访问类成员而不复制此变量或互斥变量

Access class members without copying this or mutex variable

本文关键字：变量 复制 成员 访问      更新时间：2023-10-16

我正在尝试创建一个简单的Worker类，它将创建多个线程和一组任务供线程运行。这很有教育意义，但我已经尝试过询问同学并寻找不同的答案，我认为我缺少一些"简单"/基本的东西。

关于互斥变量的设置位置，我遇到了一些问题。我相信我希望每个Worker对象都有一个，因为每个对象创建的线程完全不相关。现在，我正试图将this传递给lambda方法，该方法会添加到线程集，但这会复制互斥体，互斥体会删除它。有没有办法独立复制我需要的每个类成员，或者我应该创建一个复制构造函数？我只是想把我需要的类变量传递给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:中的错误消息

>------ Build All started: Project: TDAT2004Task2, Configuration: x64-Debug ------
[1/2] C:PROGRA~2MICROS~22019COMMUN~1VCToolsMSVC1424~1.283binHostX64x64cl.exe  /nologo /TP   /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /FoTDAT2004Task2CMakeFilesTDAT2004Task2.dirTDAT2004Task2.cpp.obj /FdTDAT2004Task2CMakeFilesTDAT2004Task2.dir /FS -c ......TDAT2004Task2TDAT2004Task2.cpp
FAILED: TDAT2004Task2/CMakeFiles/TDAT2004Task2.dir/TDAT2004Task2.cpp.obj 
C:PROGRA~2MICROS~22019COMMUN~1VCToolsMSVC1424~1.283binHostX64x64cl.exe  /nologo /TP   /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /FoTDAT2004Task2CMakeFilesTDAT2004Task2.dirTDAT2004Task2.cpp.obj /FdTDAT2004Task2CMakeFilesTDAT2004Task2.dir /FS -c ......TDAT2004Task2TDAT2004Task2.cpp
C:UsersEvensourcereposTDAT2004Task2TDAT2004Task2TDAT2004Task2.cpp(74): error C2280: 'Worker::Worker(const Worker &)': attempting to reference a deleted function
......TDAT2004Task2TDAT2004Task2.cpp(69): note: compiler has generated 'Worker::Worker' here
......TDAT2004Task2TDAT2004Task2.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 Studio2019CommunityVCToolsMSVC14.24.28314includemutex(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:UsersEvensourcereposTDAT2004Task2TDAT2004Task2>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;
^~~~~

感谢您的回复！请指出我犯过的任何愚蠢的错误，我只是想学习最好的做法。

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。

我看不出如何拥有正确的复制构造函数，因为std::thread不能复制，因此std::vector<std::thread>不能复制。

默认的move构造函数应该适用于您的案例。

所以要明确地添加行：

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

你的工人阶级。

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