add original source

source/listing_7.20.cpp

@@ -0,0 +1,33 @@
+template<typename T>
+class lock_free_queue
+{
+private:
+    struct node
+    {
+        std::atomic<T*> data;
+        std::atomic<node_counter> count;
+        std::atomic<counted_node_ptr> next;
+    };
+public:
+    std::unique_ptr<T> pop()
+    {
+        counted_node_ptr old_head=head.load(std::memory_order_relaxed);
+        for(;;)
+        {
+            increase_external_count(head,old_head);
+            node* const ptr=old_head.ptr;
+            if(ptr==tail.load().ptr)
+            {
+                return std::unique_ptr<T>();
+            }
+            counted_node_ptr next=ptr->next.load();
+            if(head.compare_exchange_strong(old_head,next))
+            {
+                T* const res=ptr->data.exchange(nullptr);
+                free_external_counter(old_head);
+                return std::unique_ptr<T>(res);
+            }
+            ptr->release_ref();
+        }
+    }
+};