_associated_allocator)(handler_,
+                asio::recycling_allocator<void>()));
+
+      std::ptrdiff_t off;
+      std::memcpy(&off, static_cast<unsigned char*>(p) + size, sizeof(off));
+      unsigned char* base = static_cast<unsigned char*>(p) - off;
+
+      std::allocator_traits<decltype(alloc)>::deallocate(
+          alloc, base, size + align -1 + sizeof(std::ptrdiff_t));
+    }
+  }
+
+  template <typename... Args>
+  void call(Args&&... args)
+  {
+    deleter d{
+        (get_associated_allocator)(handler_,
+          asio::recycling_allocator<void>())};
+
+    std::unique_ptr<any_completion_handler_impl, deleter> ptr(this, d);
+    Handler handler(static_cast<Handler&&>(handler_));
+    ptr.reset();
+
+    static_cast<Handler&&>(handler)(
+        static_cast<Args&&>(args)...);
+  }
+
+private:
+  Handler handler_;
+};
+
+template <typename Signature>
+class any_completion_handler_call_fn;
+
+template <typename R, typename... Args>
+class any_completion_handler_call_fn<R(Args...)>
+{
+public:
+  using type = void(*)(any_completion_handler_impl_base*, Args...);
+
+  constexpr any_completion_handler_call_fn(type fn)
+    : call_fn_(fn)
+  {
+  }
+
+  void call(any_completion_handler_impl_base* impl, Args... args) const
+  {
+    call_fn_(impl, static_cast<Args&&>(args)...);
+  }
+
+  template <typename Handler>
+  static void impl(any_completion_handler_impl_base* impl, Args... args)
+  {
+    static_cast<any_completion_handler_impl<Handler>*>(impl)->call(
+        static_cast<Args&&>(args)...);
+  }
+
+private:
+  type call_fn_;
+};
+
+template <typename... Signatures>
+class any_completion_handler_call_fns;
+
+template <typename Signature>
+class any_completion_handler_call_fns<Signature> :
+  public any_completion_handler_call_fn<Signature>
+{
+public:
+  using any_completion_handler_call_fn<
+    Signature>::any_completion_handler_call_fn;
+  using any_completion_handler_call_fn<Signature>::call;
+};
+
+template <typename Signature, typename... Signatures>
+class any_completion_handler_call_fns<Signature, Signatures...> :
+  public any_completion_handler_call_fn<Signature>,
+  public any_completion_handler_call_fns<Signatures...>
+{
+public:
+  template <typename CallFn, typename... CallFns>
+  constexpr any_completion_handler_call_fns(CallFn fn, CallFns... fns)
+    : any_completion_handler_call_fn<Signature>(fn),
+      any_completion_handler_call_fns<Signatures...>(fns...)
+  {
+  }
+
+  using any_completion_handler_call_fn<Signature>::call;
+  using any_completion_handler_call_fns<Signatures...>::call;
+};
+
+class any_completion_handler_destroy_fn
+{
+public:
+  using type = void(*)(any_completion_handler_impl_base*);
+
+  constexpr any_completion_handler_destroy_fn(type fn)
+    : destroy_fn_(fn)
+  {
+  }
+
+  void destroy(any_completion_handler_impl_base* impl) const
+  {
+    destroy_fn_(impl);
+  }
+
+  template <typename Handler>
+  static void impl(any_completion_handler_impl_base* impl)
+  {
+    static_cast<any_completion_handler_impl<Handler>*>(impl)->destroy();
+  }
+
+private:
+  type destroy_fn_;
+};
+
+class any_completion_handler_executor_fn
+{
+public:
+  using type = any_completion_executor(*)(
+      any_completion_handler_impl_base*, const any_completion_executor&);
+
+  constexpr any_completion_handler_executor_fn(type fn)
+    : executor_fn_(fn)
+  {
+  }
+
+  any_completion_executor executor(any_completion_handler_impl_base* impl,
+      const any_completion_executor& candidate) const
+  {
+    return executor_fn_(impl, candidate);
+  }
+
+  template <typename Handler>
+  static any_completion_executor impl(any_completion_handler_impl_base* impl,
