diff --git a/include/libconcur/data_model.h b/include/libconcur/data_model.h
index 5c5b5c4..514b26d 100644
--- a/include/libconcur/data_model.h
+++ b/include/libconcur/data_model.h
@@ -77,18 +77,15 @@ private:
   };
 
   data *init(index_t circ_size) noexcept {
-    if (!data_allocator_) {
-      return nullptr;
-    }
     void *data_ptr = nullptr;
     LIBIMP_TRY {
-      data_ptr = data_allocator_.alloc(data::size_of(circ_size));
+      data_ptr = data_allocator_.allocate(data::size_of(circ_size));
       if (data_ptr == nullptr) {
         return nullptr;
       }
       return ::LIBIMP::construct<data>(data_ptr, circ_size);
     } LIBIMP_CATCH(...) {
-      data_allocator_.dealloc(data_ptr, data::size_of(circ_size));
+      data_allocator_.deallocate(data_ptr, data::size_of(circ_size));
       return nullptr;
     }
   }
@@ -128,7 +125,7 @@ public:
     if (valid()) {
       auto sz = data_->byte_size();
       (void)::LIBIMP::destroy<data>(data_);
-      data_allocator_.dealloc(data_, sz);
+      data_allocator_.deallocate(data_, sz);
     }
   }
 
@@ -148,7 +145,7 @@ public:
     : data_model(default_circle_buffer_size) {}
 
   bool valid() const noexcept {
-    return (data_ != nullptr) && data_allocator_.valid();
+    return data_ != nullptr;
   }
 
   explicit operator bool() const noexcept {
diff --git a/include/libconcur/element.h b/include/libconcur/element.h
index ba08690..7c816ec 100644
--- a/include/libconcur/element.h
+++ b/include/libconcur/element.h
@@ -64,7 +64,7 @@ public:
   using value_type = T;
 
   template <typename... A>
-  element(A &&... args) 
+  element(A &&...args) 
     noexcept(noexcept(T{std::forward<A>(args)...}))
     : f_ct_{state::invalid_value}
     , data_{std::forward<A>(args)...} {}
diff --git a/include/libimp/byte.h b/include/libimp/byte.h
index da646be..982d173 100644
--- a/include/libimp/byte.h
+++ b/include/libimp/byte.h
@@ -44,8 +44,7 @@ class byte {
   std::uint8_t bits_;
 
 public:
-  constexpr byte() noexcept
-    : byte(0) {}
+  byte() noexcept = default;
 
   template <typename T, typename = detail::is_integral<T>>
   constexpr byte(T v) noexcept
diff --git a/include/libimp/construct.h b/include/libimp/construct.h
index 99af50c..851c657 100644
--- a/include/libimp/construct.h
+++ b/include/libimp/construct.h
@@ -23,7 +23,7 @@ LIBIMP_NAMESPACE_BEG_
 */
 
 template <typename T, typename... A>
-auto construct(void *p, A &&... args)
+auto construct(void *p, A &&...args)
   -> std::enable_if_t<::std::is_constructible<T, A...>::value, T *> {
 #if defined(LIBIMP_CPP_20)
   return std::construct_at(static_cast<T *>(p), std::forward<A>(args)...);
@@ -33,7 +33,7 @@ auto construct(void *p, A &&... args)
 }
 
 template <typename T, typename... A>
-auto construct(void *p, A &&... args)
+auto construct(void *p, A &&...args)
   -> std::enable_if_t<!::std::is_constructible<T, A...>::value, T *> {
   return ::new (p) T{std::forward<A>(args)...};
 }
diff --git a/include/libimp/log.h b/include/libimp/log.h
index 4231a40..bf6c1a1 100644
--- a/include/libimp/log.h
+++ b/include/libimp/log.h
@@ -122,7 +122,7 @@ public:
     , level_limit_(level_limit) {}
 
   template <typename... A>
-  grip const &operator()(log::level l, A &&... args) const noexcept {
+  grip const &operator()(log::level l, A &&...args) const noexcept {
     if (underlyof(l) < underlyof(level_limit_)) {
       return *this;
     }
diff --git a/include/libimp/pimpl.h b/include/libimp/pimpl.h
index 63992d0..df7787a 100644
--- a/include/libimp/pimpl.h
+++ b/include/libimp/pimpl.h
@@ -24,7 +24,7 @@ struct is_comfortable {
 };
 
