Merge pull request #198 from boostorg/feature/concurrent_unordered_interop

Feature/concurrent unordered interop

doc/unordered/changes.adoc

@@ -10,6 +10,8 @@
 
 * Added `[c]visit_while` operations to `boost::concurrent_map`,
 with serial and parallel variants.
+* Added efficient move construction of `boost::unordered_flat_map` from
+`boost::concurrent_flat_map` and vice versa.
 
 == Release 1.83.0 - Major update
 

doc/unordered/concurrent.adoc

@@ -201,3 +201,29 @@ and the user need not take any special precaution, but overall performance may b
 Another blocking operation is _rehashing_, which happens explicitly via `rehash`/`reserve`
 or during insertion when the table's load hits `max_load()`. As with non-concurrent containers,
 reserving space in advance of bulk insertions will generally speed up the process.
+
+== Interoperability with non-concurrent containers
+
+As their internal data structure is basically the same, `boost::unordered_flat_map` can
+be efficiently move-constructed from `boost::concurrent_flat_map` and vice versa.
+This interoperability comes handy in multistage scenarios where parts of the data processing happen
+in parallel whereas other steps are non-concurrent (or non-modifying). In the following example,
+we want to construct a histogram from a huge input vector of words:
+the population phase can be done in parallel with `boost::concurrent_flat_map` and results
+then transferred to the final container.
+
+[source,c++]
+----
+std::vector<std::string> words = ...;
+
+// Insert words in parallel
+boost::concurrent_flat_map<std::string_view, std::size_t> m0;
+std::for_each(
+  std::execution::par, words.begin(), words.end(),
+  [&](const auto& word) {
+    m0.try_emplace_or_visit(word, 1, [](auto& x) { ++x.second; });
+  });
+
+// Transfer to a regular unordered_flat_map
+boost::unordered_flat_map m=std::move(m0);
+----

doc/unordered/concurrent_flat_map.adoc

@@ -69,6 +69,7 @@ namespace boost {
     explicit xref:#concurrent_flat_map_allocator_constructor[concurrent_flat_map](const Allocator& a);
     xref:#concurrent_flat_map_copy_constructor_with_allocator[concurrent_flat_map](const concurrent_flat_map& other, const Allocator& a);
     xref:#concurrent_flat_map_move_constructor_with_allocator[concurrent_flat_map](concurrent_flat_map&& other, const Allocator& a);
+    xref:#concurrent_flat_map_move_constructor_from_unordered_flat_map[concurrent_flat_map](unordered_flat_map<Key, T, Hash, Pred, Allocator>&& other);
     xref:#concurrent_flat_map_initializer_list_constructor[concurrent_flat_map](std::initializer_list<value_type> il,
                         size_type n = _implementation-defined_
                         const hasher& hf = hasher(),
@@ -501,6 +502,21 @@ Concurrency:;; Blocking on `other`.
 
 ---
 
+==== Move Constructor from unordered_flat_map
+
+```c++
+concurrent_flat_map(unordered_flat_map<Key, T, Hash, Pred, Allocator>&& other);
+```
+
+Move construction from a xref:#unordered_flat_map[`unordered_flat_map`].
+The internal bucket array of `other` is transferred directly to the new container.
+The hash function, predicate and allocator are moved-constructed from `other`.
+
+[horizontal]
+Complexity:;; O(`bucket_count()`) 
+
+---
+
 ==== Initializer List Constructor
 [source,c++,subs="+quotes"]
 ----

doc/unordered/unordered_flat_map.adoc

@@ -77,6 +77,7 @@ namespace boost {
     explicit xref:#unordered_flat_map_allocator_constructor[unordered_flat_map](const Allocator& a);
     xref:#unordered_flat_map_copy_constructor_with_allocator[unordered_flat_map](const unordered_flat_map& other, const Allocator& a);
     xref:#unordered_flat_map_move_constructor_with_allocator[unordered_flat_map](unordered_flat_map&& other, const Allocator& a);
+    xref:#unordered_flat_map_move_constructor_from_concurrent_flat_map[unordered_flat_map](concurrent_flat_map<Key, T, Hash, Pred, Allocator>&& other);
     xref:#unordered_flat_map_initializer_list_constructor[unordered_flat_map](std::initializer_list<value_type> il,
                        size_type n = _implementation-defined_
                        const hasher& hf = hasher(),
@@ -472,6 +473,22 @@ from `other`, and the allocator is copy-constructed from `a`.
 
