为什么会产生正确的输出？

Question

I was just working on a multithreading/concurrency tutorial, and the following code (I think) is supposed to not work correctly, because 5 different threads operate on the same object.

But it prints out exactly 500 every time. How does that work? I'm not using mutexes, so there is no protection against accessing the same data by multiple threads...

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

struct Counter {

    int value;

    void increment() {
        value++;
    }
};

int main(){

    Counter counter;
    counter.value = 0;

    vector <thread> threads;

    for (int i = 0; i < 5; i++){
        threads.push_back(thread([&counter](){
            for (int i = 0; i < 100; ++i){
                counter.increment();
            }
        }));
    }


    for (auto& thread : threads)
        thread.join();

    cout << counter.value << endl;

    cin.get();
    return 0;
}

Answer 1

Depending on the compiler, the increment ++i will result in a single instruction, which means that the increment will likely be performed atomically.

However, execution still results in a data race when multiple threads write to the same memory without any form of synchronization and will lead to undefined behaviour . UB means that just about anything can happen, including showing the correct answer.

Answer 2

Multi-Threaded processing can be a strange beast as Snps said, i think you add a few print statements to your so you can detect how it is behaving while its running. I normally do that when i run into something confusing with any thing threaded.