 template <typename T, typename... A>
-auto make(A &&... args) -> std::enable_if_t<is_comfortable<T>::value, T *> {
+auto make(A &&...args) -> std::enable_if_t<is_comfortable<T>::value, T *> {
   T *buf {};
   // construct an object using memory of a pointer
   construct<T>(&buf, std::forward<A>(args)...);
@@ -42,7 +42,7 @@ auto clear(T *p) noexcept -> std::enable_if_t<is_comfortable<T>::value> {
 }
 
 template <typename T, typename... A>
-auto make(A &&... args) -> std::enable_if_t<!is_comfortable<T>::value, T *> {
+auto make(A &&...args) -> std::enable_if_t<!is_comfortable<T>::value, T *> {
   return new T{std::forward<A>(args)...};
 }
 
@@ -60,7 +60,7 @@ template <typename T>
 class Obj {
 public:
   template <typename... A>
-  static T *make(A &&... args) {
+  static T *make(A &&...args) {
     return pimpl::make<T>(std::forward<A>(args)...);
   }
 
diff --git a/include/libimp/result.h b/include/libimp/result.h
index aea6738..3960f88 100644
--- a/include/libimp/result.h
+++ b/include/libimp/result.h
@@ -142,7 +142,7 @@ public:
             typename = not_match<result, A...>,
             typename = decltype(type_traits_t::init_code(std::declval<storage_t &>()
                                                        , std::declval<A>()...))>
-  result(A &&... args) noexcept {
+  result(A &&...args) noexcept {
     type_traits_t::init_code(code_, std::forward<A>(args)...);
   }
 
@@ -171,7 +171,7 @@ public:
             typename = not_match<result, A...>,
             typename = decltype(type_traits_t::init_code(std::declval<storage_t &>()
                                                        , std::declval<A>()...))>
-  result(A &&... args) noexcept {
+  result(A &&...args) noexcept {
     type_traits_t::init_code(code_, std::forward<A>(args)...);
   }
 
diff --git a/include/libpmr/allocator.h b/include/libpmr/allocator.h
index fb61efe..d24bab2 100644
--- a/include/libpmr/allocator.h
+++ b/include/libpmr/allocator.h
@@ -32,63 +32,54 @@ LIBPMR_NAMESPACE_BEG_
  */
 class LIBIMP_EXPORT allocator {
 
-  class holder_base {
+  class holder_mr_base {
   public:
-    virtual ~holder_base() noexcept = default;
-    virtual void *alloc(std::size_t) = 0;
-    virtual void  dealloc(void *, std::size_t) = 0;
-    virtual bool  valid() const noexcept = 0;
+    virtual ~holder_mr_base() noexcept = default;
+    virtual void *alloc(std::size_t, std::size_t) const = 0;
+    virtual void  dealloc(void *, std::size_t, std::size_t) const = 0;
   };
 
-  class holder_null : public holder_base {
-  public:
-    void *alloc(std::size_t) override { return nullptr; }
-    void  dealloc(void *, std::size_t) override {}
-    bool  valid() const noexcept override { return false; }
-  };
-
-  template <typename MemRes, typename = bool>
-  class holder_memory_resource;
-
-  /**
-   * \brief A memory resource pointer holder class for type erasure.
-   * \tparam MR memory resource type
-   */
-  template <typename MR>
-  class holder_memory_resource<MR, verify_memory_resource<MR>> : public holder_base {
-    MR *p_mem_res_;
-
-  public:
-    holder_memory_resource(MR *p_mr) noexcept
-      : p_mem_res_(p_mr) {}
-
-    void *alloc(std::size_t s) override {
-      return p_mem_res_->allocate(s);
-    }
-
-    void dealloc(void *p, std::size_t s) override {
-      p_mem_res_->deallocate(p, s);
-    }
-  
-    bool valid() const noexcept override {
-      return p_mem_res_ != nullptr;
-    }
-  };
+  template <typename MR, typename = bool>
+  class holder_mr;
 
   /**
    * \brief An empty holding class used to calculate a reasonable memory size for the holder.
    * \tparam MR cannot be converted to the type of memory resource
    */
   template <typename MR, typename U>
-  class holder_memory_resource : public holder_null {
-    MR *p_dummy_;
+  class holder_mr : public holder_mr_base {
+  protected:
+    MR *res_;
+
+  public:
+    holder_mr(MR *p_mr) noexcept
+      : res_(p_mr) {}
   };
 
