Invariant gadget.

Invariant.h

@@ -0,0 +1,44 @@
+static_assert( __cplusplus > 2023'00 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <cassert>
+
+namespace Alepha::Hydrogen  ::detail::  Invariant_m
+{
+	inline namespace exports
+	{
+		template< typename T >
+		class Invariant
+		{
+			private:
+				const T *this_;
+
+				void
+				check() const
+				{
+					assert( this_->invariant() );
+				}
+
+			public:
+				~Invariant()
+				{
+					check();
+				}
+
+				explicit
+				Invariant( const T *const this_ )
+					: this_( this_ )
+				{
+					check();
+				}
+		};
+	}
+}
+
+namespace Alepha::Hydrogen::inline exports::inline Invariant_m
+{
+	using namespace detail::Invariant_m::exports;
+}