Question

ipc 那么,《刑法》中是否有数据竞赛? 如果该表尚未完全建成,那么,则Mlambda可打电话Base:Handler(),但如果届时将打电话Derived:Handler(,右?

#include <thread>
#include <chrono>
#include <functional>
#include <iostream>
#include <mutex>
#include <condition_variable>

struct IPC
{
    IPC() : t([&]{
        std::unique_lock lk(mut);
        cv.wait(lk, [&]{ return (bool)f; });
        f();
    }) {}

    void SetHandler(std::function<void()> handler_) {
        {
            std::unique_lock lk(mut);
            f = handler_;
        }
        cv.notify_all();
    }

    std::mutex mut;
    std::condition_variable cv;
    std::function<void()> f;
    std::thread t;
};

struct Base {
    Base() {
        ipc.SetHandler([this]{ Handle(); });

        // Maybe construction of Base takes some time.
        // Removing this sleep makes the output of the program stabilize on "Derived".
        std::this_thread::sleep_for(std::chrono::microseconds(10));
    }
    virtual void Handle() { std::cout << "Base" << std::endl; }
    IPC ipc;
};

struct Derived : Base {
    void Handle() override { std::cout << "Derived" << std::endl; }
};

int main()
{
    Derived b;
    // I only care about interaction with constructors so join here
    // to delay destruction of b until the test is finished.
    b.ipc.t.join();
}

Running this code multiple times in succession on my laptop produces randomly alternating outputs of "Base" and "Derived".

developer@WDX591VWD3:~$ g++-10 -std=c++20 hi.cpp -lpthread
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Derived
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Derived
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Derived
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Base
developer@WDX591VWD3:~$ ./a.out
Derived
developer@WDX591VWD3:~$

因此,我对问题持怀疑态度:这种不明确的行为吗? 我需要等待多长时间才能打电话给手,以便把衍生的班手提叫起来? 是否需要同步?

Answer 1

That s going to have undefined behavior.

While I think that the standard currently lacks some wording to that effect, it is in my opinion intended that the object may be used in another thread only after the construction of the most-derived object has finished.

[qu.cdtor]/2,在试图通过一个并非从this的点子接触该物体时,也作了类似的规定。

I also expect that it isn t intended to matter whether or not the class is polymorphic. The restriction linked above also doesn t apply only to polymorphic classes.

友情链接