-  using void_holder_type = holder_memory_resource<void>;
-  alignas(void_holder_type) std::array<::LIBIMP::byte, sizeof(void_holder_type)> holder_;
+  /**
+   * \brief A memory resource pointer holder class for type erasure.
+   * \tparam MR memory resource type
+   */
+  template <typename MR>
+  class holder_mr<MR, verify_memory_resource<MR>> : public holder_mr<MR, void> {
+  public:
+    holder_mr(MR *p_mr) noexcept
+      : holder_mr<MR, void>{p_mr} {}
 
-  holder_base &      get_holder() noexcept;
-  holder_base const &get_holder() const noexcept;
+    void *alloc(std::size_t s, std::size_t a) const override {
+      return res_->allocate(s, a);
+    }
+
+    void dealloc(void *p, std::size_t s, std::size_t a) const override {
+      res_->deallocate(p, s, a);
+    }
+  };
+
+  using void_holder_t = holder_mr<void *>;
+  alignas(void_holder_t) std::array<::LIBIMP::byte, sizeof(void_holder_t)> holder_;
+
+  holder_mr_base &      get_holder() noexcept;
+  holder_mr_base const &get_holder() const noexcept;
 
 public:
   allocator() noexcept;
@@ -97,24 +88,25 @@ public:
   allocator(allocator const &other) noexcept = default;
   allocator &operator=(allocator const &other) & noexcept = default;
 
-  allocator(allocator &&other) noexcept;
-  allocator &operator=(allocator &&other) & noexcept;
+  allocator(allocator &&other) noexcept = default;
+  allocator &operator=(allocator &&other) & noexcept = default;
 
   /// \brief Constructs a allocator from a memory resource pointer
   /// The lifetime of the pointer must be longer than that of allocator.
   template <typename T, verify_memory_resource<T> = true>
-  allocator(T *p_mr) : allocator() {
-    if (p_mr == nullptr) return;
-    ::LIBIMP::construct<holder_memory_resource<T>>(holder_.data(), p_mr);
+  allocator(T *p_mr) noexcept {
+    if (p_mr == nullptr) {
+      ::LIBIMP::construct<holder_mr<new_delete_resource>>(holder_.data(), new_delete_resource::get());
+      return;
+    }
+    ::LIBIMP::construct<holder_mr<T>>(holder_.data(), p_mr);
   }
 
   void swap(allocator &other) noexcept;
-  bool valid() const noexcept;
-  explicit operator bool() const noexcept;
 
   /// \brief Allocate/deallocate memory.
-  void *alloc(std::size_t s);
-  void  dealloc(void *p, std::size_t s);
+  void *allocate(std::size_t s, std::size_t = alignof(std::max_align_t)) const;
+  void  deallocate(void *p, std::size_t s, std::size_t = alignof(std::max_align_t)) const;
 };
 
