从多个线程调用 std::shuffle

Question

我正在尝试在多个线程中打乱矢量副本。我的代码：

int main(int argc, const char *argv[]) {
    std::vector<int> rw{1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1};
    std::mutex m;
    auto multithread_shuffle = [rw, &m]() {
        unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
        std::shuffle (rw.begin(), rw.end(), std::default_random_engine(seed));
        std::lock_guard lock(m);
        for (size_t i = 0; i < rw.size(); i++) {
            std::cout << "rw[" << i << "] = " << rw[i] << std::endl;
        }
    };
    std::thread t1(multithread_shuffle);
    std::thread t2(multithread_shuffle);
    t1.join();
    t2.join();
    return 0;
}

但不幸的是，我收到了很多警告和这两个错误：

error: no matching function for call to 'swap(const int&, const int&)'
   swap(*__a, *__b);
   ~~~~^~~~~~~~~~~~
error: no type named 'type' in 'struct std::enable_if<false, void>'

如有任何建议，我将不胜感激。

Answer 1

问题是向量在复制到 lambda 的捕获参数中时变为 const。考虑：

int main() {
    int x = 15;
    auto func = [x]{//Capture by value
        x = 20;//Does not compile!
    };
}

就所有意图和目的而言，x 是 const int，而不是 int。

您可以在代码中遇到同样的问题，只需稍作更改：

int main(int argc, const char *argv[]) {
    std::vector<int> rw{1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1};
    std::mutex m;
    auto multithread_shuffle = [rw, &m]() {
        rw[0] = 3; //Does not compile!
        rw = std::vector<int>{4,3,2,1}; //Does not compile!
        unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
        std::shuffle (rw.begin(), rw.end(), std::default_random_engine(seed));
        std::lock_guard lock(m);
        for (size_t i = 0; i < rw.size(); i++) {
            std::cout << "rw[" << i << "] = " << rw[i] << std::endl;
        }
    };
    std::thread t1(multithread_shuffle);
    std::thread t2(multithread_shuffle);
    t1.join();
    t2.join();
    return 0;
}

如果您希望向量可变，则需要显式分配向量的本地副本。

int main(int argc, const char *argv[]) {
    std::vector<int> rw{1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1};
    std::mutex m;
    auto multithread_shuffle = [rw, &m]() {
        auto rw_mutable = rw;
        unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
        std::shuffle (rw_mutable.begin(), rw_mutable.end(), std::default_random_engine(seed));
        std::lock_guard lock(m);
        for (size_t i = 0; i < rw.size(); i++) {
            std::cout << "rw_mutable[" << i << "] = " << rw_mutable[i] << std::endl;
        }
    };
    std::thread t1(multithread_shuffle);
    std::thread t2(multithread_shuffle);
    t1.join();
    t2.join();
    return 0;
}

您也可以只声明 lambda mutable:

int main(int argc, const char *argv[]) {
    std::vector<int> rw{1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1};
    std::mutex m;
    auto multithread_shuffle = [rw, &m]() mutable {
        unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
        std::shuffle (rw.begin(), rw.end(), std::default_random_engine(seed));
        std::lock_guard lock(m);
        for (size_t i = 0; i < rw.size(); i++) {
            std::cout << "rw[" << i << "] = " << rw[i] << std::endl;
        }
    };
    std::thread t1(multithread_shuffle);
    std::thread t2(multithread_shuffle);
    t1.join();
    t2.join();
    return 0;
}

从多个线程调用 std::shuffle

Calling std::shuffle from multiple threads

c++

multithreading

stl

stdvector

c++14