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+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+namespace container_internal {
+
+// Defines how slots are initialized/destroyed/moved.
+template <class Policy, class = void>
+struct hash_policy_traits {
+ private:
+ struct ReturnKey {
+ // We return `Key` here.
+ // When Key=T&, we forward the lvalue reference.
+ // When Key=T, we return by value to avoid a dangling reference.
+ // eg, for string_hash_map.
+ template <class Key, class... Args>
+ Key operator()(Key&& k, const Args&...) const {
+ return std::forward<Key>(k);
+ }
+ };
+
+ template <class P = Policy, class = void>
+ struct ConstantIteratorsImpl : std::false_type {};
+
+ template <class P>
+ struct ConstantIteratorsImpl<P, absl::void_t<typename P::constant_iterators>>
+ : P::constant_iterators {};
+
+ public:
+ // The actual object stored in the hash table.
+ using slot_type = typename Policy::slot_type;
+
+ // The type of the keys stored in the hashtable.
+ using key_type = typename Policy::key_type;
+
+ // The argument type for insertions into the hashtable. This is different
+ // from value_type for increased performance. See initializer_list constructor
+ // and insert() member functions for more details.
+ using init_type = typename Policy::init_type;
+
+ using reference = decltype(Policy::element(std::declval<slot_type*>()));
+ using pointer = typename std::remove_reference<reference>::type*;
+ using value_type = typename std::remove_reference<reference>::type;
+
+ // Policies can set this variable to tell raw_hash_set that all iterators
+ // should be constant, even `iterator`. This is useful for set-like
+ // containers.
+ // Defaults to false if not provided by the policy.
+ using constant_iterators = ConstantIteratorsImpl<>;
+
+ // PRECONDITION: `slot` is UNINITIALIZED
+ // POSTCONDITION: `slot` is INITIALIZED
+ template <class Alloc, class... Args>
+ static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
+ Policy::construct(alloc, slot, std::forward<Args>(args)...);
+ }
+
+ // PRECONDITION: `slot` is INITIALIZED
+ // POSTCONDITION: `slot` is UNINITIALIZED
+ template <class Alloc>
+ static void destroy(Alloc* alloc, slot_type* slot) {
+ Policy::destroy(alloc, slot);
+ }
+
+ // Transfers the `old_slot` to `new_slot`. Any memory allocated by the
+ // allocator inside `old_slot` to `new_slot` can be transfered.
+ //
+ // OPTIONAL: defaults to:
+ //
+ // clone(new_slot, std::move(*old_slot));
+ // destroy(old_slot);
+ //
+ // PRECONDITION: `new_slot` is UNINITIALIZED and `old_slot` is INITIALIZED
+ // POSTCONDITION: `new_slot` is INITIALIZED and `old_slot` is
+ // UNINITIALIZED
+ template <class Alloc>
+ static void transfer(Alloc* alloc, slot_type* new_slot, slot_type* old_slot) {
+ transfer_impl(alloc, new_slot, old_slot, 0);
+ }
+
+ // PRECONDITION: `slot` is INITIALIZED
+ // POSTCONDITION: `slot` is INITIALIZED
+ template <class P = Policy>
+ static auto element(slot_type* slot) -> decltype(P::element(slot)) {
+ return P::element(slot);
+ }
+
+ // Returns the amount of memory owned by `slot`, exclusive of `sizeof(*slot)`.
+ //
+ // If `slot` is nullptr, returns the constant amount of memory owned by any
+ // full slot or -1 if slots own variable amounts of memory.
+ //
+ // PRECONDITION: `slot` is INITIALIZED or nullptr
+ template <class P = Policy>
+ static size_t space_used(const slot_type* slot) {
+ return P::space_used(slot);
+ }
+
+ // Provides generalized access to the key for elements, both for elements in
+ // the table and for elements that have not yet been inserted (or even
+ // constructed). We would like an API that allows us to say: `key(args...)`
+ // but we cannot do that for all cases, so we use this more general API that
+ // can be used for many things, including the following:
+ //
+ // - Given an element in a table, get its key.
+ // - Given an element initializer, get its key.
+ // - Given `emplace()` arguments, get the element key.
+ //
+ // Implementations of this must adhere to a very strict technical
+ // specification around aliasing and consuming arguments:
+ //
+ // Let `value_type` be the result type of `element()` without ref- and
+ // cv-qualifiers. The first argument is a functor, the rest are constructor
+ // arguments for `value_type`. Returns `std::forward<F>(f)(k, xs...)`, where
+ // `k` is the element key, and `xs...` are the new constructor arguments for
+ // `value_type`. It's allowed for `k` to alias `xs...`, and for both to alias
+ // `ts...`. The key won't be touched once `xs...` are used to construct an
+ // element; `ts...` won't be touched at all, which allows `apply()` to consume
+ // any rvalues among them.
+ //
+ // If `value_type` is constructible from `Ts&&...`, `Policy::apply()` must not
+ // trigger a hard compile error unless it originates from `f`. In other words,
+ // `Policy::apply()` must be SFINAE-friendly. If `value_type` is not
+ // constructible from `Ts&&...`, either SFINAE or a hard compile error is OK.
+ //
+ // If `Ts...` is `[cv] value_type[&]` or `[cv] init_type[&]`,
+ // `Policy::apply()` must work. A compile error is not allowed, SFINAE or not.
+ template <class F, class... Ts, class P = Policy>
+ static auto apply(F&& f, Ts&&... ts)
+ -> decltype(P::apply(std::forward<F>(f), std::forward<Ts>(ts)...)) {
+ return P::apply(std::forward<F>(f), std::forward<Ts>(ts)...);
+ }
+
+ // Returns the "key" portion of the slot.
+ // Used for node handle manipulation.
+ template <class P = Policy>
+ static auto key(slot_type* slot)
+ -> decltype(P::apply(ReturnKey(), element(slot))) {
+ return P::apply(ReturnKey(), element(slot));
+ }
+
+ // Returns the "value" (as opposed to the "key") portion of the element. Used
+ // by maps to implement `operator[]`, `at()` and `insert_or_assign()`.
+ template <class T, class P = Policy>
+ static auto value(T* elem) -> decltype(P::value(elem)) {
+ return P::value(elem);
+ }
+
+ private:
+ // Use auto -> decltype as an enabler.
+ template <class Alloc, class P = Policy>
+ static auto transfer_impl(Alloc* alloc, slot_type* new_slot,
+ slot_type* old_slot, int)
+ -> decltype((void)P::transfer(alloc, new_slot, old_slot)) {
+ P::transfer(alloc, new_slot, old_slot);
+ }
+ template <class Alloc>
+ static void transfer_impl(Alloc* alloc, slot_type* new_slot,
+ slot_type* old_slot, char) {
+ construct(alloc, new_slot, std::move(element(old_slot)));
+ destroy(alloc, old_slot);
+ }
+};
+
+} // namespace container_internal
+} // namespace absl
+
+#endif // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_