Add a new mailbox alternative

Atomic/DeadDrop.h

@@ -0,0 +1,91 @@
+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <cassert>
+
+#include <queue>
+
+#include <Alepha/Thread.h>
+
+namespace Alepha::Hydrogen::Atomic  ::detail::  DeadDrop_m
+{
+	inline namespace exports
+	{
+		template< typename Item >
+		class DeadDrop;
+	}
+
+	template< typename Item >
+	class exports::DeadDrop
+	{
+		private:
+			Alepha::mutex access;
+
+			struct Box
+			{
+				private:
+					Alepha::mutex *access= nullptr;
+					Alepha::condition_variable ready;
+
+					Item original;
+
+					Item *active= &original;
+					std::queue< Item * > next;
+
+					Box *companion= nullptr;
+
+					friend DeadDrop;
+
+					void
+					setup( Alepha::mutex *const access, Box *const companion )
+					{
+						this->access= access;
+						this->companion= companion;
+					}
+
+				public:
+					Item &
+					acquireBox()
+					{
+						if( not active )
+						{
+							[[unlikely]];
+							std::unique_lock lock{ *access };
+							ready.wait( lock, [&]{ return not next.empty(); } );
+							active= next.front();
+							next.pop();
+						}
+						assert( active );
+						return *active;
+					}
+
+					void
+					releaseBox()
+					{
+						std::unique_lock lock{ *access };
+						companion->next.push( active );
+						active= nullptr;
+						companion->ready.notify_one();
+					}
+			};
+
+		public:
+			Box producer;
+			Box consumer;
+
+			explicit
+			DeadDrop()
+			{
+				producer.setup( &this->access, &consumer );
+				consumer.setup( &this->access, &producer );
+			}
+	};
+}
+
+namespace Alepha::Hydrogen::Atomic::inline exports::inline DeadDrop_m
+{
+	using namespace detail::DeadDrop_m::exports;
+}