+      const any_completion_executor& candidate)
+  {
+    return static_cast<any_completion_handler_impl<Handler>*>(impl)->executor(
+        candidate);
+  }
+
+private:
+  type executor_fn_;
+};
+
+class any_completion_handler_immediate_executor_fn
+{
+public:
+  using type = any_completion_executor(*)(
+      any_completion_handler_impl_base*, const any_io_executor&);
+
+  constexpr any_completion_handler_immediate_executor_fn(type fn)
+    : immediate_executor_fn_(fn)
+  {
+  }
+
+  any_completion_executor immediate_executor(
+      any_completion_handler_impl_base* impl,
+      const any_io_executor& candidate) const
+  {
+    return immediate_executor_fn_(impl, candidate);
+  }
+
+  template <typename Handler>
+  static any_completion_executor impl(any_completion_handler_impl_base* impl,
+      const any_io_executor& candidate)
+  {
+    return static_cast<any_completion_handler_impl<Handler>*>(
+        impl)->immediate_executor(candidate);
+  }
+
+private:
+  type immediate_executor_fn_;
+};
+
+class any_completion_handler_allocate_fn
+{
+public:
+  using type = void*(*)(any_completion_handler_impl_base*,
+      std::size_t, std::size_t);
+
+  constexpr any_completion_handler_allocate_fn(type fn)
+    : allocate_fn_(fn)
+  {
+  }
+
+  void* allocate(any_completion_handler_impl_base* impl,
+      std::size_t size, std::size_t align) const
+  {
+    return allocate_fn_(impl, size, align);
+  }
+
+  template <typename Handler>
+  static void* impl(any_completion_handler_impl_base* impl,
+      std::size_t size, std::size_t align)
+  {
+    return static_cast<any_completion_handler_impl<Handler>*>(impl)->allocate(
+        size, align);
+  }
+
+private:
+  type allocate_fn_;
+};
+
+class any_completion_handler_deallocate_fn
+{
+public:
+  using type = void(*)(any_completion_handler_impl_base*,
+      void*, std::size_t, std::size_t);
+
+  constexpr any_completion_handler_deallocate_fn(type fn)
+    : deallocate_fn_(fn)
+  {
+  }
+
+  void deallocate(any_completion_handler_impl_base* impl,
+      void* p, std::size_t size, std::size_t align) const
+  {
+    deallocate_fn_(impl, p, size, align);
+  }
+
+  template <typename Handler>
+  static void impl(any_completion_handler_impl_base* impl,
+      void* p, std::size_t size, std::size_t align)
+  {
+    static_cast<any_completion_handler_impl<Handler>*>(impl)->deallocate(
+        p, size, align);
+  }
+
+private:
+  type deallocate_fn_;
+};
+
+template <typename... Signatures>
+class any_completion_handler_fn_table
+  : private any_completion_handler_destroy_fn,
+    private any_completion_handler_executor_fn,
+    private any_completion_handler_immediate_executor_fn,
+    private any_completion_handler_allocate_fn,
+    private any_completion_handler_deallocate_fn,
+    private any_completion_handler_call_fns<Signatures...>
+{
+public:
+  template <typename... CallFns>
+  constexpr any_completion_handler_fn_table(
+      any_completion_handler_destroy_fn::type destroy_fn,
+      any_completion_handler_executor_fn::type executor_fn,
+      any_completion_handler_immediate_executor_fn::type immediate_executor_fn,
+      any_completion_handler_allocate_fn::type allocate_fn,
+      any_completion_handler_deallocate_fn::type deallocate_fn,
+      CallFns... call_fns)
+    : any_completion_handler_destroy_fn(destroy_fn),
+      any_completion_handler_executor_fn(executor_fn),
+      any_completion_handler_immediate_executor_fn(immediate_executor_fn),
+      any_completion_handler_allocate_fn(allocate_fn),
+      any_completion_handler_deallocate_fn(deallocate_fn),
+      any_completion_handler_call_fns<Signatures...>(call_fns...)
