抛出的对象不能在多线程解决方案中捕获 [英] Thrown object cannot be caught in a multi-threaded solution

问题描述

我有一个RAII类解决内部线程中的问题：

I have a RAII class that solves a problem in an inner thread:

#include <iostream>
#include <thread>
using namespace std;

struct solution_using_thread {
    solution_using_thread()
     : alive_(true), thread_() {
        thread_ = thread([this]() {
            while(alive_);
        });
    }
    ~solution_using_thread() {
        alive_ = false;
        thread_.join();
    }
private:
    bool alive_;
    thread thread_;
};

int main() {
    cout << 0 << endl;
    try {
        solution_using_thread solution;
        throw 1;
    } catch (int i ) {
        cout << i << endl;
    }
    cout << 2 << endl;
}

创建它的实例后，main会抛出异常。问题是在一些执行中，输出是：

After creating an instance of it, the main happens to throw an exception. The problem is in some of the executions, the output is:

0

当它应该是：

0
1
2

在 coliru ，它总是只有 0 。

我在这里缺少什么？

PS：对不起，

推荐答案

您假设线程将（最终）看到更新的 alive _ 值。但这不是保证。没有同步的对象不保证在任何有限的时间内对其他线程可见。 alive _ 的值可能会被缓存到某处，从不更新。

You're assuming that the thread will (eventually) see the updated value of alive_. But that's not guaranteed. Objects without synchronisation are not guaranteed to be made visible to other threads in any finite amount of time. It's possible the value of alive_ is cached somewhere and never updated.

您需要一些同步才能使其工作。使用互斥或​​另一个同步原语，或使 alive _ 原子。以下是后者的工作示例：

You need some synchronisation to make it work—either use a mutex or another synchronisation primitive, or make alive_ atomic. Here's a working example of the latter:

#include <atomic>

struct solution_using_thread {
    solution_using_thread()
     : alive_(true), thread_() {
        thread_ = thread([this]() {
            while(alive_);
        });
    }
    ~solution_using_thread() {
        alive_ = false;
        thread_.join();
    }
private:
    atomic<bool> alive_;
    thread thread_;
};

没有任何同步，程序有未定义行为，因为两个线程在 alive _ 。

Without any synchronisation, the program has Undefined Behaviour, because the two threads enter a data race over alive_.

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