如何在多线程c ++ 17程序中交换两个指针？ [英] How to swap two pointers in multi-threaded c++ 17 program?

问题描述

我有两个指针：pA和pB。他们指向两个大的哈希图对象。
由pB指向的哈希映射被完全更新时，我想交换pB和pA。

I have two pointers: pA and pB. They points to two big hash map objects. when the hash map pointed by pB is updated completely, I want to swap pB and pA.

在C ++ 17中，如何快速交换它们和线程安全？原子？我是c ++ 17的新手。

In C++ 17, how to swap them fast and thread safe? Atomic? I am new to c++ 17.

推荐答案

2个指针的原子无等待交换可以通过以下方式实现：

Atomic wait-free exchange of 2 pointers can be implemented in the following manner:

#include <atomic>
#include <cstdint>
#include <cassert>

template<class T>
class Pointers2 {
    uintptr_t const ab_;
    std::atomic<uintptr_t> a_;

public:
    Pointers2(T* a, T* b)
        : ab_(reinterpret_cast<uintptr_t>(a) ^ reinterpret_cast<uintptr_t>(b))
        , a_(reinterpret_cast<uintptr_t>(a))
    {}

    T* a() const { return reinterpret_cast<T*>(a_.load(std::memory_order_acquire)); }
    T* b() const { return reinterpret_cast<T*>(a_.load(std::memory_order_acquire) ^ ab_); }
    void exchange() { a_.fetch_xor(ab_, std::memory_order_release); }
};

int main() {
    int a = 1, b = 2;
    Pointers2<int> p2(&a, &b);
    assert(p2.a() == &a);
    assert(p2.b() == &b);
    p2.exchange();
    assert(p2.a() == &b);
    assert(p2.b() == &a);
    p2.exchange();
    assert(p2.a() == &a);
    assert(p2.b() == &b);
}

需要获取/释放内存顺序，以确保写入共享数据 T 不会在交换之后重新排序。

Acquire/release memory ordering is required to make sure that writes to shared data T do not get reordered past exchange.

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