Question

我试图在多个线程中混淆矢量副本。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>'

我将不胜感激任何建议。

Answer 1

问题是当复制到lambda的捕获参数时，向量变为Random。考虑：

const

出于所有意图和目的，int main() { int x = 15; auto func = [x]{//Capture by value x = 20;//Does not compile! }; }是x，而不是const int。

只需稍加改动，您就可以在代码中遇到同样的问题：

int

如果您希望它可变，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]() {
        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;
}

您也可以声明lambda 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; }：

mutable

从多个线程调用std :: shuffle

1 个答案: