无法将std::mutex与std::vector配合使用的编译错误排查

问题代码 #

#include <vector>
#include <mutex>

class A
{
public:
  A(){};
private:
  std::mutex a_mutex;
};

int main()
{
   std::vector<A> v;
   A a;
   v.push_back(a);
   return 0;
}

编译错误信息 #

$ g++ test__use_of_deleted_function.cpp
In file included from /usr/include/c++/8/x86_64-redhat-linux/bits/c++allocator.h:33,
                 from /usr/include/c++/8/bits/allocator.h:46,
                 from /usr/include/c++/8/string:41,
                 from test__use_of_delete_function.cpp:1:
/usr/include/c++/8/ext/new_allocator.h: In instantiation of ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = A; _Args = {const A&}; _Tp = A]’:
/usr/include/c++/8/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = A; _Args = {const A&}; _Tp = A; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<A>]’:
/usr/include/c++/8/bits/stl_vector.h:1079:30:   required from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = A; _Alloc = std::allocator<A>; std::vector<_Tp, _Alloc>::value_type = A]’:
test__use_of_delete_function.cpp:19:17:   required from here
/usr/include/c++/8/ext/new_allocator.h:136:4: error: use of deleted function ‘A::A(const A&)’
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test__use_of_delete_function.cpp:6:7: note: ‘A::A(const A&)’ is implicitly deleted because the default definition would be ill-formed:
 class A
       ^
test__use_of_delete_function.cpp:6:7: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’
In file included from /usr/include/c++/8/mutex:43,
                 from test__use_of_delete_function.cpp:3:
/usr/include/c++/8/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;
     ^~~~~
In file included from /usr/include/c++/8/vector:62,
                 from test__use_of_delete_function.cpp:2:
/usr/include/c++/8/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = A; _Args = {A}]’:
/usr/include/c++/8/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<A*>; _ForwardIterator = A*; bool _TrivialValueTypes = false]’:
/usr/include/c++/8/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<A*>; _ForwardIterator = A*]’:
/usr/include/c++/8/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::move_iterator<A*>; _ForwardIterator = A*; _Tp = A]’:
/usr/include/c++/8/bits/stl_uninitialized.h:311:2:   required from ‘_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = A*; _ForwardIterator = A*; _Allocator = std::allocator<A>]’:
/usr/include/c++/8/bits/vector.tcc:447:6:   required from ‘void std::vector<_Tp, _Alloc>::_M_realloc_insert(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const A&}; _Tp = A; _Alloc = std::allocator<A>; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<A*, std::vector<A> >; typename std::_Vector_base<_Tp, _Alloc>::pointer = A*]’:
/usr/include/c++/8/bits/stl_vector.h:1085:4:   required from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = A; _Alloc = std::allocator<A>; std::vector<_Tp, _Alloc>::value_type = A]’:
test__use_of_delete_function.cpp:19:17:   required from here
/usr/include/c++/8/bits/stl_construct.h:75:7: error: use of deleted function ‘A::A(A&&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test__use_of_delete_function.cpp:6:7: note: ‘A::A(A&&)’ is implicitly deleted because the default definition would be ill-formed:
 class A
       ^
test__use_of_delete_function.cpp:6:7: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’
In file included from /usr/include/c++/8/mutex:43,
                 from test__use_of_delete_function.cpp:3:
/usr/include/c++/8/bits/std_mutex.h:97:5: note: declared here
     mutex(const mutex&) = delete;
     ^~~~~

错误原因 #

std::mutex的拷贝构造函数被显式删除（mutex(const mutex&) = delete;），C++编译器原本会为类自动生成默认的拷贝/移动构造函数，但如果类成员包含不可拷贝/移动的对象，编译器会隐式删除类的默认拷贝构造函数和移动构造函数。

当调用std::vector::push_back(const A&)时，需要拷贝A对象到容器中，但A的拷贝构造函数已被删除，因此触发编译错误。

解决方案 #

方案1：在容器中存储智能指针（推荐）

指针可以被拷贝，而std::mutex属于对象本身，不会被拷贝，同时智能指针能自动管理内存，避免泄漏：

#include <vector>
#include <mutex>
#include <memory>

class A
{
public:
  A(){};
private:
  std::mutex a_mutex;
};

int main()
{
   std::vector<std::unique_ptr<A>> v;
   // 直接在容器内构造对象
   v.push_back(std::make_unique<A>());
   // 或者转移外部对象的所有权
   // auto a = std::make_unique<A>();
   // v.push_back(std::move(a));
   return 0;
}

如果需要共享对象所有权，可以使用std::shared_ptr替代std::unique_ptr。

方案2：直接在容器内构造对象

使用emplace_back直接在容器内存中构造A对象，完全避免拷贝/移动操作：

#include <vector>
#include <mutex>

class A
{
public:
  A(){};
private:
  std::mutex a_mutex;
};

int main()
{
   std::vector<A> v;
   // 直接在vector的内存空间中构造A对象
   v.emplace_back();
   return 0;
}

这种方式适用于不需要先创建外部对象的场景。

方案3：手动实现类A的拷贝/移动构造函数（不推荐）

由于std::mutex不能被拷贝，需要为拷贝构造函数创建新的互斥锁，需确保逻辑符合业务需求：

#include <vector>
#include <mutex>

class A
{
public:
  A(){};
  // 拷贝构造：为新对象创建独立的mutex
  A(const A&) : a_mutex() {}
  // 移动构造：同样需要手动实现，因为mutex不可移动
  A(A&&) noexcept : a_mutex() {}
private:
  std::mutex a_mutex;
};

int main()
{
   std::vector<A> v;
   A a;
   v.push_back(a);
   return 0;
}

注意：拷贝后的对象拥有独立互斥锁，若你的业务逻辑需要共享锁，这种方式会引发线程安全问题。

内容的提问来源于stack exchange，提问作者user180574