diff --git a/CMakeLists.txt b/CMakeLists.txt
index 253bfc5..68a857e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
 
-project(UECS VERSION 0.12.0)
+project(UECS VERSION 0.12.1)
 message(STATUS "[Project] ${PROJECT_NAME}")
 
 include(cmake/InitUCMake.cmake)
diff --git a/doc/todo.md b/doc/todo.md
index ba6eb64..b90cde9 100644
--- a/doc/todo.md
+++ b/doc/todo.md
@@ -28,6 +28,7 @@
 - [x] serialize (`IListner`)
 - [x] system base -> `System` 
 - [x] singleton
+- [ ] doxygen
 
 ### maybe support in future
 
diff --git a/include/UECS/CmptPtr.h b/include/UECS/CmptPtr.h
index 71b0af4..13c7068 100644
--- a/include/UECS/CmptPtr.h
+++ b/include/UECS/CmptPtr.h
@@ -8,9 +8,10 @@ namespace Ubpa::UECS {
 	// CmptType + void*
 	class CmptPtr {
 	public:
-		constexpr CmptPtr(CmptType type, void* p) noexcept : type{ type }, p{ p }{}
+		constexpr CmptPtr(CmptType type, void* p) noexcept : type{ type }, p{ p } {}
 		template<typename Cmpt>
-		constexpr CmptPtr(Cmpt* p) noexcept : type{ CmptType::Of<Cmpt> }, p{ p }{}
+		constexpr CmptPtr(Cmpt* p) noexcept : type{ CmptType::Of<Cmpt> }, p{ p } {}
+		constexpr CmptPtr() noexcept : CmptPtr{ Invalid() } {}
 
 		constexpr void* Ptr() const noexcept { return p; }
 
@@ -35,6 +36,7 @@ namespace Ubpa::UECS {
 		template<typename TaggedCmpt>
 		constexpr CmptAccessPtr(TaggedCmpt p) noexcept : accessType{ CmptAccessType::Of<TaggedCmpt> }, p{ CastToVoidPointer(p) } {}
 		explicit constexpr CmptAccessPtr(CmptPtr p) noexcept : CmptAccessPtr{ p, AccessMode::LATEST } {}
+		explicit constexpr CmptAccessPtr() noexcept : CmptAccessPtr{ Invalid() } {}
 
 		explicit constexpr operator CmptPtr() const noexcept { return { CmptType{accessType}, p }; }
 
diff --git a/include/UECS/CmptType.h b/include/UECS/CmptType.h
index e0b432b..471f274 100644
--- a/include/UECS/CmptType.h
+++ b/include/UECS/CmptType.h
@@ -13,6 +13,7 @@ namespace Ubpa::UECS {
 	public:
 		explicit constexpr CmptType(size_t id) noexcept : hashcode{ id } {}
 		explicit constexpr CmptType(std::string_view type_name) noexcept : hashcode{ RuntimeTypeID(type_name) } {}
+		explicit constexpr CmptType() noexcept : CmptType{ Invalid() } {}
 
 		template<typename Cmpt, std::enable_if_t<!IsTaggedCmpt_v<Cmpt>, int> = 0>
 		static constexpr CmptType Of = CmptType{ TypeID<Cmpt> };
@@ -45,6 +46,7 @@ namespace Ubpa::UECS {
 		constexpr CmptAccessType(CmptType type, AccessMode mode) noexcept
 			: type{ type }, mode{ mode } {}
 		explicit constexpr CmptAccessType(CmptType type) noexcept : CmptAccessType{ type, AccessMode::LATEST } {}
+		explicit constexpr CmptAccessType() noexcept : CmptAccessType{ Invalid() } {}
 
 		template<typename Cmpt, AccessMode mode = AccessMode::LATEST>
 		static constexpr CmptAccessType Of = CmptAccessType{ CmptType::Of<RemoveTag_t<Cmpt>>, AccessModeOf_default<Cmpt, mode> };
diff --git a/include/UECS/Entity.h b/include/UECS/Entity.h
index 6af3618..2a5ea70 100644
--- a/include/UECS/Entity.h
+++ b/include/UECS/Entity.h
@@ -6,20 +6,20 @@ namespace Ubpa::UECS {
 	// index + version
 	class Entity {
 	public:
-		size_t Idx() const noexcept { return idx; }
-		size_t Version() const noexcept { return version; }
+		constexpr Entity(size_t idx, size_t version) noexcept : idx(idx), version(version) {}
+		constexpr Entity() noexcept : Entity{ Invalid() } {}
+		constexpr size_t Idx() const noexcept { return idx; }
+		constexpr size_t Version() const noexcept { return version; }
 		static constexpr Entity Invalid() noexcept { return { size_t_invalid,size_t_invalid }; }
 		constexpr bool IsValid() const noexcept { return idx == size_t_invalid; }
-		bool operator==(const Entity& rhs) const noexcept {
+		constexpr bool operator==(const Entity& rhs) const noexcept {
 			return idx == rhs.idx && version == rhs.version;
 		}
-		bool operator<(const Entity& rhs) const noexcept {
+		constexpr bool operator<(const Entity& rhs) const noexcept {
 			return idx < rhs.idx || (idx == rhs.idx && version < rhs.version);
 		}
 	private:
 		friend class EntityMngr;
-		friend class Archetype;
-		constexpr Entity(size_t idx, size_t version) noexcept : idx(idx), version(version) {}
 		size_t idx;
 		size_t version;
 	};
diff --git a/include/UECS/EntityMngr.h b/include/UECS/EntityMngr.h
index 58af4d9..6b090a9 100644
--- a/include/UECS/EntityMngr.h
+++ b/include/UECS/EntityMngr.h
@@ -24,6 +24,8 @@ namespace Ubpa::UECS {
 	// [important]
 	// - some API with CmptType need RTDCmptTraits to get {size|alignment|lifecycle function} (throw std::logic_error)
 	// - API with Entity require Entity exist  (throw std::invalid_argument)
+	// [details]
+	// - when free entries is empty, use new entity entry (version is 0)
 	class EntityMngr {
 	public:
 		RTDCmptTraits cmptTraits;
@@ -64,7 +66,11 @@ namespace Ubpa::UECS {
 
 		void Destroy(Entity);
 
+		size_t TotalEntityNum() const noexcept { return entityTable.size() - entityTableFreeEntry.size(); }
 		size_t EntityNum(const EntityQuery&) const;
+		// use entry in reverse
+		const std::vector<size_t>& GetEntityFreeEntries() const noexcept { return entityTableFreeEntry; }
+		size_t GetEntityVersion(size_t idx) const noexcept { return entityTable.at(idx).version; }
 
 		std::tuple<bool, std::vector<CmptAccessPtr>> LocateSingletons(const SingletonLocator&) const;