#include #include template class LockFreeQueue { public: LockFreeQueue() : head_(new Node), tail_(head_.load()) {} ~LockFreeQueue() { while (Node *old_head = head

这个队列实现上可能存在ABA问题。具体来说，如果一个节点在某一时刻被弹出，然后又有一个新节点插入并且其指针恰好和该节点相同，那么后续的操作会将该新节点误认为是之前弹出的节点，从而导致问题。

解决方法是使用带有版本号的指针，在CAS操作时同时比较版本号。以下给出改进后的代码：

#include #include

template class LockFreeQueue { public:

LockFreeQueue() : head_(new Node), tail_(head_.load()) {}
~LockFreeQueue()
{
    while (Node *old_head = head_.load())
    {
        Node *next = old_head->next_;
        if (head_.compare_exchange_weak(old_head, next))
        {
            delete old_head;
            break;
        }
    }
}

void push(const T &value)
{
    Node *new_node = new Node(value);
    while (true)
    {
        Tail tail = tail_.load();
        Node *tail_ptr = tail.ptr;
        uint64_t tail_version = tail.version;
        Node *next = tail_ptr->next_.load(std::memory_order_relaxed);
        if (!next)
        {
            if (tail_ptr->next_.compare_exchange_weak(next, new_node,
                                                      std::memory_order_release,
                                                      std::memory_order_relaxed))
            {
                Tail new_tail{new_node, tail_version + 1};
                tail_.compare_exchange_weak(tail, new_tail,
                                            std::memory_order_release,
                                            std::memory_order_relaxed);
                size_.fetch_add(1, std::memory_order_release);
                return;
            }
        }
        else
        {
            Tail new_tail{next, tail_version + 1};
            tail_.compare_exchange_weak(tail, new_tail,
                                        std::memory_order_release,
                                        std::memory_order_relaxed);
        }
    }
}

bool try_pop(T &value)
{
    while (true)
    {
        Head head = head_.load();
        Tail tail = tail_.load();
        Node *head_ptr = head.ptr;
        uint64_t head_version = head.version;
        uint64_t tail_version = tail.version;
        Node *next = head_ptr->next_.load(std::memory_order_relaxed);
        if (head.ptr == tail.ptr)
        {
            if (!next)
            {
                return false;
            }
            Tail new_tail{next, tail_version + 1};
            tail_.compare_exchange_weak(tail, new_tail,
                                        std::memory_order_release,
                                        std::memory_order_relaxed);
        }
        else
        {
            value = next->value_;
            Head new_head{next, head_version + 1};
            if (head_.compare_exchange_weak(head, new_head,
                                             std::memory_order_release,
                                             std::memory_order_relaxed))
            {
                size_.fetch_sub(1, std::memory_order_release);
                delete head_ptr;
                return true;
            }
        }
    }
}

bool empty() const
{
    return size_.load(std::memory_order_acquire) == 0;
}

private:

struct Node
{
    T value_;
    std::atomic<Node *> next_;
    Node() : value_(T()), next_(nullptr) {}
    explicit Node(const T &value) : value_(value), next_(nullptr) {}
};

struct Head
{
    Node *ptr;
    uint64_t version;
};

struct Tail
{
    Node *ptr;
    uint64_t version;
};

std::atomic<Head> head_;
std::atomic<Tail> tail_;
std::atomic<size_t> size_{0};

};