 ---
 
+==== Move Constructor from concurrent_flat_map
+
+```c++
+unordered_flat_map(concurrent_flat_map<Key, T, Hash, Pred, Allocator>&& other);
+```
+
+Move construction from a xref:#concurrent_flat_map[`concurrent_flat_map`].
+The internal bucket array of `other` is transferred directly to the new container.
+The hash function, predicate and allocator are moved-constructed from `other`.
+
+[horizontal]
+Complexity:;; Constant time.
+Concurrency:;; Blocking on `other`.
+
+---
+
 ==== Initializer List Constructor
 [source,c++,subs="+quotes"]
 ----

include/boost/unordered/concurrent_flat_map.hpp

@@ -15,6 +15,7 @@
 #include <boost/unordered/detail/foa/concurrent_table.hpp>
 #include <boost/unordered/detail/foa/flat_map_types.hpp>
 #include <boost/unordered/detail/type_traits.hpp>
+#include <boost/unordered/unordered_flat_map_fwd.hpp>
 
 #include <boost/container_hash/hash.hpp>
 #include <boost/core/allocator_access.hpp>
@@ -84,6 +85,9 @@ namespace boost {
       template <class Key2, class T2, class Hash2, class Pred2,
         class Allocator2>
       friend class concurrent_flat_map;
+      template <class Key2, class T2, class Hash2, class Pred2,
+        class Allocator2>
+      friend class unordered_flat_map;
 
       using type_policy = detail::foa::flat_map_types<Key, T>;
 
@@ -223,6 +227,13 @@ namespace boost {
       {
       }
 
+
+      concurrent_flat_map(
+        unordered_flat_map<Key, T, Hash, Pred, Allocator>&& other)
+          : table_(std::move(other.table_))
+      {
+      }
+
       ~concurrent_flat_map() = default;
 
       concurrent_flat_map& operator=(concurrent_flat_map const& rhs)

include/boost/unordered/detail/foa/concurrent_table.hpp

@@ -252,7 +252,21 @@ struct concurrent_table_arrays:table_arrays<Value,Group,SizePolicy>
   template<typename Allocator>
   static concurrent_table_arrays new_(Allocator& al,std::size_t n)
   {
-    concurrent_table_arrays arrays{super::new_(al,n),nullptr};
+    super x{super::new_(al,n)};
+    BOOST_TRY{
+      return new_group_access(al,x);
+    }
+    BOOST_CATCH(...){
+      super::delete_(al,x);
+      BOOST_RETHROW
+    }
+    BOOST_CATCH_END
+  }
+
+  template<typename Allocator>
+  static concurrent_table_arrays new_group_access(Allocator& al,const super& x)
+  {
+    concurrent_table_arrays arrays{x,nullptr};
     if(!arrays.elements){
       arrays.group_accesses=dummy_group_accesses<SizePolicy::min_size()>();
     }
@@ -261,26 +275,26 @@ struct concurrent_table_arrays:table_arrays<Value,Group,SizePolicy>
         typename boost::allocator_rebind<Allocator,group_access>::type;
       using access_traits=boost::allocator_traits<access_alloc>;
 
-      BOOST_TRY{
-        auto aal=access_alloc(al);
-        arrays.group_accesses=boost::to_address(
-          access_traits::allocate(aal,arrays.groups_size_mask+1));
+      auto aal=access_alloc(al);
+      arrays.group_accesses=boost::to_address(
+        access_traits::allocate(aal,arrays.groups_size_mask+1));
 
