mtp, mpi: Add docstings for the atomic counter

src/core/model/atomic-counter.h

@@ -1,4 +1,22 @@
-/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2023 State Key Laboratory for Novel Software Technology
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Songyuan Bai <i@f5soft.site>
+ */
+
 #ifndef ATOMIC_COUNTER_H
 #define ATOMIC_COUNTER_H
 
@@ -7,35 +25,69 @@
 namespace ns3
 {
 
+/**
+ * @brief
+ * The implementation of the atomic counter used for reference counting.
+ *
+ * It overrides operators of existing atomic variables with a more relaxed
+ * memory order to improve reference counting performance.
+ */
 class AtomicCounter
 {
   public:
+    /** Constructor */
     inline AtomicCounter()
     {
         m_count.store(0, std::memory_order_release);
     }
 
+    /**
+     * @brief Construct a new atomic counter object.
+     * 
+     * @param count The initialization count number
+     */
     inline AtomicCounter(uint32_t count)
     {
         m_count.store(count, std::memory_order_release);
     }
 
+    /**
+     * @brief Read the counter value with a more relaxed memory order.
+     * 
+     * @return The counter value.
+     */
     inline operator uint32_t() const
     {
         return m_count.load(std::memory_order_acquire);
     }
 
+    /**
+     * @brief Set the counter value with a more relaxed memory order.
+     * 
+     * @param count The counter value to be set
+     * @return The counter value to be set
+     */
     inline uint32_t operator=(const uint32_t count)
     {
         m_count.store(count, std::memory_order_release);
         return count;
     }
 
+    /**
+     * @brief Increment the counter by one with a more relaxed memory order.
+     * 
+     * @return The old counter value
+     */
     inline uint32_t operator++(int)
     {
         return m_count.fetch_add(1, std::memory_order_relaxed);
     }
 
+    /**
+     * @brief Decrement the counter by one with a more relaxed memory order.
+     * 
+     * @return The old counter value
+     */
     inline uint32_t operator--(int)
     {
         return m_count.fetch_sub(1, std::memory_order_release);