+  {
+  }
+
+  using any_completion_handler_destroy_fn::destroy;
+  using any_completion_handler_executor_fn::executor;
+  using any_completion_handler_immediate_executor_fn::immediate_executor;
+  using any_completion_handler_allocate_fn::allocate;
+  using any_completion_handler_deallocate_fn::deallocate;
+  using any_completion_handler_call_fns<Signatures...>::call;
+};
+
+template <typename Handler, typename... Signatures>
+struct any_completion_handler_fn_table_instance
+{
+  static constexpr any_completion_handler_fn_table<Signatures...>
+    value = any_completion_handler_fn_table<Signatures...>(
+        &any_completion_handler_destroy_fn::impl<Handler>,
+        &any_completion_handler_executor_fn::impl<Handler>,
+        &any_completion_handler_immediate_executor_fn::impl<Handler>,
+        &any_completion_handler_allocate_fn::impl<Handler>,
+        &any_completion_handler_deallocate_fn::impl<Handler>,
+        &any_completion_handler_call_fn<Signatures>::template impl<Handler>...);
+};
+
+template <typename Handler, typename... Signatures>
+constexpr any_completion_handler_fn_table<Signatures...>
+any_completion_handler_fn_table_instance<Handler, Signatures...>::value;
+
+} // namespace detail
+
+template <typename... Signatures>
+class any_completion_handler;
+
+/// An allocator type that forwards memory allocation operations through an
+/// instance of @c any_completion_handler.
+template <typename T, typename... Signatures>
+class any_completion_handler_allocator
+{
+private:
+  template <typename...>
+  friend class any_completion_handler;
+
+  template <typename, typename...>
+  friend class any_completion_handler_allocator;
+
+  const detail::any_completion_handler_fn_table<Signatures...>* fn_table_;
+  detail::any_completion_handler_impl_base* impl_;
+
+  constexpr any_completion_handler_allocator(int,
+      const any_completion_handler<Signatures...>& h) noexcept
+    : fn_table_(h.fn_table_),
+      impl_(h.impl_)
+  {
+  }
+
+public:
+  /// The type of objects that may be allocated by the allocator.
+  typedef T value_type;
+
+  /// Rebinds an allocator to another value type.
+  template <typename U>
+  struct rebind
+  {
+    /// Specifies the type of the rebound allocator.
+    typedef any_completion_handler_allocator<U, Signatures...> other;
+  };
+
+  /// Construct from another @c any_completion_handler_allocator.
+  template <typename U>
+  constexpr any_completion_handler_allocator(
+      const any_completion_handler_allocator<U, Signatures...>& a)
+    noexcept
+    : fn_table_(a.fn_table_),
+      impl_(a.impl_)
+  {
+  }
+
+  /// Equality operator.
+  constexpr bool operator==(
+      const any_completion_handler_allocator& other) const noexcept
+  {
+    return fn_table_ == other.fn_table_ && impl_ == other.impl_;
+  }
+
+  /// Inequality operator.
+  constexpr bool operator!=(
+      const any_completion_handler_allocator& other) const noexcept
+  {
+    return fn_table_ != other.fn_table_ || impl_ != other.impl_;
+  }
+
+  /// Allocate space for @c n objects of the allocator's value type.
+  T* allocate(std::size_t n) const
+  {
+    if (fn_table_)
+    {
+      return static_cast<T*>(
+          fn_table_->allocate(
+            impl_, sizeof(T) * n, alignof(T)));
+    }
+    std::bad_alloc ex;
+    asio::detail::throw_exception(ex);
+    return nullptr;
+  }
+
+  /// Deallocate space for @c n objects of the allocator's value type.
+  void deallocate(T* p, std::size_t n) const
+  {
+    fn_table_->deallocate(impl_, p, sizeof(T) * n, alignof(T));
+  }
+};
+
+/// A protoco-allocator type that may be rebound to obtain an allocator that
+/// forwards memory allocation operations through an instance of
+/// @c any_completion_handler.
+template <typename... Signatures>
+class any_completion_handler_allocator<void, Signatures...>
+{
+private:
+  template <typename...>
+  friend class any_completion_handler;
+
+  template <typename, typename...>
+  friend class any_completion_handler_allocator;
+
+  const detail::any_completion_handler_fn_table<Signatures...>* fn_table_;
+  detail::any_completion_handler_impl_base* impl_;
+
+  constexpr any_completion_handler_allocator(int,
+      const any_completion_handler<Signatures...>& h) noexcept
+    : fn_table_(h.fn_table_),
+      impl_(h.impl_)
+  {
+  }
+
+public:
+  /// @c void as no objects can be allocated through a proto-allocator.