-        for(std::size_t i=0;i<arrays.groups_size_mask+1;++i){
-          ::new (arrays.group_accesses+i) group_access();
-        }
+      for(std::size_t i=0;i<arrays.groups_size_mask+1;++i){
+        ::new (arrays.group_accesses+i) group_access();
       }
-      BOOST_CATCH(...){
-        super::delete_(al,arrays);
-        BOOST_RETHROW
-      }
-      BOOST_CATCH_END
     }
     return arrays;
   }
 
   template<typename Allocator>
   static void delete_(Allocator& al,concurrent_table_arrays& arrays)noexcept
+  {
+    delete_group_access(al,arrays);
+    super::delete_(al,arrays);
+  }
+
+  template<typename Allocator>
+  static void delete_group_access(Allocator& al,concurrent_table_arrays& arrays)noexcept
   {
     if(arrays.elements){
       using access_alloc=
@@ -294,7 +308,6 @@ struct concurrent_table_arrays:table_arrays<Value,Group,SizePolicy>
         aal,pointer_traits::pointer_to(*arrays.group_accesses),
         arrays.groups_size_mask+1);
     }
-    super::delete_(al,arrays);
   }
 
   group_access *group_accesses;
@@ -307,7 +320,7 @@ struct atomic_size_control
 
   atomic_size_control(std::size_t ml_,std::size_t size_):
     pad0_{},ml{ml_},pad1_{},size{size_}{}
-  atomic_size_control(atomic_size_control& x):
+  atomic_size_control(const atomic_size_control& x):
     pad0_{},ml{x.ml.load()},pad1_{},size{x.size.load()}{}
 
   /* padding to avoid false sharing internally and with sorrounding data */
@@ -359,7 +372,7 @@ inline void swap(atomic_size_control& x,atomic_size_control& y)
  *   - Parallel versions of [c]visit_all(f) and erase_if(f) are provided based
  *     on C++17 stdlib parallel algorithms.
  * 
- * Consult boost::unordered_flat_map docs for the full API reference.
+ * Consult boost::concurrent_flat_map docs for the full API reference.
  * Heterogeneous lookup is suported by default, that is, without checking for
  * any ::is_transparent typedefs --this checking is done by the wrapping
  * containers.
@@ -391,6 +404,9 @@ inline void swap(atomic_size_control& x,atomic_size_control& y)
  *       over.
  */
 
+template<typename,typename,typename,typename>
+class table; /* concurrent/non-concurrent interop */
+
 template <typename TypePolicy,typename Hash,typename Pred,typename Allocator>
 using concurrent_table_core_impl=table_core<
   TypePolicy,group15<atomic_integral>,concurrent_table_arrays,
@@ -412,10 +428,10 @@ class concurrent_table:
   using group_type=typename super::group_type;
   using super::N;
   using prober=typename super::prober;
-
-  template<
-    typename TypePolicy2,typename Hash2,typename Pred2,typename Allocator2>
-  friend class concurrent_table;
+  using arrays_type=typename super::arrays_type;
+  using size_ctrl_type=typename super::size_ctrl_type;
+  using compatible_nonconcurrent_table=table<TypePolicy,Hash,Pred,Allocator>;
+  friend compatible_nonconcurrent_table;
 
 public:
   using key_type=typename super::key_type;
@@ -450,6 +466,21 @@ public:
     concurrent_table(x,al_,x.exclusive_access()){}
   concurrent_table(concurrent_table&& x,const Allocator& al_):
     concurrent_table(std::move(x),al_,x.exclusive_access()){}
+
+  concurrent_table(compatible_nonconcurrent_table&& x):
+    super{
+      std::move(x.h()),std::move(x.pred()),std::move(x.al()),
+      arrays_type(arrays_type::new_group_access(
+        x.al(),
+        typename arrays_type::super{
+          x.arrays.groups_size_index,x.arrays.groups_size_mask,
+          reinterpret_cast<group_type*>(x.arrays.groups),
+          reinterpret_cast<value_type*>(x.arrays.elements)})),
+      size_ctrl_type{x.size_ctrl.ml,x.size_ctrl.size}}
+  {
+    x.empty_initialize();
+  }
+
   ~concurrent_table()=default;
 
   concurrent_table& operator=(const concurrent_table& x)
@@ -875,6 +906,8 @@ public:
   }
 
 private:
+  template<typename,typename,typename,typename> friend class concurrent_table;
+
   using mutex_type=rw_spinlock;
   using multimutex_type=multimutex<mutex_type,128>; // TODO: adapt 128 to the machine
   using shared_lock_guard=shared_lock<mutex_type>;

