c++ - std::thread 在我调用 join() 之前是否运行？

Question

在我实例化 std::thread 之后它会开始运行吗？或者线程是否仅在我调用 join() 时才开始运行？这在文档中有点不清楚。

Answer 1

它会在您实例化时执行。

Joining 用于让当前线程等待直到您的其他线程完成执行。

来自 http://en.cppreference.com/w/cpp/thread/thread/thread 的一些示例代码

    void f1(int n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 1 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}

void f2(int& n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 2 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}

int main()
{
    int n = 0;
    std::thread t1; // t1 is not a thread
    std::thread t2(f1, n + 1); // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread
    t2.join();
    t4.join();
    std::cout << "Final value of n is " << n << '\n';
}

    Possible output:
Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 2 executing
Thread 1 executing
Final value of n is 5