 LIBPMR_NAMESPACE_END_
diff --git a/include/libpmr/holder_base.h b/include/libpmr/holder_base.h
new file mode 100644
index 0000000..fc52c6d
--- /dev/null
+++ b/include/libpmr/holder_base.h
@@ -0,0 +1,122 @@
+/**
+ * \file libpmr/small_storage.h
+ * \author mutouyun (orz@orzz.org)
+ * \brief Unified SSO (Small Size Optimization) holder base.
+ * \date 2023-09-02
+ */
+#pragma once
+
+#include <type_traits>
+#include <array>
+#include <typeinfo>
+#include <utility>
+#include <cstddef>
+
+#include "libimp/export.h"
+#include "libimp/construct.h"
+#include "libimp/byte.h"
+#include "libimp/generic.h"
+
+#include "libpmr/def.h"
+
+LIBPMR_NAMESPACE_BEG_
+
+class LIBIMP_EXPORT allocator;
+
+/**
+ * \class holder_base
+ * \brief Data holder base class.
+*/
+class holder_base {
+public:
+  virtual ~holder_base() noexcept = default;
+  virtual bool valid() const noexcept = 0;
+  virtual std::type_info const &type() const noexcept = 0;
+  virtual std::size_t sizeof_type() const noexcept = 0;
+  virtual std::size_t count() const noexcept = 0;
+  virtual std::size_t size () const noexcept = 0;
+  virtual void *get() noexcept = 0;
+  virtual void const *get() const noexcept = 0;
+  virtual void move_to(allocator const &, void *) noexcept = 0;
+  virtual void copy_to(allocator const &, void *) const noexcept(false) = 0;
+};
+
+/**
+ * \class holder_null
+ * \brief A holder implementation that does not hold any data objects.
+*/
+class holder_null : public holder_base {
+public:
+  bool valid() const noexcept override { return false; }
+  std::type_info const &type() const noexcept override { return typeid(nullptr);}
+  std::size_t sizeof_type() const noexcept override { return 0; }
+  std::size_t count() const noexcept override { return 0; }
+  std::size_t size () const noexcept override { return 0; }
+  void *get() noexcept override { return nullptr; }
+  void const *get() const noexcept override { return nullptr; }
+
+  /// \brief The passed destination pointer is never null, 
+  /// and the memory to which it points is uninitialized.
+  void move_to(allocator const &, void *) noexcept override {}
+  void copy_to(allocator const &, void *) const noexcept(false) override {}
+};
+
+template <typename Value, bool/*is on stack*/>
+class holder;
+
+/**
+ * \class template <typename Value> holder<Value, true>
+ * \brief A holder implementation that holds a data object if type `Value` on stack memory.
+ * \tparam Value The storage type of the holder.
+ */
+template <typename Value>
+class holder<Value, true> : public holder_base {
+
+  alignas(alignof(Value)) std::array<::LIBIMP::byte, sizeof(Value)> value_storage_;
+
+public:
+  holder() = default; // uninitialized
+
+  template <typename... A>
+  holder(::LIBIMP::in_place_t, A &&...args) {
+    ::LIBIMP::construct<Value>(value_storage_.data(), std::forward<A>(args)...);
+  }
+
+  bool valid() const noexcept override {
+    return true;
+  }
+
+  std::type_info const &type() const noexcept override {
+    return typeid(Value);
+  }
+
+  std::size_t sizeof_type() const noexcept override {
+    return sizeof(Value);
+  }
+
+  std::size_t count() const noexcept override {
+    return 1;
+  }
+
+  std::size_t size() const noexcept override {
+    return value_storage_.size();
+  }
+
+  void *get() noexcept override {
+    return value_storage_.data();
+  }
+
+  void const *get() const noexcept override {
+    return value_storage_.data();
+  }
+
+  void move_to(allocator const &, void *p) noexcept override {
+    ::LIBIMP::construct<holder>(p, ::LIBIMP::in_place, std::move(*static_cast<Value *>(get())));
+  }
+
+  void copy_to(allocator const &, void *p) const noexcept(false) override {
+    ::LIBIMP::construct<holder>(p, ::LIBIMP::in_place, *static_cast<Value *>(get()));
+  }
+};
+
+LIBPMR_NAMESPACE_END_
diff --git a/include/libpmr/small_storage.h b/include/libpmr/small_storage.h
new file mode 100644
index 0000000..918dc93
--- /dev/null
+++ b/include/libpmr/small_storage.h
@@ -0,0 +1,35 @@
+/**
+ * \file libpmr/small_storage.h
+ * \author mutouyun (orz@orzz.org)
+ * \brief Unified SSO (Small Size Optimization).
+ * \date 2023-09-02
+ */
+#pragma once
+
+#include <type_traits>
+#include <array>
+#include <typeinfo>
+#include <utility>
+#include <memory>
+#include <cstddef>
+
+#include "libimp/export.h"
+#include "libimp/construct.h"
+#include "libimp/byte.h"
+#include "libimp/generic.h"
+
+#include "libpmr/def.h"
+#include "libpmr/holder_base.h"
+
+LIBPMR_NAMESPACE_BEG_
+
+/**
+ * \class template <typename Value> holder<Value, false>
+ * \brief A holder implementation that holds a data object if type `Value` on heap memory.
+ * \tparam Value The storage type of the holder.
+ */
+template <typename Value>
+class holder<Value, false> : public holder_base {
+};
+
+LIBPMR_NAMESPACE_END_
diff --git a/src/libpmr/allocator.cpp b/src/libpmr/allocator.cpp
index 4760347..68eb9fc 100644
--- a/src/libpmr/allocator.cpp
+++ b/src/libpmr/allocator.cpp
@@ -5,54 +5,31 @@
 
