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include/asio/detail/op_queue.hpp

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+//
+// detail/op_queue.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_DETAIL_OP_QUEUE_HPP
+#define ASIO_DETAIL_OP_QUEUE_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/noncopyable.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+template <typename Operation>
+class op_queue;
+
+class op_queue_access
+{
+public:
+  template <typename Operation>
+  static Operation* next(Operation* o)
+  {
+    return static_cast<Operation*>(o->next_);
+  }
+
+  template <typename Operation1, typename Operation2>
+  static void next(Operation1*& o1, Operation2* o2)
+  {
+    o1->next_ = o2;
+  }
+
+  template <typename Operation>
+  static void destroy(Operation* o)
+  {
+    o->destroy();
+  }
+
+  template <typename Operation>
+  static Operation*& front(op_queue<Operation>& q)
+  {
+    return q.front_;
+  }
+
+  template <typename Operation>
+  static Operation*& back(op_queue<Operation>& q)
+  {
+    return q.back_;
+  }
+};
+
+template <typename Operation>
+class op_queue
+  : private noncopyable
+{
+public:
+  // Constructor.
+  op_queue()
+    : front_(0),
+      back_(0)
+  {
+  }
+
+  // Destructor destroys all operations.
+  ~op_queue()
+  {
+    while (Operation* op = front_)
+    {
+      pop();
+      op_queue_access::destroy(op);
+    }
+  }
+
+  // Get the operation at the front of the queue.
+  Operation* front()
+  {
+    return front_;
+  }
+
+  // Pop an operation from the front of the queue.
+  void pop()
+  {
+    if (front_)
+    {
+      Operation* tmp = front_;
+      front_ = op_queue_access::next(front_);
+      if (front_ == 0)
+        back_ = 0;
+      op_queue_access::next(tmp, static_cast<Operation*>(0));
+    }
+  }
+
+  // Push an operation on to the back of the queue.
+  void push(Operation* h)
+  {
+    op_queue_access::next(h, static_cast<Operation*>(0));
+    if (back_)
+    {
+      op_queue_access::next(back_, h);
+      back_ = h;
+    }
+    else
+    {
+      front_ = back_ = h;
+    }
+  }
+
+  // Push all operations from another queue on to the back of the queue. The
+  // source queue may contain operations of a derived type.
+  template <typename OtherOperation>
+  void push(op_queue<OtherOperation>& q)
+  {
+    if (Operation* other_front = op_queue_access::front(q))
+    {
+      if (back_)
+        op_queue_access::next(back_, other_front);
+      else
+        front_ = other_front;
+      back_ = op_queue_access::back(q);
+      op_queue_access::front(q) = 0;
+      op_queue_access::back(q) = 0;
+    }
+  }
+
+  // Whether the queue is empty.
+  bool empty() const
+  {
+    return front_ == 0;
+  }
+
+  // Test whether an operation is already enqueued.
+  bool is_enqueued(Operation* o) const
+  {
+    return op_queue_access::next(o) != 0 || back_ == o;
+  }
+
+private:
+  friend class op_queue_access;
+
+  // The front of the queue.
+  Operation* front_;
+
+  // The back of the queue.
+  Operation* back_;
+};
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_DETAIL_OP_QUEUE_HPP