访问 class 个成员而不复制此变量或互斥变量

Question

我正在尝试创建一个简单的 Worker class，它将创建多个线程和一组线程任务运行。这是有教育意义的，但我已经尝试询问 class 伙伴并寻找不同的答案，我认为这里缺少一些 "simple"/基础知识。

我在设置互斥量变量的位置时遇到了一些问题。我相信我希望我的每个 Worker 对象都有一个，因为每个对象创建的线程是完全不相关的。现在，我正在尝试将 this 传递给添加到线程集的 lambda 方法，但这会复制互斥量，后者会删除它。有没有办法独立复制我需要的每个 class 成员，或者我应该创建一个复制构造函数？我只是想将我需要的 class 变量传递给 lambda。这是我的代码：

// TDAT2004Task2.cpp : Defines the entry point for the application.
//

#include <functional>
#include <iostream>
#include <list>
#include <vector>
#include <mutex>
#include <thread>

using namespace std;

class Worker {
    int threads;

    mutable mutex tasks_mutex;

    vector<thread> worker_threads;
    list<function<void()>> tasks;

public:

    Worker(int threadAmount) {
        threads = threadAmount;
    }


    void start() {
        for (int i = 0; i < threads; i++) {
            //This is the place errors occur. I'm copying the this element, and thus deleting my 
            //mutex variable.
            worker_threads.emplace_back([this] {
                while (true) {
                    function<void()> task;
                    {
                        lock_guard<mutex> lock(tasks_mutex);
                        if (!tasks.empty()) {
                            task = *tasks.begin();
                            tasks.pop_front();
                        }
                    }
                    if (task) {
                        task();
                    }
                }
                });
        }
        for (auto& thread : worker_threads) {
            thread.join();
        }
    }

    void post(function<void()> function) {
        lock_guard<mutex> lock(tasks_mutex);
        tasks.emplace_back(function);
    }
};


int main()
{
    //This does not compile. 
    Worker worker_threads = Worker(4);
}

我在 VS 中的错误信息：

>------ Build All started: Project: TDAT2004Task2, Configuration: x64-Debug ------
  [1/2] C:\PROGRA~2\MICROS~219\COMMUN~1\VC\Tools\MSVC24~1.283\bin\HostX64\x64\cl.exe  /nologo /TP   /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /FoTDAT2004Task2\CMakeFiles\TDAT2004Task2.dir\TDAT2004Task2.cpp.obj /FdTDAT2004Task2\CMakeFiles\TDAT2004Task2.dir\ /FS -c ..\..\..\TDAT2004Task2\TDAT2004Task2.cpp
  FAILED: TDAT2004Task2/CMakeFiles/TDAT2004Task2.dir/TDAT2004Task2.cpp.obj 
  C:\PROGRA~2\MICROS~219\COMMUN~1\VC\Tools\MSVC24~1.283\bin\HostX64\x64\cl.exe  /nologo /TP   /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /FoTDAT2004Task2\CMakeFiles\TDAT2004Task2.dir\TDAT2004Task2.cpp.obj /FdTDAT2004Task2\CMakeFiles\TDAT2004Task2.dir\ /FS -c ..\..\..\TDAT2004Task2\TDAT2004Task2.cpp
C:\Users\Even\source\repos\TDAT2004Task2\TDAT2004Task2\TDAT2004Task2.cpp(74): error C2280: 'Worker::Worker(const Worker &)': attempting to reference a deleted function
  ..\..\..\TDAT2004Task2\TDAT2004Task2.cpp(69): note: compiler has generated 'Worker::Worker' here
  ..\..\..\TDAT2004Task2\TDAT2004Task2.cpp(69): note: 'Worker::Worker(const Worker &)': function was implicitly deleted because a data member invokes a deleted or inaccessible function 'std::mutex::mutex(const std::mutex &)'
  C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.24.28314\include\mutex(92): note: 'std::mutex::mutex(const std::mutex &)': function was explicitly deleted
  ninja: build stopped: subcommand failed.

控制台中来自 cpp 的错误消息：

C:\Users\Even\source\repos\TDAT2004Task2\TDAT2004Task2>gcc TDAT2004Task2.cpp
TDAT2004Task2.cpp: In function 'int main()':
TDAT2004Task2.cpp:74:34: error: use of deleted function 'Worker::Worker(Worker&&)'
  Worker worker_threads = Worker(4);
                                  ^
TDAT2004Task2.cpp:18:7: note: 'Worker::Worker(Worker&&)' is implicitly deleted because the default definition would be ill-formed:
 class Worker {
       ^~~~~~
TDAT2004Task2.cpp:18:7: error: use of deleted function 'std::mutex::mutex(const std::mutex&)'
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.4.0/include/c++/mutex:43:0,
                 from TDAT2004Task2.cpp:13:
/usr/lib/gcc/x86_64-pc-cygwin/7.4.0/include/c++/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;
     ^~~~~

感谢您的任何回复！请指出我犯过的任何愚蠢错误，我只是想学习最佳实践。

Answer 1

问题是，您定义了一个复制构造函数。因此，您没有默认的移动构造函数。

我不知道你怎么能有一个正确的复制构造函数，因为 std::thread 不能被复制，所以 std::vector<std::thread> 不能被复制。

默认移动构造函数应该适合您的情况。

所以要明确添加行：

public:

   Worker(const Worker&) = delete;
   Worker& operator=(const Worker&) = delete;
   Worker(Worker&&) = default;
   Worker& operator=(Worker&&) = default;

致您的工作人员 class。

访问 class 个成员而不复制此变量或互斥变量

Access class members without copying this or mutex variable

c++

lambda

multithreading

mutex

copy-constructor