1 files changed, 60 insertions, 6 deletions

diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
index a94a82a0..a3e36e05 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -26,18 +26,25 @@
 //
 // In most cases, your default choice for a hash set should be a set of type
 // `flat_hash_set`.
+//
+// `flat_hash_set` is not exception-safe.
+
 #ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
 #define ABSL_CONTAINER_FLAT_HASH_SET_H_
 
+#include <cstddef>
+#include <memory>
 #include <type_traits>
 #include <utility>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
+#include "absl/container/hash_container_defaults.h"
 #include "absl/container/internal/container_memory.h"
-#include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
 #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
 #include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -58,7 +65,7 @@ struct FlatHashSetPolicy;
 // * Requires keys that are CopyConstructible
 // * Supports heterogeneous lookup, through `find()` and `insert()`, provided
 //   that the set is provided a compatible heterogeneous hashing function and
-//   equality operator.
+//   equality operator. See below for details.
 // * Invalidates any references and pointers to elements within the table after
 //   `rehash()` and when the table is moved.
 // * Contains a `capacity()` member function indicating the number of element
@@ -76,6 +83,19 @@ struct FlatHashSetPolicy;
 // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
 // be randomized across dynamically loaded libraries.
 //
+// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
+// parameters can be used or `T` should have public inner types
+// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
+// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
+// well-formed. Both types are basically functors:
+// * `Hash` should support `size_t operator()(U val) const` that returns a hash
+// for the given `val`.
+// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
+// if `lhs` is equal to `rhs`.
+//
+// In most cases `T` needs only to provide the `absl_container_hash`. In this
+// case `std::equal_to<void>` will be used instead of `eq` part.
+//
 // NOTE: A `flat_hash_set` stores its keys directly inside its implementation
 // array to avoid memory indirection. Because a `flat_hash_set` is designed to
 // move data when rehashed, set keys will not retain pointer stability. If you
@@ -99,10 +119,10 @@ struct FlatHashSetPolicy;
 //  if (ducks.contains("dewey")) {
 //    std::cout << "We found dewey!" << std::endl;
 //  }
-template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
-          class Eq = absl::container_internal::hash_default_eq<T>,
+template <class T, class Hash = DefaultHashContainerHash<T>,
+          class Eq = DefaultHashContainerEq<T>,
           class Allocator = std::allocator<T>>
-class flat_hash_set
+class ABSL_INTERNAL_ATTRIBUTE_OWNER flat_hash_set
     : public absl::container_internal::raw_hash_set<
           absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq, Allocator> {
   using Base = typename flat_hash_set::raw_hash_set;
@@ -460,6 +480,33 @@ typename flat_hash_set<T, H, E, A>::size_type erase_if(
 
 namespace container_internal {
 
+// c_for_each_fast(flat_hash_set<>, Function)
+//
+// Container-based version of the <algorithm> `std::for_each()` function to
+// apply a function to a container's elements.
+// There is no guarantees on the order of the function calls.
+// Erasure and/or insertion of elements in the function is not allowed.
+template <typename T, typename H, typename E, typename A, typename Function>
+decay_t<Function> c_for_each_fast(const flat_hash_set<T, H, E, A>& c,
+                                  Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>& c, Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+template <typename T, typename H, typename E, typename A, typename Function>
+decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>&& c, Function&& f) {
+  container_internal::ForEach(f, &c);
+  return f;
+}
+
+}  // namespace container_internal
+
+namespace container_internal {
+
 template <class T>
 struct FlatHashSetPolicy {
   using slot_type = T;
@@ -473,9 +520,11 @@ struct FlatHashSetPolicy {
                                                  std::forward<Args>(args)...);
   }
 
+  // Return std::true_type in case destroy is trivial.
   template <class Allocator>
-  static void destroy(Allocator* alloc, slot_type* slot) {
+  static auto destroy(Allocator* alloc, slot_type* slot) {
     absl::allocator_traits<Allocator>::destroy(*alloc, slot);
+    return IsDestructionTrivial<Allocator, slot_type>();
   }
 
   static T& element(slot_type* slot) { return *slot; }
@@ -489,6 +538,11 @@ struct FlatHashSetPolicy {
   }
 
   static size_t space_used(const T*) { return 0; }
+
+  template <class Hash>
+  static constexpr HashSlotFn get_hash_slot_fn() {
+    return &TypeErasedApplyToSlotFn<Hash, T>;
+  }
 };
 }  // namespace container_internal