C++ LRU cache 实现

LRU

简单回顾 LRU( Least Recently Used)，最近最久未使用。
LRU 缓存要具备以下特点：

新增缓存时，应该被放到 Cache 的最前面
访问某个缓存之后，也应该被挪到 Cache 的最前面
容量不够，擦除尾端的Cache

最近翻了下mediapipe的源码，发现里面有个image_multi_pool.cc，基本原理就是使用LRU cache 对image对象池的管理，不过里面LRU实现用的是deque+unordered_map。
主要代码

class ImageMultiPool {public:ImageMultiPool() {}explicit ImageMultiPool(void* ignored) {}~ImageMultiPool();// Obtains a buffer. May either be reused or created anew.Image GetBuffer(int width, int height, bool use_gpu,ImageFormat::Format format /*= ImageFormat::SRGBA*/){IBufferSpec key(width, height, format);auto pool_it = pools_cpu_.find(key);if (pool_it == pools_cpu_.end()) {// Discard the least recently used pool in LRU cache.if (pools_cpu_.size() >= kMaxPoolCount) {auto old_spec = buffer_specs_cpu_.front();  // Front has LRU.buffer_specs_cpu_.pop_front();pools_cpu_.erase(old_spec);}buffer_specs_cpu_.push_back(key);  // Push new spec to back.std::tie(pool_it, std::ignore) = pools_cpu_.emplace(std::piecewise_construct, std::forward_as_tuple(key),std::forward_as_tuple(MakeSimplePoolCpu(key)));} else {// Find and move current 'key' spec to back, keeping others in same order.auto specs_it = buffer_specs_cpu_.begin();while (specs_it != buffer_specs_cpu_.end()) {if (*specs_it == key) {buffer_specs_cpu_.erase(specs_it);break;}++specs_it;}buffer_specs_cpu_.push_back(key);}return GetBufferFromSimplePool(pool_it->first, pool_it->second);}struct IBufferSpec {IBufferSpec(int w, int h, mediapipe::ImageFormat::Format f): width(w), height(h), format(f) {}int width;int height;mediapipe::ImageFormat::Format format;};private:std::unordered_map<IBufferSpec, SimplePoolcpu, IBufferSpecHash> pools_cpu_std::deque<IBufferSpec> buffer_specs_cpu_;
};

问题

对LRU管理对象池，没问题，但是使用deque实现LRU管理最近使用的池感觉不太合适，里面的遍历deque查找最近使用的池也不是最优解。

实现

其实就是把deque用list代替，list 元素插删可以保证o(1),deque只保证头尾插删的大部分情况下O(1)复杂度，触发resize时还是要做整体迁移O(n)。
另外一个就是存储迭代器，方便擦除LRU ，这个也是不能使用deque的原因，插入和擦除都可能会导致deque的迭代器失效，而list则只有在删除元素时，该元素的迭代器失效。

#include <iostream>
#include <unordered_map>
#include <list>
#include <mutex>
#include <thread>template <typename KeyType, typename ValueType, int capacity = 10>
class LRUCache {private:std::unordered_map<KeyType, std::pair<ValueType, typename std::list<KeyType>::iterator>> cache;std::list<KeyType> lruList;std::mutex mtx;  // Mutex for synchronizationpublic:ValueType get(const KeyType& key) {std::lock_guard<std::mutex> lock(mtx);  // Lock for thread safetyif (cache.find(key) != cache.end()) {// Move the accessed item to the front of the listlruList.erase(cache[key].second);lruList.push_front(key);cache[key].second = lruList.begin();return cache[key].first;}return ValueType();  // Return a default-constructed value if the key is not in the cache}void put(const KeyType& key, const ValueType& value) {std::lock_guard<std::mutex> lock(mtx);  // Lock for thread safetyif (cache.find(key) != cache.end()) {// If key exists, update its value and move to the frontcache[key].first = value;lruList.erase(cache[key].second);lruList.push_front(key);cache[key].second = lruList.begin();} else {// If key does not existif (cache.size() >= capacity) {// Remove the least recently used itemKeyType lruKey = lruList.back();cache.erase(lruKey);lruList.pop_back();}// Add the new key-value pairlruList.push_front(key);cache[key] = std::make_pair(value, lruList.begin());}}friend std::ostream& operator<<(std::ostream& os,const LRUCache<KeyType, ValueType, capacity>& rhs) {for (auto p : rhs.lruList) {os << p << " ";}return os;}
};int main() {LRUCache<std::string, int, 2> cache;  // Capacity is set to 2std::thread t1([&]() {cache.put("one", 1);cache.put("two", 2);std::cout << "Thread 1 cache: " << cache << std::endl;std::cout << "Thread 1: " << cache.get("one") << std::endl;std::cout << "Thread 1 cache: " << cache << std::endl;});std::thread t2([&]() {cache.put("three", 3);std::cout << "Thread 2 cache: " << cache << std::endl;std::cout << "Thread 2: " << cache.get("two") << std::endl;std::cout << "Thread 2 cache: " << cache << std::endl;std::cout << "Thread 2: " << cache.get("one") << std::endl;std::cout << "Thread 2 cache: " << cache << std::endl;});t1.join();t2.join();return 0;
}