include/boost/unordered/detail/foa/core.hpp

@@ -1282,6 +1282,17 @@ public:
     size_ctrl{initial_max_load(),0}
     {}
 
+  /* bare transfer ctor for concurrent/non-concurrent interop */
+
+  table_core(
+    Hash&& h_,Pred&& pred_,Allocator&& al_,
+    const arrays_type& arrays_,const size_ctrl_type& size_ctrl_):
+    hash_base{empty_init,std::move(h_)},
+    pred_base{empty_init,std::move(pred_)},
+    allocator_base{empty_init,std::move(al_)},
+    arrays(arrays_),size_ctrl(size_ctrl_)
+  {}
+
   table_core(const table_core& x):
     table_core{x,alloc_traits::select_on_container_copy_construction(x.al())}{}
 
@@ -1290,14 +1301,11 @@ public:
       std::is_nothrow_move_constructible<Hash>::value&&
       std::is_nothrow_move_constructible<Pred>::value&&
       std::is_nothrow_move_constructible<Allocator>::value):
-    hash_base{empty_init,std::move(x.h())},
-    pred_base{empty_init,std::move(x.pred())},
-    allocator_base{empty_init,std::move(x.al())},
-    arrays(x.arrays),size_ctrl(x.size_ctrl)
+    table_core{
+      std::move(x.h()),std::move(x.pred()),std::move(x.al()),
+      x.arrays,x.size_ctrl}
   {
-    x.arrays=x.new_arrays(0);
-    x.size_ctrl.ml=x.initial_max_load();
-    x.size_ctrl.size=0;
+    x.empty_initialize();
   }
 
   table_core(const table_core& x,const Allocator& al_):
@@ -1336,6 +1344,13 @@ public:
     delete_arrays(arrays);
   }
 
+  void empty_initialize()noexcept
+  {
+    arrays=new_arrays(0);
+    size_ctrl.ml=initial_max_load();
+    size_ctrl.size=0;
+  }
+
   table_core& operator=(const table_core& x)
   {
     BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED(Hash, Pred)
@@ -1804,7 +1819,8 @@ private:
     pred_base{empty_init,std::move(pred_)},
     allocator_base{empty_init,al_},arrays(new_arrays(0)),
     size_ctrl{initial_max_load(),0}
-    {}
+  {
+  }
 
   arrays_type new_arrays(std::size_t n)
   {

include/boost/unordered/detail/foa/table.hpp

@@ -264,6 +264,9 @@ private:
  * checking is done by boost::unordered_(flat|node)_(map|set).
  */
 
+template<typename,typename,typename,typename>
+class concurrent_table; /* concurrent/non-concurrent interop */
+
 template <typename TypePolicy,typename Hash,typename Pred,typename Allocator>
 using table_core_impl=
   table_core<TypePolicy,group15<plain_integral>,table_arrays,
@@ -284,7 +287,12 @@ class table:table_core_impl<TypePolicy,Hash,Pred,Allocator>
   using group_type=typename super::group_type;
   using super::N;
   using prober=typename super::prober;
+  using arrays_type=typename super::arrays_type;
+  using size_ctrl_type=typename super::size_ctrl_type;
   using locator=typename super::locator;
+  using compatible_concurrent_table=
+    concurrent_table<TypePolicy,Hash,Pred,Allocator>;
+  friend compatible_concurrent_table;
 
 public:
   using key_type=typename super::key_type;
@@ -323,6 +331,8 @@ public:
   table(table&& x)=default;
   table(const table& x,const Allocator& al_):super{x,al_}{}
   table(table&& x,const Allocator& al_):super{std::move(x),al_}{}
+  table(compatible_concurrent_table&& x):
+    table(std::move(x),x.exclusive_access()){}
   ~table()=default;
 
   table& operator=(const table& x)=default;
@@ -496,6 +506,22 @@ public:
   friend bool operator!=(const table& x,const table& y){return !(x==y);}
 
