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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