为什么我不能将带有互斥量的仿函数传递给线程？
Why can't I pass in a functor with a mutex to a thread?
class test
{
std::mutex m1;
public:
inline static int i{0};
void operator()()
{
m1.lock();
++i;
m1.unlock();
}
};
int main()
{
test t;
std::thread t1{t}; // doesn't work
// std::thread t1{std::ref(t)}; // works
t1.join();
cout << test::i << endl;
}
错误：
In file included from test.cpp:19:
/Library/Developer/CommandLineTools/usr/include/c++/v1/thread:365:17: error: no matching constructor for initialization of
'_Gp' (aka 'tuple<unique_ptr<std::__1::__thread_struct>, test>')
new _Gp(std::move(__tsp),
^ ~~~~~~~~~~~~~~~~~
test.cpp:53:17: note: in instantiation of function template specialization 'std::__1::thread::thread<test &, void>' requested
here
std::thread t1{t}; // doesn't work
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:625:5: note: candidate template ignored: requirement
'__lazy_and<is_same<allocator_arg_t, unique_ptr<__thread_struct, default_delete<__thread_struct> > >,
__lazy_all<__dependent_type<is_default_constructible<unique_ptr<__thread_struct, default_delete<__thread_struct> > >,
true>, __dependent_type<is_default_constructible<test>, true> > >::value' was not satisfied [with _AllocArgT =
std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, _Alloc = test,
_Dummy = true]
tuple(_AllocArgT, _Alloc const& __a)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:641:5: note: candidate template ignored: requirement
'_CheckArgsConstructor<true>::template __enable_implicit<const std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> > &, const test &>()' was not satisfied [with _Dummy = true]
tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:659:14: note: candidate template ignored: requirement
'_CheckArgsConstructor<true>::template __enable_explicit<const std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> > &, const test &>()' was not satisfied [with _Dummy = true]
explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:723:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !false>::template
__enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>() || _CheckArgsConstructor<_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
!false>::template __enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> >, test>()' was not satisfied [with _Up =
<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, test>,
_PackIsTuple = false]
tuple(_Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:756:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) <= sizeof...(_Tp) && !_PackExpandsToThisTuple<unique_ptr<__thread_struct,
default_delete<__thread_struct> >, test>::value>::template
__enable_explicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>() || _CheckArgsConstructor<!_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
!_PackExpandsToThisTuple<unique_ptr<__thread_struct, default_delete<__thread_struct> >, test>::value>::template
__enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>()' was not satisfied [with _Up = <std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> >, test>]
tuple(_Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:783:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !_PackExpandsToThisTuple<>::value>::template
__enable_implicit<>()' was not satisfied [with _Alloc = test, _Up = <>]
tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:803:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !_PackExpandsToThisTuple<>::value>::template
__enable_explicit<>()' was not satisfied [with _Alloc = test, _Up = <>]
tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:612:23: note: candidate constructor template not viable: requires
0 arguments, but 2 were provided
_LIBCPP_CONSTEXPR tuple()
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:677:7: note: candidate constructor template not viable: requires 4
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:697:7: note: candidate constructor template not viable: requires 4
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:822:9: note: candidate constructor template not viable: requires
single argument '__t', but 2 arguments were provided
tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:837:9: note: candidate constructor template not viable: requires
single argument '__t', but 2 arguments were provided
tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:850:9: note: candidate constructor template not viable: requires 3
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:864:9: note: candidate constructor template not viable: requires 3
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:615:5: note: candidate constructor not viable: requires 1
argument, but 2 were provided
tuple(tuple const&) = default;
^
In file included from test.cpp:1:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/iostream:38:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/ios:216:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/__locale:15:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string:500:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string_view:176:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/__string:56:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:640:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/initializer_list:47:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/cstddef:110:
/Library/Developer/CommandLineTools/usr/include/c++/v1/type_traits:2360:12: error: call to implicitly-deleted copy constructor
of 'typename decay<test &>::type' (aka 'test')
return _VSTD::forward<_Tp>(__t);
^~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/__config:508:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/thread:366:21: note: in instantiation of function template
specialization 'std::__1::__decay_copy<test &>' requested here
__decay_copy(_VSTD::forward<_Fp>(__f)),
^
test.cpp:53:17: note: in instantiation of function template specialization 'std::__1::thread::thread<test &, void>' requested
here
std::thread t1{t}; // doesn't work
^
test.cpp:36:16: note: copy constructor of 'test' is implicitly deleted because field 'm1' has an inaccessible copy constructor
std::mutex m1;
^
2 errors generated.
