使用可变数量 std::thread 的引用问题

Question

我使用了 posix 线程 (pthread)，但我正在尝试使用标准 C++11 线程（最有可能也是底层的 pthread）。我去过这里和其他论坛好几个 posts，但我无法弄清楚我的问题。如果有人能给我一盏灯，我会很高兴。

问题是我正在尝试将数据传递给线程，修改内部数据并且修改必须在抛出的线程完成（加入）后被主线程看到。问题是在某些线程中没有正确读取数据（int），因此输出结果不正确。

我可以编写的用于重现该问题的最少代码是：

#include <iostream>
#include <vector>
#include <thread>

using namespace std;

void threadCallbackInt(int const & x)
{
    int & y = const_cast<int &>(x);
    y += 10;
    std::cout<<"Inside Thread x = "<<y << " at position = " << &y <<std::endl;
}

int main(int argc, char* argv[])
{
    unsigned int nthreads = 3;
    vector<int> ints;
    vector<std::thread> threads(nthreads);
    threads.reserve(nthreads);
    for(unsigned int i = 0; i < nthreads; i++){
        ints.push_back(i);

        std::cout<<"In Main Thread : Before Thread Start x = "<<ints.at(i)<< " at position = " << &ints.at(i) << std::endl;
        threads[i] = std::thread(threadCallbackInt,std::ref(ints[i]));
    }

    cout << "size = " << threads.size() << endl;

    for(unsigned int i = 0; i < nthreads; i++){
        threads[i].join();
    }

    for(unsigned int i = 0; i < nthreads; i++){
        std::cout<<"In Main Thread : After Thread Start x = "<<ints[i]<< " at position = " << &ints[i] << std::endl;
    }
    return 0;
}

在几个论坛上有几个类似的 post，但我认为最接近我的问题的是 C++ 11 std::thread strange behavior

注意我使用reserve方法是为了避免引用失效和编号失效，但是没有用。

根据执行情况，我得到不同的输出。这是输出之一：

In Main Thread : Before Thread Start x = 0 at position = 0x21d7c40

In Main Thread : Before Thread Start x = 1 at position = 0x21d81e4

In Main Thread : Before Thread Start x = 2 at position = 0x21d7c48

Inside Thread x = 10 at position = 0x21d81e4

size = 3

Inside Thread x = 12 at position = 0x21d7c48

Inside Thread x = 10 at position = 0x21d7c40

In Main Thread : After Thread Start x = 10 at position = 0x21d7c40

In Main Thread : After Thread Start x = 1 at position = 0x21d7c44

In Main Thread : After Thread Start x = 12 at position = 0x21d7c48

请注意，其中一个 x 仍未更新，即使在线程结束后也是如此。这是因为用于此的内存不正确（在线程之前是 0x21d81e4，在线程之后是 0x21d7c40）。如果您运行代码，您会看到结果会有所不同，并且任何时候引用不匹配，我都会得到错误的结果。

任何关于可能解决方案的信息都将非常受欢迎。抱歉，如果这个 post 重复（我找不到这个确切问题的实际答案）。但如果是，请注明原文post.

Answer 1

当 ints 被 ints.push_back(i); 调整大小时，引用 std::ref(ints[i]) 无效。如果你想这样做，你还需要在 ints 上使用 reserve。

在使用 gcc 进行测试时，

当i == 0时，则ints.capacity() == 1
当i == 1时，则ints.capacity() == 2
当i == 2时，则ints.capacity() == 4

这表明确实正在调整大小。

Answer 2

执行ints.push_back(i)后，a[0]的地址正在改变。在打印 'In Main Thread...' 之前添加 std::cout << "ints.size()=" << ints.size() << " &ints[0]=" << &ints[0] << endl; ，输出如下（我正在使用 Visual Studio 2017 测试代码并添加 EnterCriticalSection/LeaveCriticalSection 以使输出不混淆。）

ints.size()=1 &ints[0]=0095BA28
In Main Thread : Before Thread Start x = 0 at position = 0095BA28
Inside Thread x = 10 at position = 0095BA28
ints.size()=2 &ints[0]=0095BA38
In Main Thread : Before Thread Start x = 1 at position = 0095BA3C
Inside Thread x = 11 at position = 0095BA3C
ints.size()=3 &ints[0]=0095E940
In Main Thread : Before Thread Start x = 2 at position = 0095E948
Inside Thread x = 12 at position = 0095E948
size = 3
In Main Thread : After Thread Start x = 10 at position = 0095E940
In Main Thread : After Thread Start x = 11 at position = 0095E944
In Main Thread : After Thread Start x = 12 at position = 0095E948

vector对象被调整大小，int[0]指向新分配的对象。因此，添加 ints.reserve(nthreads); 会保留指向 ints 向量的指针。

ints.size()=1 &ints[0]=00DD8608
In Main Thread : Before Thread Start x = 0 at position = 00DD8608
Inside Thread x = 10 at position = 00DD8608
ints.size()=2 &ints[0]=00DD8608
In Main Thread : Before Thread Start x = 1 at position = 00DD860C
Inside Thread x = 11 at position = 00DD860C
ints.size()=3 &ints[0]=00DD8608
In Main Thread : Before Thread Start x = 2 at position = 00DD8610
Inside Thread x = 12 at position = 00DD8610
size = 3
In Main Thread : After Thread Start x = 10 at position = 00DD8608
In Main Thread : After Thread Start x = 11 at position = 00DD860C
In Main Thread : After Thread Start x = 12 at position = 00DD8610

使用可变数量 std::thread 的引用问题

Problems with references using variable number of std::thread

c++

multithreading

c++11

stdthread