+  typedef void value_type;
+
+  /// Rebinds an allocator to another value type.
+  template <typename U>
+  struct rebind
+  {
+    /// Specifies the type of the rebound allocator.
+    typedef any_completion_handler_allocator<U, Signatures...> other;
+  };
+
+  /// Construct from another @c any_completion_handler_allocator.
+  template <typename U>
+  constexpr any_completion_handler_allocator(
+      const any_completion_handler_allocator<U, Signatures...>& a)
+    noexcept
+    : fn_table_(a.fn_table_),
+      impl_(a.impl_)
+  {
+  }
+
+  /// Equality operator.
+  constexpr bool operator==(
+      const any_completion_handler_allocator& other) const noexcept
+  {
+    return fn_table_ == other.fn_table_ && impl_ == other.impl_;
+  }
+
+  /// Inequality operator.
+  constexpr bool operator!=(
+      const any_completion_handler_allocator& other) const noexcept
+  {
+    return fn_table_ != other.fn_table_ || impl_ != other.impl_;
+  }
+};
+
+/// Polymorphic wrapper for completion handlers.
+/**
+ * The @c any_completion_handler class template is a polymorphic wrapper for
+ * completion handlers that propagates the associated executor, associated
+ * allocator, and associated cancellation slot through a type-erasing interface.
+ *
+ * When using @c any_completion_handler, specify one or more completion
+ * signatures as template parameters. These will dictate the arguments that may
+ * be passed to the handler through the polymorphic interface.
+ *
+ * Typical uses for @c any_completion_handler include:
+ *
+ * @li Separate compilation of asynchronous operation implementations.
+ *
+ * @li Enabling interoperability between asynchronous operations and virtual
+ *     functions.
+ */
+template <typename... Signatures>
+class any_completion_handler
+{
+#if !defined(GENERATING_DOCUMENTATION)
+private:
+  template <typename, typename...>
+  friend class any_completion_handler_allocator;
+
+  template <typename, typename>
+  friend struct associated_executor;
+
+  template <typename, typename>
+  friend struct associated_immediate_executor;
+
+  const detail::any_completion_handler_fn_table<Signatures...>* fn_table_;
+  detail::any_completion_handler_impl_base* impl_;
+#endif // !defined(GENERATING_DOCUMENTATION)
+
+public:
+  /// The associated allocator type.
+  using allocator_type = any_completion_handler_allocator<void, Signatures...>;
+
+  /// The associated cancellation slot type.
+  using cancellation_slot_type = cancellation_slot;
+
+  /// Construct an @c any_completion_handler in an empty state, without a target
+  /// object.
+  constexpr any_completion_handler()
+    : fn_table_(nullptr),
+      impl_(nullptr)
+  {
+  }
+
+  /// Construct an @c any_completion_handler in an empty state, without a target
+  /// object.
+  constexpr any_completion_handler(nullptr_t)
+    : fn_table_(nullptr),
+      impl_(nullptr)
+  {
+  }
+
+  /// Construct an @c any_completion_handler to contain the specified target.
+  template <typename H, typename Handler = decay_t<H>>
+  any_completion_handler(H&& h,
+      constraint_t<
+        !is_same<decay_t<H>, any_completion_handler>::value
+      > = 0)
+    : fn_table_(
+        &detail::any_completion_handler_fn_table_instance<
+          Handler, Signatures...>::value),
+      impl_(detail::any_completion_handler_impl<Handler>::create(
+            (get_associated_cancellation_slot)(h), static_cast<H&&>(h)))
+  {
+  }
+
+  /// Move-construct an @c any_completion_handler from another.
+  /**
+   * After the operation, the moved-from object @c other has no target.
+   */
+  any_completion_handler(any_completion_handler&& other) noexcept
+    : fn_table_(other.fn_table_),
+      impl_(other.impl_)
+  {
+    other.fn_table_ = nullptr;
+    other.impl_ = nullptr;
+  }
+
+  /// Move-assign an @c any_completion_handler from another.
+  /**
+   * After the operation, the moved-from object @c other has no target.
