Recording how I _want_ it to work.

It looks like the move operation on allocators is actually a copy!

Memory/CMakeLists.txt

@@ -1 +1,2 @@
 add_subdirectory( Blob.test )
+add_subdirectory( ThreadSlab.test )

Memory/ThreadSlab.h

@@ -0,0 +1,151 @@
+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <string>
+
+#include <Alepha/Blob.h>
+
+namespace Alepha::Hydrogen::Memory  ::detail::  ThreadSlab_m
+{
+	inline namespace exports
+	{
+		template< typename T >
+		class ThreadSlab;
+
+		using ThreadSlabString= std::basic_string< char, std::char_traits< char >, ThreadSlab< char > >;
+	}
+
+	namespace C
+	{
+		const std::size_t slabSize= 64 * 1024 * 1024;
+	}
+
+	template< typename T >
+	class exports::ThreadSlab
+	{
+		private:
+			class Impl
+			{
+				public:
+					std::vector< Blob > leases;
+
+					inline static thread_local Blob slab;
+
+					Impl() { leases.reserve( 8 ); }
+			};
+
+			std::shared_ptr< Impl > pimpl;
+
+		public:
+			using value_type= T;
+			using propagate_on_container_copy_assignment= std::true_type;
+			using propagate_on_container_move_assignment= std::true_type;
+			using propagate_on_container_swap= std::true_type;
+			using is_always_equal= std::false_type;
+
+			ThreadSlab select_on_container_copy_construction() { auto rv= ThreadSlab{}; std::cerr << "selection creation at " << (void *) &rv << std::endl; return rv; }
+
+			ThreadSlab()
+				: pimpl( std::make_unique< Impl >() )
+			{
+				std::cerr << (void *) this << " owns " << (void *) pimpl.get() << " (creation)" << std::endl;
+			}
+
+			~ThreadSlab() { std::cerr << "Retired allocator external " << (void *) this << " which held " << (void *) pimpl.get() << std::endl; }
+
+			ThreadSlab &
+			operator= ( ThreadSlab &&other )
+			{
+				std::cerr << "Allocator move assign from " << (void *) &other << " which owns " << (void *) other.pimpl.get()
+						<< " to " << (void *) this << " which owns " << (void *) pimpl.get() << std::endl;
+
+				pimpl= std::move( other.pimpl );
+				other.pimpl.reset();
+				std::cerr << "After the move assign from " << (void *) &other << " it now owns " << (void *) other.pimpl.get()
+						<< " and I'm still " << (void *) this << " which now owns " << (void *) pimpl.get() << std::endl;
+
+				return *this;
+			}
+
+			ThreadSlab &
+			operator= ( const ThreadSlab &other )
+			{
+				std::cerr << "Allocator copy assign from " << (void *) &other << " which owns " << (void *) other.pimpl.get()
+						<< " to " << (void *) this << " which owns " << (void *) pimpl.get() << std::endl;
+
+				pimpl= other.pimpl;
+				std::cerr << "After the copy assign from " << (void *) &other << " it now owns " << (void *) other.pimpl.get()
+						<< " and I'm still " << (void *) this << " which now owns " << (void *) pimpl.get() << std::endl;
+
+				return *this;
+			}
+
+			ThreadSlab( const ThreadSlab &other )
+				: pimpl( other.pimpl )
+			{
+				std::cerr << "Allocator copy construction created " << (void *) this << " (from " << (void *) &other << ") which held " << (void *) pimpl.get() << std::endl;
+			}
+
+			ThreadSlab( ThreadSlab &&other )
+				: pimpl( std::move( other.pimpl ) )
+			{
+				std::cerr << "Allocator move construction created " << (void *) this << " (from " << (void *) &other << ") which held " << (void *) pimpl.get() << std::endl;
+				assert( pimpl );
+				std::cerr << "Allocator move construction still holds " << pimpl.get() << std::endl;
+			}
+
+			[[nodiscard]] T *
+			allocate( const std::size_t amt )
+			{
+				assert( pimpl );
+				std::cerr << (void *) this << " owns " << (void *) pimpl.get() << " (alloc)" << std::endl;
+				if( pimpl->slab.size() < amt ) pimpl->slab.reset( std::max( amt, C::slabSize ) );
+
+				auto next= pimpl->slab.carveHead( amt );
+				const auto rv= &next.template as< T >();
+				std::cerr << "Carved off " << amt << " bytes to address " << (void *) rv << " as allocator at " << (void *) pimpl.get() << std::endl;
+
+				pimpl->leases.push_back( std::move( next ) );
+
+				std::cerr << "The allocator at 0x" << (void *) pimpl.get() << " has " << pimpl->leases.size() << " leases. (alloc)" << std::endl;
+				assert( pimpl );
+
+				return rv;
+			}
+
+			void
+			deallocate( T *const p, const std::size_t /* ignored */ ) noexcept
+			{
+				assert( pimpl );
+				std::cerr << (void *) this << " owns " << (void *) pimpl.get() << " (dealloc)" << std::endl;
+				std::cerr << "Deallocation attempt of " << (void *) p << " by " << (void *) pimpl.get() << " when there are " << pimpl->leases.size() << " leases left." << std::endl;
+				auto found= std::find_if( begin( pimpl->leases ), end( pimpl->leases ), [p]( const auto &x ) { return x.data() == p; } );
+				if( found != end( pimpl->leases ) )
+				{
+					pimpl->leases.erase( found );
+					std::cerr << "The allocator at 0x" << (void *) pimpl.get() << " has " << pimpl->leases.size() << " leases. (dealloc)" << std::endl;
+				}
+				else
+				{
+					abort(); // Panic because it wasn't found in the list!
+				}
+				assert( pimpl );
+			}
+
+			friend constexpr bool
+			operator == ( const ThreadSlab &a, const ThreadSlab &b ) noexcept
+			{
+				assert( a.pimpl );
+				assert( b.pimpl );
+				return a.pimpl == b.pimpl;
+			}
+	};
+}
+
+namespace Alepha::Hydrogen::Memory::inline exports::inline ThreadSlab_m
+{
+	using namespace detail::ThreadSlab_m::exports;
+}

Memory/ThreadSlab.test/0.cc

@@ -0,0 +1,32 @@
+static_assert( __cplusplus > 2020'99 );
+
+#include "../ThreadSlab.h"
+
+#include <Alepha/Testing/test.h>
+
+#include <Alepha/Utility/evaluation_helpers.h>
+
+static auto init= Alepha::Utility::enroll <=[]
+{
+	using namespace Alepha::Testing::literals;
+
+	using namespace Alepha::Memory::exports::ThreadSlab_m;
+	using String= ThreadSlabString;
+
+	"Can we work with simple `ThreadSlabStrings` without errors?"_test <=[]
+	{
+		String s;
+		std::cerr << "s is empty" << std::endl;
+
+		String s2= "Hello World";
+		std::cerr << "small hello world string." << std::endl;
+
+		String s3= s2 + ": and bob";
+		std::cerr << "appended..." << std::endl;
+		s3= s3 + s3 + s2;
+
+		s2= std::move( s3 );
+
+		std::cout << s3 << std::endl;
+	};
+};

Memory/ThreadSlab.test/CMakeLists.txt

@@ -0,0 +1 @@
+unit_test( 0 )