Start to migrate delimiter code to common helper.

IOStreams/StreamState.h

@@ -0,0 +1,101 @@
+static_assert( __cplusplus > 2020'00 );
+
+#pragma once
+
+#include <ios>
+#include <exception>
+
+#include <Alepha/Alepha.h>
+
+namespace Alepha::Hydrogen::IOStreams  ::detail::  stream_state
+{
+	inline namespace exports
+	{
+		template< typename Tag, typename Type >
+		class StreamState;
+	}
+
+	template< typename Tag, typename Type >
+	class exports::StreamState
+	{
+		private:
+			static auto
+			index()
+			{
+				static const auto rv= std::ios::xalloc();
+				return rv;
+			}
+
+			static Type *&
+			get_ptr( std::ios_base &ios )
+			{
+				return reinterpret_cast< Type *& >( ios.pword( index() ) );
+			}
+
+			static void
+			destroy( std::ios_base &ios )
+			{
+				delete get_ptr( ios );
+				get_ptr( ios )= nullptr;
+			}
+
+			static void
+			callback_impl( const std::ios_base::event event, std::ios_base &ios, const int idx )
+			{
+				if( idx != index() ) throw std::logic_error( "Wrong index." );
+
+				if( event == std::ios_base::erase_event ) destroy( ios );
+				else if( event == std::ios_base::imbue_event )
+				{
+					// Nothing to do... until I develop locale support.
+				}
+				else if( event == std::ios_base::copyfmt_event )
+				{
+					get_ptr( ios )= new Type{ get( ios ) };
+				}
+			}
+
+			static void
+			callback( const std::ios_base::event event, std::ios_base &ios, const int idx ) noexcept
+			{
+				return callback_impl( event, ios, idx );
+			}
+
+			static void
+			init( std::ios_base &ios )
+			{
+				if( not ios.iword( index() ) )
+				{
+					ios.iword( index() )= 1;
+					ios.register_callback( callback, index() );
+				}
+				auto *&ptr= get_ptr( ios );
+				if( not ptr ) ptr= new Type{};
+			}
+
+		public:
+			static Type &
+			get( std::ios_base &ios )
+			{
+				init( ios );
+				return *get_ptr( ios );
+			}
+
+			struct Setter
+			{
+				const Type val;
+
+				friend std::ostream &
+				operator << ( std::ostream &os, const Setter &s )
+				{
+					StreamState::get( os )= s.val;
+					return os;
+				}
+			};
+	};
+}
+
+namespace Alepha::Hydrogen::IOStreams::inline exports::inline stream_state
+{
+	using namespace detail::stream_state::exports;
+}