hash_policy_traits.h

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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
//
//      https://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 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
  //                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_