std::weak_ptr 和相应的 std::shared_ptr 之间是否存在数据竞争？

Question

According to cppref, accessing const members of shared_ptr across multiple threads is safe. But is this statement valid when we have a weak_ptr that corresponds to a shared_ptr ?

As an example assume the following code:

#include <memory>
#include <iostream>
#include <thread>

std::shared_ptr<int> sp;
std::weak_ptr<int> gw;

int main()
{
    sp = std::make_shared<int>(42);
    gw = sp;
    auto th1 = std::thread([]{
        for (int i = 0; i < 200; i++) {
            if (sp.use_count() > 1) {
                std::cout << i << "\n";
                std::this_thread::yield();
            }
        }
    });
    auto th2 = std::thread([]{
        for (int i = 0; i < 20; i++) {
            if (auto l = gw.lock()) {
                std::cout << "locked ->" << l.use_count() << "\n";
                std::this_thread::yield();
            }
        }
    });
    th1.join();
    th2.join();
}

This code creates 2 threads. One checks use_count() of shared_ptr() which is a const method and the other one use lock() to lock weak_ptr() which also is a const method too. But in reality, when I call lock on weak_ptr , I practically increase reference count of shared_ptr which is not thread safe unless reference count is internally guarded. I wonder if I will have a data race in situations like this. Is this supposed to be thread-safe by standard?

Answer 1

Yes. The reference counter is atomic, so there are no data races in your example.

That being said, mutable operations on objects pointed by std::shared_ptr are not atomic, so they must be guarded as you would guard access via a plain pointer.