 LIBPMR_NAMESPACE_BEG_
 
-allocator::holder_base &allocator::get_holder() noexcept {
-  return *reinterpret_cast<holder_base *>(holder_.data());
+allocator::holder_mr_base &allocator::get_holder() noexcept {
+  return *reinterpret_cast<holder_mr_base *>(holder_.data());
 }
 
-allocator::holder_base const &allocator::get_holder() const noexcept {
-  return *reinterpret_cast<holder_base const *>(holder_.data());
+allocator::holder_mr_base const &allocator::get_holder() const noexcept {
+  return *reinterpret_cast<holder_mr_base const *>(holder_.data());
 }
 
-allocator::allocator() noexcept {
-  ::LIBIMP::construct<holder_null>(holder_.data());
-}
+allocator::allocator() noexcept
+  : allocator(new_delete_resource::get()) {}
 
 allocator::~allocator() noexcept {
   ::LIBIMP::destroy(&get_holder());
 }
 
-allocator::allocator(allocator &&other) noexcept
-  : allocator(other) /*copy*/ {
-  ::LIBIMP::construct<holder_null>(other.holder_.data());
-}
-
-allocator &allocator::operator=(allocator &&other) & noexcept {
-  if (this == &other) return *this;
-  this->holder_ = other.holder_;
-  ::LIBIMP::construct<holder_null>(other.holder_.data());
-  return *this;
-}
-
 void allocator::swap(allocator &other) noexcept {
   std::swap(this->holder_, other.holder_);
 }
 
-bool allocator::valid() const noexcept {
-  return get_holder().valid();
+void *allocator::allocate(std::size_t s, std::size_t a) const {
+  return get_holder().alloc(s, a);
 }
 
-allocator::operator bool() const noexcept {
-  return valid();
-}
-
-void *allocator::alloc(std::size_t s) {
-  if (!valid()) return nullptr;
-  return get_holder().alloc(s);
-}
-
-void allocator::dealloc(void *p, std::size_t s) {
-  if (!valid()) return;
-  get_holder().dealloc(p, s);
+void allocator::deallocate(void *p, std::size_t s, std::size_t a) const {
+  get_holder().dealloc(p, s, a);
 }
 
 LIBPMR_NAMESPACE_END_
diff --git a/test/pmr/test_pmr_allocator.cpp b/test/pmr/test_pmr_allocator.cpp
index a1d2368..8173c82 100644
--- a/test/pmr/test_pmr_allocator.cpp
+++ b/test/pmr/test_pmr_allocator.cpp
@@ -8,71 +8,71 @@
 
 TEST(allocator, construct) {
   pmr::allocator alc;
-  EXPECT_FALSE(alc.valid());
-  EXPECT_FALSE(alc);
+  SUCCEED();
 }
 
-TEST(allocator, construct_with_memory_resource) {
-  pmr::new_delete_resource mem_res;
-  pmr::allocator alc {&mem_res};
-  EXPECT_TRUE(alc.valid());
-  EXPECT_TRUE(alc);
-
-  auto p = alc.alloc(128);
+TEST(allocator, construct_value_initialization) {
+  pmr::allocator alc{};
+  auto p = alc.allocate(128);
   EXPECT_NE(p, nullptr);
-  EXPECT_NO_THROW(alc.dealloc(p, 128));
+  EXPECT_NO_THROW(alc.deallocate(p, 128));
 }
 
+namespace {
+
+class dummy_resource {
+public:
+  void *allocate(std::size_t, std::size_t = 0) noexcept {
+    return nullptr;
+  }
+  void deallocate(void *, std::size_t, std::size_t = 0) noexcept {
+    return;
+  }
+};
+
+} // namespace
+
 TEST(allocator, construct_copy_move) {
   pmr::new_delete_resource mem_res;
+  dummy_resource dummy_res;
+  pmr::allocator alc1{&mem_res}, alc2{&dummy_res};
+  auto p = alc1.allocate(128);
+  ASSERT_NE(p, nullptr);
+  ASSERT_NO_THROW(alc1.deallocate(p, 128));
+  ASSERT_EQ(alc2.allocate(128), nullptr);
 