+   */
+  any_completion_handler& operator=(
+      any_completion_handler&& other) noexcept
+  {
+    any_completion_handler(
+        static_cast<any_completion_handler&&>(other)).swap(*this);
+    return *this;
+  }
+
+  /// Assignment operator that sets the polymorphic wrapper to the empty state.
+  any_completion_handler& operator=(nullptr_t) noexcept
+  {
+    any_completion_handler().swap(*this);
+    return *this;
+  }
+
+  /// Destructor.
+  ~any_completion_handler()
+  {
+    if (impl_)
+      fn_table_->destroy(impl_);
+  }
+
+  /// Test if the polymorphic wrapper is empty.
+  constexpr explicit operator bool() const noexcept
+  {
+    return impl_ != nullptr;
+  }
+
+  /// Test if the polymorphic wrapper is non-empty.
+  constexpr bool operator!() const noexcept
+  {
+    return impl_ == nullptr;
+  }
+
+  /// Swap the content of an @c any_completion_handler with another.
+  void swap(any_completion_handler& other) noexcept
+  {
+    std::swap(fn_table_, other.fn_table_);
+    std::swap(impl_, other.impl_);
+  }
+
+  /// Get the associated allocator.
+  allocator_type get_allocator() const noexcept
+  {
+    return allocator_type(0, *this);
+  }
+
+  /// Get the associated cancellation slot.
+  cancellation_slot_type get_cancellation_slot() const noexcept
+  {
+    return impl_ ? impl_->get_cancellation_slot() : cancellation_slot_type();
+  }
+
+  /// Function call operator.
+  /**
+   * Invokes target completion handler with the supplied arguments.
+   *
+   * This function may only be called once, as the target handler is moved from.
+   * The polymorphic wrapper is left in an empty state.
+   *
+   * Throws @c std::bad_function_call if the polymorphic wrapper is empty.
+   */
+  template <typename... Args>
+  auto operator()(Args&&... args)
+    -> decltype(fn_table_->call(impl_, static_cast<Args&&>(args)...))
+  {
+    if (detail::any_completion_handler_impl_base* impl = impl_)
+    {
+      impl_ = nullptr;
+      return fn_table_->call(impl, static_cast<Args&&>(args)...);
+    }
+    std::bad_function_call ex;
+    asio::detail::throw_exception(ex);
+  }
+
+  /// Equality operator.
+  friend constexpr bool operator==(
+      const any_completion_handler& a, nullptr_t) noexcept
+  {
+    return a.impl_ == nullptr;
+  }
+
+  /// Equality operator.
+  friend constexpr bool operator==(
+      nullptr_t, const any_completion_handler& b) noexcept
+  {
+    return nullptr == b.impl_;
+  }
+
+  /// Inequality operator.
+  friend constexpr bool operator!=(
+      const any_completion_handler& a, nullptr_t) noexcept
+  {
+    return a.impl_ != nullptr;
+  }
+
+  /// Inequality operator.
+  friend constexpr bool operator!=(
+      nullptr_t, const any_completion_handler& b) noexcept
+  {
+    return nullptr != b.impl_;
+  }
+};
+
+template <typename... Signatures, typename Candidate>
+struct associated_executor<any_completion_handler<Signatures...>, Candidate>
+{
+  using type = any_completion_executor;
+
+  static type get(const any_completion_handler<Signatures...>& handler,
+      const Candidate& candidate = Candidate()) noexcept
+  {
+    any_completion_executor any_candidate(std::nothrow, candidate);
+    return handler.fn_table_
+      ? handler.fn_table_->executor(handler.impl_, any_candidate)
+      : any_candidate;
+  }
+};
+
+template <typename... Signatures, typename Candidate>
+struct associated_immediate_executor<
+    any_completion_handler<Signatures...>, Candidate>
+{
+  using type = any_completion_executor;
+
+  static type get(const any_completion_handler<Signatures...>& handler,
+      const Candidate& candidate = Candidate()) noexcept
+  {
+    any_io_executor any_candidate(std::nothrow, candidate);
+    return handler.fn_table_
+      ? handler.fn_table_->immediate_executor(handler.impl_, any_candidate)
+      : any_candidate;
+  }
+};
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_ANY_COMPLETION_HANDLER_HPP