Add a unique pointer dynamic cast utility.

Utility/derived_pointer_cast.h

@@ -0,0 +1,41 @@
+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <memory>
+
+namespace Alepha::Hydrogen::Utility  ::detail::  derived_pointer_cast_m
+{
+	inline namespace exports
+	{
+		template< typename Target, typename Source >
+		std::unique_ptr< Target > derived_pointer_cast( std::unique_ptr< Source > source );
+	}
+
+	template< typename Target, typename Source >
+	std::unique_ptr< Target >
+	exports::derived_pointer_cast( std::unique_ptr< Source > source )
+	{
+		if( Target *const conv= dynamic_cast< Target * >( source.get() ); not conv )
+		{
+			// TODO: More precise Alepha exception here.
+			throw std::bad_cast{};
+		}
+		else
+		{
+			// It's safe to release here, because we're taking ownership below.
+			source.release();
+
+			// We reuse the conversion pointer so we can avoid a double-trip thru the
+			// RTTI pathway.
+			return std::unique_ptr< Target >{ conv };
+		}
+	}
+}
+
+namespace Alepha::Hydrogen::Utility::inline exports::inline derived_pointer_cast_m
+{
+	using namespace detail::derived_pointer_cast_m::exports;
+}