-  pmr::allocator alc1 {&mem_res}, alc2;
-  EXPECT_TRUE (alc1.valid());
-  EXPECT_TRUE (alc1);
-  EXPECT_FALSE(alc2.valid());
-  EXPECT_FALSE(alc2);
+  pmr::allocator alc3{alc1}, alc4{alc2}, alc5{std::move(alc1)};
 
-  pmr::allocator alc3 {alc1}, alc4{alc2}, alc5 {std::move(alc1)};
-  EXPECT_TRUE (alc3.valid());
-  EXPECT_TRUE (alc3);
-  EXPECT_FALSE(alc4.valid());
-  EXPECT_FALSE(alc4);
-  EXPECT_TRUE (alc5.valid());
-  EXPECT_TRUE (alc5);
-  EXPECT_FALSE(alc1.valid());
-  EXPECT_FALSE(alc1);
+  p = alc3.allocate(128);
+  ASSERT_NE(p, nullptr);
+  ASSERT_NO_THROW(alc3.deallocate(p, 128));
+
+  ASSERT_EQ(alc4.allocate(128), nullptr);
+
+  p = alc5.allocate(128);
+  ASSERT_NE(p, nullptr);
+  ASSERT_NO_THROW(alc5.deallocate(p, 128));
 }
 
 TEST(allocator, swap) {
   pmr::new_delete_resource mem_res;
-
-  pmr::allocator alc1 {&mem_res}, alc2;
-  EXPECT_TRUE (alc1.valid());
-  EXPECT_TRUE (alc1);
-  EXPECT_FALSE(alc2.valid());
-  EXPECT_FALSE(alc2);
-
+  dummy_resource dummy_res;
+  pmr::allocator alc1{&mem_res}, alc2{&dummy_res};
   alc1.swap(alc2);
-  EXPECT_FALSE(alc1.valid());
-  EXPECT_FALSE(alc1);
-  EXPECT_TRUE (alc2.valid());
-  EXPECT_TRUE (alc2);
+  auto p = alc2.allocate(128);
+  ASSERT_NE(p, nullptr);
+  ASSERT_NO_THROW(alc2.deallocate(p, 128));
+  ASSERT_EQ(alc1.allocate(128), nullptr);
 }
 
 TEST(allocator, invalid_alloc_free) {
-  pmr::new_delete_resource mem_res;
+  pmr::allocator alc1;
+  EXPECT_EQ(alc1.allocate(0), nullptr);
+  EXPECT_NO_THROW(alc1.deallocate(nullptr, 128));
+  EXPECT_NO_THROW(alc1.deallocate(nullptr, 0));
+  EXPECT_NO_THROW(alc1.deallocate(&alc1, 0));
+}
 
-  pmr::allocator alc1 {&mem_res}, alc2;
-  EXPECT_EQ(alc1.alloc(0), nullptr);
-  EXPECT_NO_THROW(alc1.dealloc(nullptr, 128));
-  EXPECT_NO_THROW(alc1.dealloc(nullptr, 0));
-  EXPECT_NO_THROW(alc1.dealloc(&alc1, 0));
-
-  EXPECT_EQ(alc2.alloc(0), nullptr);
-  EXPECT_NO_THROW(alc2.dealloc(nullptr, 128));
-  EXPECT_NO_THROW(alc2.dealloc(nullptr, 0));
-  EXPECT_NO_THROW(alc2.dealloc(&alc1, 0));
-
-  EXPECT_EQ(alc2.alloc(1024), nullptr);
-  EXPECT_NO_THROW(alc2.dealloc(&alc1, sizeof(alc1)));
-}
\ No newline at end of file
+TEST(allocator, sizeof) {
+  EXPECT_EQ(sizeof(pmr::allocator), sizeof(void *) * 2);
+}
diff --git a/test/pmr/test_pmr_small_storage.cpp b/test/pmr/test_pmr_small_storage.cpp
new file mode 100644
index 0000000..e3629a3
--- /dev/null
+++ b/test/pmr/test_pmr_small_storage.cpp
@@ -0,0 +1,7 @@
+
+#include "gtest/gtest.h"
+
+#include "libpmr/small_storage.h"
+
+TEST(small_storage, construct) {
+}