带有 thread_local 的 C++ Schwarz 计数器

Question

我可以使用Schwarz counter (aka Nifty counter)吗？成语，用thread_local？ (假设我将所有 static 替换为 thread_local)

我需要这个(java jni 线程的助手):

class ThisThread{
    JNIEnv* jni_env{nullptr};
public:
    JNIEnv* getEnv(){
        if (!jni_env){
            // Attach thread
            java_vm->GetEnv((void**)&jni_env, JNI_VERSION);
            java_vm->AttachCurrentThread(&jni_env, NULL);
        }

        return jni_env;
    }

    ~ThisThread(){
        if (!jni_env) return;
        // Deattach thread
        java_vm->DetachCurrentThread();
    }
};

static thread_local ThisThread this_thread;

在每个线程中首先构造，最后析构。我可以从其他静态或 thread_local 对象的析构函数/构造函数调用 this_thread->getEnv()。

更新

https://stackoverflow.com/a/30200992 - 这里，标准说 thread_local 析构函数调用了 BEFORE static，我需要这个在之后。

Answer 1

我认为最好的解决方案是像往常一样实现 schwartz 计数器，但是根据 thread_local static Impl 实现 ThisThread 类.

带有输出的完整示例:

// header file
#include <memory>
#include <mutex>
#include <iostream>
#include <thread>

std::mutex emit_mutex;

template<class...Ts>
void emit(Ts&&...ts)
{
    auto action = [](auto&&x) { std::cout << x; };
    auto lock = std::unique_lock<std::mutex>(emit_mutex);

    using expand = int[];
    expand{ 0,
        (action(std::forward<Ts>(ts)), 0)...
    };
}


struct ThisThread
{
    struct Impl
    {
        Impl()
        {
            emit("ThisThread created on thread ", std::this_thread::get_id(), '\n');
        }
        ~Impl()
        {
            emit("ThisThread destroyed on thread ", std::this_thread::get_id(), '\n');
        }
        void foo() 
        { 
            emit("foo on thread ", std::this_thread::get_id(), '\n');
        }
    };

    decltype(auto) foo() { return get_impl().foo(); }

private:
    static Impl& get_impl() { return impl_; }
    static thread_local Impl impl_;
};

struct ThisThreadInit
{

    ThisThreadInit();
    ~ThisThreadInit();

    static int initialised;
};

extern ThisThread& thisThread;
static ThisThreadInit thisThreadInit;



// cppfile

static std::aligned_storage_t<sizeof(ThisThread), alignof(ThisThread)> storage;
ThisThread& thisThread = *reinterpret_cast<ThisThread*>(std::addressof(storage));
int ThisThreadInit::initialised;
thread_local ThisThread::Impl ThisThread::impl_;

ThisThreadInit::ThisThreadInit()
{
    if (0 == initialised++)
    {
        new (std::addressof(storage)) ThisThread ();    
    }
}

ThisThreadInit::~ThisThreadInit()
{
    if (0 == --initialised)
    {
        thisThread.~ThisThread();
    }
}


// now use the object

#include <thread>

int main()
{
    thisThread.foo();

    auto t = std::thread([]{ thisThread.foo(); });
    t.join();
}

示例输出:

ThisThread created on thread 140475785611072
foo on thread 140475785611072
ThisThread created on thread 140475768067840
foo on thread 140475768067840
ThisThread destroyed on thread 140475768067840
ThisThread destroyed on thread 140475785611072