为什么不能将带有互斥量的仿函数传递给线程？

2024-04-29 • 问答

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,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,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,test>() || _CheckArgsConstructor<_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
      !false>::template __enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct,test>()' was not satisfied [with _Up =
      <std::__1::unique_ptr<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,test>() || _CheckArgsConstructor<!_EnableImplicitReducedArityExtension && sizeof...(_Up) < sizeof...(_Tp) &&
      !_PackExpandsToThisTuple<unique_ptr<__thread_struct,test>::value>::template
      __enable_implicit<std::__1::unique_ptr<std::__1::__thread_struct,test>()' was not satisfied [with _Up = <std::__1::unique_ptr<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&&... __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 _Tp& ... __t)
      ^
/library/Developer/CommandLineTools/usr/include/c++/v1/tuple:697:7: note: candidate constructor template not viable: requires 4
      arguments,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',_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,_Tuple&& __t)
        ^
/library/Developer/CommandLineTools/usr/include/c++/v1/tuple:864:9: note: candidate constructor template not viable: requires 3
      arguments,_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.

当我将函子传递到线程中时，该程序无法编译。但是当我用std::ref包装它时，它就可以工作。看来类成员互斥锁是问题所在，但我不确定为什么。有人可以解释为什么std::ref包装器允许对此进行编译，但是如果没有包装，程序将无法编译？

编译器错误消息似乎没有帮助。

iCMS 回答：为什么不能将带有互斥量的仿函数传递给线程？

thread的默认行为是进行复制。除其他外，这避免了在线程完成之前必须跨线程同步数据和超出范围的数据的麻烦。

在这种特定情况下，要复制的对象包含mutex，并且mutex s无法复制或移动。 mutex使多个线程脱离同一关键代码段。如果不同的线程具有mutex的不同副本，则它们都可以锁定其副本并进入关键部分，从而使mutex无效。它们必须都共享相同的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也是可行的解决方案。

注意：由于i是public且任何人都可以访问，因此无论i为何，任何人都可以轻松访问mutex。

注意：首选使用std::lock_guard或std::scoped_lock而不是手动调用锁定和解锁。 lock_guard或scoped_lock会在构造时锁定mutex，并在破坏时解锁，从而保证mutex在锁定对象超出范围时将被解锁。

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。

c++functor multithreading mutex

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为什么不能将带有互斥量的仿函数传递给线程？

iCMS 回答：为什么不能将带有互斥量的仿函数传递给线程？

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