 private:
+  template<typename ExclusiveLockGuard>
+  table(compatible_concurrent_table&& x,ExclusiveLockGuard):
+    super{
+      std::move(x.h()),std::move(x.pred()),std::move(x.al()),
+      arrays_type{
+        x.arrays.groups_size_index,x.arrays.groups_size_mask,
+        reinterpret_cast<group_type*>(x.arrays.groups),
+        reinterpret_cast<value_type*>(x.arrays.elements)},
+      size_ctrl_type{
+        x.size_ctrl.ml,x.size_ctrl.size}}
+  {
+    compatible_concurrent_table::arrays_type::delete_group_access(
+      this->al(),x.arrays);
+    x.empty_initialize();
+  }
+
   struct erase_on_exit
   {
     erase_on_exit(table& x_,const_iterator it_):x{x_},it{it_}{}

include/boost/unordered/unordered_flat_map.hpp

@@ -10,6 +10,7 @@
 #pragma once
 #endif
 
+#include <boost/unordered/concurrent_flat_map_fwd.hpp>
 #include <boost/unordered/detail/foa/flat_map_types.hpp>
 #include <boost/unordered/detail/foa/table.hpp>
 #include <boost/unordered/detail/type_traits.hpp>
@@ -36,6 +37,10 @@ namespace boost {
     template <class Key, class T, class Hash, class KeyEqual, class Allocator>
     class unordered_flat_map
     {
+      template <class Key2, class T2, class Hash2, class Pred2,
+        class Allocator2>
+      friend class concurrent_flat_map;
+
       using map_types = detail::foa::flat_map_types<Key, T>;
 
       using table_type = detail::foa::table<map_types, Hash, KeyEqual,
@@ -173,6 +178,12 @@ namespace boost {
       {
       }
 
+      unordered_flat_map(
+        concurrent_flat_map<Key, T, Hash, KeyEqual, Allocator>&& other)
+          : table_(std::move(other.table_))
+      {
+      }
+
       ~unordered_flat_map() = default;
 
       unordered_flat_map& operator=(unordered_flat_map const& other)

test/cfoa/assign_tests.cpp

@@ -33,6 +33,9 @@ using hasher = stateful_hash;
 using key_equal = stateful_key_equal;
 using allocator_type = stateful_allocator<std::pair<raii const, raii> >;
 
+using flat_map_type = boost::unordered::unordered_flat_map<raii, raii, hasher,
+  key_equal, allocator_type>;
+
 using map_type = boost::unordered::concurrent_flat_map<raii, raii, hasher,
   key_equal, allocator_type>;
 