当我将仿函数传入线程时，该程序编译失败。但是当我用 std::ref 包装它时它会起作用。 class 成员互斥锁似乎是问题所在，但我不确定为什么。有人可以解释为什么 std::ref 包装器允许编译但没有它，程序无法编译吗？
编译器错误消息似乎没有帮助。
threads 的默认行为是复制。除其他外，这可以防止必须跨线程同步数据以及线程完成之前数据超出范围的一般麻烦。
在此特定情况下，正在复制的对象包含一个 mutex，并且无法复制或移动 mutex。 mutex 使多个线程远离代码的同一临界区。如果不同的线程拥有 mutex 的不同副本，它们都可以锁定自己的副本并进入临界区，从而使 mutex 无用。它们必须共享相同的 mutex，在这种情况下，这意味着所有线程都必须共享相同的 test.
在这种情况下，除了通过引用传递之外，static mutex m1 将是一个可行的解决方案。
注意：因为 i 是 public 并且任何人都可以访问，所以 i 可以很容易地被任何人访问，无论 mutex.
注意：最好使用 std::lock_guard or a std::scoped_lock 而不是手动调用锁定和手动解锁。 lock_guard 或 scoped_lock 在构建时锁定 mutex 并在销毁时解锁，保证 mutex 在锁定对象超出范围时解锁。
注意：删除 mutex 并使用 std::atomic<int> 代替 int 应该可以解决大多数同步问题。
class test
{
std::mutex m1;
public:
inline static int i{0};
void operator()()
{
m1.lock();
++i;
m1.unlock();
}
};
int main()
{
test t;
std::thread t1{t}; // doesn't work
// std::thread t1{std::ref(t)}; // works
t1.join();
cout << test::i << endl;
}
错误：
In file included from test.cpp:19:
/Library/Developer/CommandLineTools/usr/include/c++/v1/thread:365:17: error: no matching constructor for initialization of
'_Gp' (aka 'tuple<unique_ptr<std::__1::__thread_struct>, test>')
new _Gp(std::move(__tsp),
^ ~~~~~~~~~~~~~~~~~
test.cpp:53:17: note: in instantiation of function template specialization 'std::__1::thread::thread<test &, void>' requested
here
std::thread t1{t}; // doesn't work
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:625:5: note: candidate template ignored: requirement
'__lazy_and<is_same<allocator_arg_t, unique_ptr<__thread_struct, default_delete<__thread_struct> > >,
__lazy_all<__dependent_type<is_default_constructible<unique_ptr<__thread_struct, default_delete<__thread_struct> > >,
true>, __dependent_type<is_default_constructible<test>, true> > >::value' was not satisfied [with _AllocArgT =
std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, _Alloc = test,
_Dummy = true]
tuple(_AllocArgT, _Alloc const& __a)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:641:5: note: candidate template ignored: requirement
'_CheckArgsConstructor<true>::template __enable_implicit<const std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> > &, const test &>()' was not satisfied [with _Dummy = true]
tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:659:14: note: candidate template ignored: requirement
'_CheckArgsConstructor<true>::template __enable_explicit<const std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> > &, const test &>()' was not satisfied [with _Dummy = true]
explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:723:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !false>::template
__enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>() || _CheckArgsConstructor<_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
!false>::template __enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> >, test>()' was not satisfied [with _Up =
<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, test>,
_PackIsTuple = false]
tuple(_Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:756:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) <= sizeof...(_Tp) && !_PackExpandsToThisTuple<unique_ptr<__thread_struct,
default_delete<__thread_struct> >, test>::value>::template
__enable_explicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>() || _CheckArgsConstructor<!_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
!_PackExpandsToThisTuple<unique_ptr<__thread_struct, default_delete<__thread_struct> >, test>::value>::template
__enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >,
test>()' was not satisfied [with _Up = <std::__1::unique_ptr<std::__1::__thread_struct,
std::__1::default_delete<std::__1::__thread_struct> >, test>]
