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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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#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 {
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 {};
// 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<>;
template <class Alloc, class... Args>
static void construct(Alloc* alloc, slot_type* slot, Args&&... args) {
Policy::construct(alloc, slot, std::forward<Args>(args)...);
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 transferred.
// 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
template <class Alloc>
static void transfer(Alloc* alloc, slot_type* new_slot, slot_type* old_slot) {
transfer_impl(alloc, new_slot, old_slot, 0);
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);
// 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