@@ -843,6 +846,136 @@ namespace {
     }
     check_raii_counts();
   }
+
+  template <class G> void flat_map_move_assign(G gen, test::random_generator rg)
+  {
+    auto values = make_random_values(1024 * 16, [&] { return gen(rg); });
+    auto reference_map =
+      boost::unordered_flat_map<raii, raii>(values.begin(), values.end());
+
+    /*
+     * basically test that a temporary container is materialized and we
+     * move-assign from that
+     *
+     * we don't need to be super rigorous here because we already have tests for
+     * container assignment, we're just testing that a temporary is materialized
+     */
+
+    {
+      raii::reset_counts();
+
+      flat_map_type flat_map(values.begin(), values.end(), values.size(),
+        hasher(1), key_equal(2), allocator_type(3));
+
+      map_type map(0, hasher(2), key_equal(1), allocator_type(3));
+
+      BOOST_TEST(flat_map.get_allocator() == map.get_allocator());
+
+      map = std::move(flat_map);
+
+      BOOST_TEST(flat_map.empty());
+      BOOST_TEST_EQ(map.size(), reference_map.size());
+
+      test_fuzzy_matches_reference(map, reference_map, rg);
+
+      BOOST_TEST_EQ(map.hash_function(), hasher(1));
+      BOOST_TEST_EQ(map.key_eq(), key_equal(2));
+
+      BOOST_TEST_EQ(raii::copy_constructor, 2 * values.size());
+      BOOST_TEST_EQ(raii::destructor, 2 * values.size());
+      BOOST_TEST_EQ(raii::move_constructor, 2 * reference_map.size());
+      BOOST_TEST_EQ(raii::copy_assignment, 0u);
+      BOOST_TEST_EQ(raii::move_assignment, 0u);
+    }
+
+    check_raii_counts();
+
+    {
+      raii::reset_counts();
+
+      map_type map(values.begin(), values.end(), values.size(), hasher(1),
+        key_equal(2), allocator_type(3));
+
+      flat_map_type flat_map(0, hasher(2), key_equal(1), allocator_type(3));
+
+      BOOST_TEST(flat_map.get_allocator() == map.get_allocator());
+
+      flat_map = std::move(map);
+
+      BOOST_TEST(map.empty());
+      BOOST_TEST_EQ(flat_map.size(), reference_map.size());
+
+      BOOST_TEST_EQ(flat_map.hash_function(), hasher(1));
+      BOOST_TEST_EQ(flat_map.key_eq(), key_equal(2));
+
+      BOOST_TEST_EQ(raii::copy_constructor, 2 * values.size());
+      BOOST_TEST_EQ(raii::destructor, 2 * values.size());
+      BOOST_TEST_EQ(raii::move_constructor, 2 * reference_map.size());
+      BOOST_TEST_EQ(raii::copy_assignment, 0u);
+      BOOST_TEST_EQ(raii::move_assignment, 0u);
+    }
+
+    check_raii_counts();
+
+    {
+      raii::reset_counts();
+
+      flat_map_type flat_map(values.begin(), values.end(), values.size(),
+        hasher(1), key_equal(2), allocator_type(3));
+
+      map_type map(0, hasher(2), key_equal(1), allocator_type(4));
+
+      BOOST_TEST(flat_map.get_allocator() != map.get_allocator());
+
+      map = std::move(flat_map);
+
+      BOOST_TEST(flat_map.empty());
+      BOOST_TEST_EQ(map.size(), reference_map.size());
+
+      test_fuzzy_matches_reference(map, reference_map, rg);
+
+      BOOST_TEST_EQ(map.hash_function(), hasher(1));
+      BOOST_TEST_EQ(map.key_eq(), key_equal(2));
+
+      BOOST_TEST_EQ(raii::copy_constructor, 2 * values.size());
+      BOOST_TEST_EQ(
+        raii::destructor, 2 * values.size() + 2 * reference_map.size());
+      BOOST_TEST_EQ(raii::move_constructor, 4 * reference_map.size());
+      BOOST_TEST_EQ(raii::copy_assignment, 0u);
+      BOOST_TEST_EQ(raii::move_assignment, 0u);
+    }
+
+    check_raii_counts();
+
+    {
+      raii::reset_counts();
+
+      map_type map(values.begin(), values.end(), values.size(), hasher(1),
+        key_equal(2), allocator_type(3));
+
+      flat_map_type flat_map(0, hasher(2), key_equal(1), allocator_type(4));
+
+      BOOST_TEST(flat_map.get_allocator() != map.get_allocator());
+
+      flat_map = std::move(map);
+
+      BOOST_TEST(map.empty());
+      BOOST_TEST_EQ(flat_map.size(), reference_map.size());
+
+      BOOST_TEST_EQ(flat_map.hash_function(), hasher(1));
+      BOOST_TEST_EQ(flat_map.key_eq(), key_equal(2));
+
+      BOOST_TEST_EQ(raii::copy_constructor, 2 * values.size());
+      BOOST_TEST_EQ(
+        raii::destructor, 2 * values.size() + 2 * reference_map.size());
+      BOOST_TEST_EQ(raii::move_constructor, 4 * reference_map.size());
+      BOOST_TEST_EQ(raii::copy_assignment, 0u);
+      BOOST_TEST_EQ(raii::move_assignment, 0u);
+    }
+
+    check_raii_counts();
+  }
+
 } // namespace
 
 // clang-format off
@@ -860,6 +993,11 @@ UNORDERED_TEST(
   insert_and_assign,
   ((init_type_generator))
   ((default_generator)(sequential)(limited_range)))
+
+UNORDERED_TEST(
+  flat_map_move_assign,
+  ((init_type_generator))
+  ((default_generator)(sequential)(limited_range)))
 // clang-format on
 
