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// Copyright 2020 The Pigweed 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, 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.
#pragma once
#include <cstddef>
#include <initializer_list>
#include <type_traits>
#include "pw_containers/internal/intrusive_list_impl.h"
namespace pw {
// IntrusiveList provides singly-linked list functionality for derived
// class items. IntrusiveList<T> is a handle to access and manipulate the
// list, and IntrusiveList<T>::Item is a base class items must inherit
// from. An instantiation of the derived class becomes a list item when inserted
// into a IntrusiveList.
// This has two important ramifications:
// - An instantiated IntrusiveList::Item must remain in scope for the
// lifetime of the IntrusiveList it has been added to.
// - A linked list item CANNOT be included in two lists, as it is part of a
// preexisting list and adding it to another implicitly breaks correctness of
// the first list.
// Usage:
// class TestItem
// : public IntrusiveList<TestItem>::Item {}
// IntrusiveList<TestItem> test_items;
// auto item = TestItem();
// test_items.push_back(item);
// for (auto& test_item : test_items) {
// // Do a thing.
// }
template <typename T>
class IntrusiveList {
class Item : public intrusive_list_impl::List::Item {
constexpr Item() = default;
using element_type = T;
using value_type = std::remove_cv_t<T>;
using pointer = T*;
using reference = T&;
using iterator = intrusive_list_impl::Iterator<T, Item>;
using const_iterator =
intrusive_list_impl::Iterator<std::add_const_t<T>, const Item>;
constexpr IntrusiveList() { CheckItemType(); }
// Constructs an IntrusiveList from an iterator over Items. The iterator may
// dereference as either Item& (e.g. from std::array<Item>) or Item* (e.g.
// from std::initializer_list<Item*>).
template <typename Iterator>
IntrusiveList(Iterator first, Iterator last) : list_(first, last) {
// Constructs an IntrusiveList from a std::initializer_list of pointers to
// items.
IntrusiveList(std::initializer_list<Item*> items)
: IntrusiveList(items.begin(), items.end()) {}
template <typename Iterator>
void assign(Iterator first, Iterator last) {
list_.assign(first, last);
void assign(std::initializer_list<Item*> items) {
list_.assign(items.begin(), items.end());
[[nodiscard]] bool empty() const noexcept { return list_.empty(); }
void push_front(T& item) { list_.insert_after(list_.before_begin(), item); }
void push_back(T& item) { list_.insert_after(list_.before_end(), item); }
iterator insert_after(iterator pos, T& item) {
list_.insert_after(&(*pos), item);
return iterator(&item);
// Removes the first item in the list. The list must not be empty.
void pop_front() { list_.erase_after(list_.before_begin()); }
// Removes the item following pos from the list. The item is not destructed.
iterator erase_after(iterator pos) {
return ++pos;
// Removes all items from the list. The items themselves are not destructed.
void clear() { list_.clear(); }
// Removes this specific item from the list, if it is present. Finds the item
// by identity (address comparison) rather than value equality. Returns true
// if the item was removed; false if it was not present.
bool remove(const T& item) { return list_.remove(item); }
// Reference to the first element in the list. Undefined behavior if empty().
T& front() { return *static_cast<T*>(list_.begin()); }
// Reference to the last element in the list. Undefined behavior if empty().
T& back() { return *static_cast<T*>(list_.before_end()); }
// As in std::forward_list, returns the iterator before the begin() iterator.
iterator before_begin() noexcept {
return iterator(static_cast<Item*>(list_.before_begin()));
const_iterator before_begin() const noexcept {
return const_iterator(static_cast<const Item*>(list_.before_begin()));
const_iterator cbefore_begin() const noexcept { return before_begin(); }
iterator begin() noexcept {
return iterator(static_cast<Item*>(list_.begin()));
const_iterator begin() const noexcept {
return const_iterator(static_cast<const Item*>(list_.begin()));
const_iterator cbegin() const noexcept { return begin(); }
iterator end() noexcept { return iterator(static_cast<Item*>(list_.end())); }
const_iterator end() const noexcept {
return const_iterator(static_cast<const Item*>(list_.end()));
const_iterator cend() const noexcept { return end(); }
// Operation is O(size).
size_t size() const { return list_.size(); }
// Check that T is an Item in a function, since the class T will not be fully
// defined when the IntrusiveList<T> class is instantiated.
static constexpr void CheckItemType() {
std::is_base_of<Item, T>(),
"IntrusiveList items must be derived from IntrusiveList<T>::Item");
intrusive_list_impl::List list_;
} // namespace pw