C++ 显式调用mutex类构造函数报错

Question

在学习多线程的过程中，我创建了两个互斥锁分别对应标准输入、输出流，创建了两个线程。

具体代码如下：

#include <bits/stdc++.h>
#include <conio.h>
// #include <windows.h>

using namespace std;

int main(void)
{
	bool finished[2] = {false};
	mutex lock_stdout = mutex(), lock_stdin = mutex();
	thread t[2] =
		{thread([&finished, &lock_stdin, &lock_stdout]() -> void
				{
					unsigned long long int ull = 0ULL;
					lock_stdin.lock();
					while (!kbhit())
					{
						lock_stdin.unlock();
						lock_stdout.lock();
						cout << "t[0] : " << (ull++) << endl;
						lock_stdout.unlock();
						// Sleep(50);
						lock_stdin.lock();
					}
					lock_stdin.unlock();
					lock_stdout.unlock();
					finished[0] = true;
				}),
		 thread([&finished, &lock_stdin, &lock_stdout]() -> void
				{
					unsigned long long int ull = 0ULL;
					lock_stdin.lock();
					while (!kbhit())
					{
						lock_stdin.unlock();
						lock_stdout.lock();
						cout << "t[1] : " << (ull++) << endl;
						lock_stdout.unlock();
						// Sleep(50);
						lock_stdin.lock();
					}
					lock_stdin.unlock();
					lock_stdout.unlock();
					finished[1] = true;
				})};
	t[0].detach();
	t[1].detach();
	cout << "All threads have finished running." << endl;
	return 0;
}

在代码第10行，我显式调用了mutex类构造函数来创建互斥锁对象，即

mutex lock_stdout = mutex(), lock_stdin = mutex();

在Win7下使用MinGW64编译，编译器报错如下：

test.cpp: In function 'int main()':
test.cpp:10:28: error: use of deleted function 'std::mutex::mutex(const std::mutex&)'
  mutex lock_stdout = mutex(), lock_stdin = mutex();
                            ^
In file included from D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/condition_variable:39,
                 from D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/x86_64-w64-mingw32/bits/stdc++.h:103,
                 from test.cpp:1:
D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;
     ^~~~~
test.cpp:10:50: error: use of deleted function 'std::mutex::mutex(const std::mutex&)'
  mutex lock_stdout = mutex(), lock_stdin = mutex();
                                                  ^
In file included from D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/condition_variable:39,
                 from D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/x86_64-w64-mingw32/bits/stdc++.h:103,
                 from test.cpp:1:
D:/MinGW64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;
     ^~~~~

显然我调用的并非复制构造函数mutex(const mutex &)，但编译器却提示我是因为使用了mutex(const mutex &)才报错的。而一旦我直接隐式调用构造函数，即直接写成

mutex lock_stdout, lock_stdin;

形式，所有问题就都解决了。

小生有点强迫症，喜欢显式调用构造函数来创建对象，奈何见识浅薄，不得其原因，敢问各位这是为什么？如何解决？

Answer 1

mutex lock_stdout = mutex()这里，mutex()调用的是无参构造函数，但是=将其赋值给lock_stdout时，调用的是拷贝构造函数（所有的=号，默认采用拷贝构造函数），但是std::mutex不允许拷贝构造，也不允许 move 拷贝，所以就会报错。而mutex lock_stdout就是直接声明了一个变量，然后采用无参构造函数创建了这个变量，没有拷贝，所以不会报错。