 RUN_TESTS()

test/cfoa/constructor_tests.cpp

@@ -775,6 +775,109 @@ namespace {
     check_raii_counts();
   }
 
+  template <class G> void flat_map_constructor(G gen, test::random_generator rg)
+  {
+    auto values = make_random_values(1024 * 16, [&] { return gen(rg); });
+    auto reference_map =
+      boost::unordered_flat_map<raii, raii, hasher, key_equal, allocator_type>(
+        values.begin(), values.end(), values.size());
+
+    raii::reset_counts();
+
+    {
+      boost::unordered_flat_map<raii, raii, hasher, key_equal, allocator_type>
+        flat_map(values.begin(), values.end(), reference_map.size(), hasher(1),
+          key_equal(2), allocator_type(3));
+
+      auto const old_dc = +raii::default_constructor;
+      auto const old_mc = +raii::move_constructor;
+      auto const old_cc = +raii::copy_constructor;
+
+      BOOST_TEST_EQ(old_dc, 0u);
+      BOOST_TEST_GT(old_mc, 0u);
+      BOOST_TEST_GT(old_cc, 0u);
+
+      map_type x(std::move(flat_map));
+
+      test_fuzzy_matches_reference(x, reference_map, rg);
+
+      BOOST_TEST_EQ(+raii::default_constructor, old_dc);
+      BOOST_TEST_EQ(+raii::move_constructor, old_mc);
+      BOOST_TEST_EQ(+raii::copy_constructor, old_cc);
+
+      BOOST_TEST_EQ(x.hash_function(), hasher(1));
+      BOOST_TEST_EQ(x.key_eq(), key_equal(2));
+      BOOST_TEST(x.get_allocator() == allocator_type(3));
+
+      BOOST_TEST(flat_map.empty());
+    }
+
+    check_raii_counts();
+
+    {
+      boost::unordered_flat_map<raii, raii, hasher, key_equal, allocator_type>
+        flat_map(0, hasher(1), key_equal(2), allocator_type(3));
+
+      map_type x(std::move(flat_map));
+
+      BOOST_TEST(x.empty());
+
+      BOOST_TEST_EQ(x.hash_function(), hasher(1));
+      BOOST_TEST_EQ(x.key_eq(), key_equal(2));
+      BOOST_TEST(x.get_allocator() == allocator_type(3));
+
+      BOOST_TEST(flat_map.empty());
+    }
+
+    check_raii_counts();
+
+    {
+      map_type flat_map(values.begin(), values.end(), reference_map.size(),
+        hasher(1), key_equal(2), allocator_type(3));
+
+      auto const old_dc = +raii::default_constructor;
+      auto const old_mc = +raii::move_constructor;
+      auto const old_cc = +raii::copy_constructor;
+
+      BOOST_TEST_EQ(old_dc, 0u);
+      BOOST_TEST_GT(old_mc, 0u);
+      BOOST_TEST_GT(old_cc, 0u);
+
+      boost::unordered_flat_map<raii, raii, hasher, key_equal, allocator_type>
+        x(std::move(flat_map));
+
+      BOOST_TEST(x == reference_map);
+
+      BOOST_TEST_EQ(+raii::default_constructor, old_dc);
+      BOOST_TEST_EQ(+raii::move_constructor, old_mc);
+      BOOST_TEST_EQ(+raii::copy_constructor, old_cc);
+
+      BOOST_TEST_EQ(x.hash_function(), hasher(1));
+      BOOST_TEST_EQ(x.key_eq(), key_equal(2));
+      BOOST_TEST(x.get_allocator() == allocator_type(3));
+
+      BOOST_TEST(flat_map.empty());
+    }
+
+    check_raii_counts();
+
+    {
+      map_type flat_map(0, hasher(1), key_equal(2), allocator_type(3));
+
+      boost::unordered_flat_map<raii, raii, hasher, key_equal, allocator_type>
+        x(std::move(flat_map));
+
+      BOOST_TEST(x.empty());
+
+      BOOST_TEST_EQ(x.hash_function(), hasher(1));
+      BOOST_TEST_EQ(x.key_eq(), key_equal(2));
+      BOOST_TEST(x.get_allocator() == allocator_type(3));
+
+      BOOST_TEST(flat_map.empty());
+    }
+
+    check_raii_counts();
+  }
 } // namespace
 
 // clang-format off
@@ -818,6 +921,11 @@ UNORDERED_TEST(
   ((value_type_generator))
   ((default_generator)(sequential)(limited_range)))
 
+UNORDERED_TEST(
+  flat_map_constructor,
+  ((value_type_generator))
+  ((default_generator)(sequential)(limited_range)))
+
 // clang-format on
 
 RUN_TESTS()