Multithreading – thread:: join() prevents it from

To learn how to use atomics in C 11, I tried the following code snippet:

#include <iostream>
#include <thread>
#include <atomic>

using namespace std;

struct solution {
    atomic<bool> alive_;
    thread thread_;

    solution() : thread_([this] {
        alive_ = true;
        while (alive_);
    }) { }
    ~solution() {
        alive_ = false;
        thread_.join();
    }
};

int main() {
    constexpr int N = 1; // or 2
    for (int i = 0; i < N; ++i) {
        solution s;
    }
    cout << "done" << endl;
}

If n equals 1, the output is complete However, if I set it to 2, the main thread will block in thread:: join() When n > Why do you think we can't see 1？

Note: if I use the following constructor:

solution() : alive_(true),thread_([this] {
        while (alive_);
    }) { }

It prints any value of n

Solution

If you do not initialize alive_ And set it only when the thread starts, you can perform the following interleaving execution:

MAIN: s::solution()
MAIN: s.thread_(/*your args*/)
MAIN: schedule(s.thread_) to run
thread: waiting to start
MAIN: s::~solution()
MAIN: s.alive_ = false
thread: alive_ = true
MAIN: s.thread_.join()
thread: while(alive_) {}