tuple(_Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:783:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !_PackExpandsToThisTuple<>::value>::template
__enable_implicit<>()' was not satisfied [with _Alloc = test, _Up = <>]
tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:803:9: note: candidate template ignored: requirement
'_CheckArgsConstructor<sizeof...(_Up) == sizeof...(_Tp) && !_PackExpandsToThisTuple<>::value>::template
__enable_explicit<>()' was not satisfied [with _Alloc = test, _Up = <>]
tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:612:23: note: candidate constructor template not viable: requires
0 arguments, but 2 were provided
_LIBCPP_CONSTEXPR tuple()
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:677:7: note: candidate constructor template not viable: requires 4
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:697:7: note: candidate constructor template not viable: requires 4
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:822:9: note: candidate constructor template not viable: requires
single argument '__t', but 2 arguments were provided
tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:837:9: note: candidate constructor template not viable: requires
single argument '__t', but 2 arguments were provided
tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:850:9: note: candidate constructor template not viable: requires 3
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:864:9: note: candidate constructor template not viable: requires 3
arguments, but 2 were provided
tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/tuple:615:5: note: candidate constructor not viable: requires 1
argument, but 2 were provided
tuple(tuple const&) = default;
^
In file included from test.cpp:1:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/iostream:38:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/ios:216:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/__locale:15:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string:500:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/string_view:176:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/__string:56:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:640:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/initializer_list:47:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/cstddef:110:
/Library/Developer/CommandLineTools/usr/include/c++/v1/type_traits:2360:12: error: call to implicitly-deleted copy constructor
of 'typename decay<test &>::type' (aka 'test')
return _VSTD::forward<_Tp>(__t);
^~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/__config:508:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/thread:366:21: note: in instantiation of function template
specialization 'std::__1::__decay_copy<test &>' requested here
__decay_copy(_VSTD::forward<_Fp>(__f)),
^
test.cpp:53:17: note: in instantiation of function template specialization 'std::__1::thread::thread<test &, void>' requested
here
std::thread t1{t}; // doesn't work
^
test.cpp:36:16: note: copy constructor of 'test' is implicitly deleted because field 'm1' has an inaccessible copy constructor
std::mutex m1;
^
2 errors generated.
当我将仿函数传入线程时，该程序编译失败。但是当我用 std::ref 包装它时它会起作用。 class 成员互斥锁似乎是问题所在，但我不确定为什么。有人可以解释为什么 std::ref 包装器允许编译但没有它，程序无法编译吗？
编译器错误消息似乎没有帮助。
threads 的默认行为是复制。除其他外，这可以防止必须跨线程同步数据以及线程完成之前数据超出范围的一般麻烦。
在此特定情况下，正在复制的对象包含一个 mutex，并且无法复制或移动 mutex。 mutex 使多个线程远离代码的同一临界区。如果不同的线程拥有 mutex 的不同副本，它们都可以锁定自己的副本并进入临界区，从而使 mutex 无用。它们必须共享相同的 mutex，在这种情况下，这意味着所有线程都必须共享相同的 test.
在这种情况下，除了通过引用传递之外，static mutex m1 将是一个可行的解决方案。
注意：因为 i 是 public 并且任何人都可以访问，所以 i 可以很容易地被任何人访问，无论 mutex.
注意：最好使用 std::lock_guard or a std::scoped_lock 而不是手动调用锁定和手动解锁。 lock_guard 或 scoped_lock 在构建时锁定 mutex 并在销毁时解锁，保证 mutex 在锁定对象超出范围时解锁。
注意：删除 mutex 并使用 std::atomic<int> 代替 int 应该可以解决大多数同步问题。