std::mutex 作为 class 成员，并将 class 对象存储到容器

Question

下面是重现错误的最少代码。

#include <iostream>
#include <mutex>
#include <vector>

class A {
    std::mutex mutex;
    public:
    A(){};
};
int main() 
{
    std::vector<std::pair<std::string,A>> aa;
    A a;
    //aa.push_back(std::make_pair(std::string("aa"),A()));
    //aa.push_back(std::make_pair(std::string("aa"),a));
    aa.push_back(std::make_pair(std::string("aa"),std::move(a)));    
}

以下是错误。

Microsoft (R) C/C++ Optimizing Compiler Version 19.16.27026.1 for x64 Copyright (C) Microsoft Corporation. All rights reserved.

>   C:\Program Files (x86)\Microsoft Visual
> Studio17\Community\VC\Tools\MSVC.16.27023\include\xlocale(319):
> warning C4530: C++ exception handler used, but unwind semantics are
> not enabled. Specify /EHsc    C:\Program Files (x86)\Microsoft Visual
> Studio17\Community\VC\Tools\MSVC.16.27023\include\utility(405):
> error C2440: '<function-style-cast>': cannot convert from 'initializer
> list' to '_Mypair'    C:\Program Files (x86)\Microsoft Visual
> Studio17\Community\VC\Tools\MSVC.16.27023\include\utility(405):
> note: No constructor could take the source type, or constructor
> overload resolution was ambiguous
>   ..\examples\json_object\json.cpp(16): note: see reference to function
> template instantiation 'std::pair<std::string,A>
> std::make_pair<std::string,A>(_Ty1 &&,_Ty2 &&)' being compiled            with
>           [
>               _Ty1=std::string,
>               _Ty2=A          ]

gcc 编译器出现类似错误。当我从 class 中删除 std::mutex 或不将对象推到 std::vector 上时，它可以正常编译。

Answer 1

根据 std::mutex 上的文档。

std::mutex is neither copyable nor movable.

由于classA包含一个std::mutex变量mutex，所以也不可移动

Answer 2

正如 P.W 所指出的和 freakish 提供的提示，我想出了以下解决方案。

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

class A {
    std::mutex mutex;
    public:
    A(){};
};
int main() 
{
    std::vector<std::pair<std::string,std::shared_ptr<A>>> aa;
    A a;
    //aa.push_back(std::make_pair(std::string("aa"),A()));
    //aa.push_back(std::make_pair(std::string("aa"),a));
    aa.push_back(std::make_pair(std::string("aa"),std::make_shared<A>()));   
}

我修改了容器以存储对象的智能指针而不是对象本身。

Answer 3

作为P.W。指出，std::mutex 既不可复制也不可移动，这是有充分理由的。拥有互斥锁的全部意义在于防止对某些数据同时进行多线程访问。 move操作本身需要保护，mutex应该被move操作使用

以下示例为 class 提供了一些可移动数据，并展示了如何在移动操作中使用互斥锁（复制操作类似）：

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

class A {
public:
  A() {};

  // Move constructor
  A(A&& other) {
    std::lock_guard<std::mutex> guard(other.m_mutex);
    m_data = std::move(other.m_data);
  }

  // Move operator
  A& operator=(A&& other) {
    if (this == &other) return *this;

    // Lock this and other in a consistent order to prevent deadlock
    std::mutex* first;
    std::mutex* second;
    if (this < &other) {
      first = &this->m_mutex;
      second = &other.m_mutex;
    } else {
      first = &other.m_mutex;
      second = &this->m_mutex;
    }
    std::lock_guard<std::mutex> guard1(*first);
    std::lock_guard<std::mutex> guard2(*second);

    // Now both this and other are safe to access.  Do the actual data move.
    m_data = std::move(other.m_data);
    return *this;
  }

private:
  std::mutex m_mutex;
  std::unique_ptr<int> m_data;
};

int main() {
  std::vector<std::pair<std::string,A>> aa;
  A a1;
  A a2;
  a1 = std::move(a2);
  aa.emplace_back("aa", std::move(a1));
}

std::mutex 作为 class 成员，并将 class 对象存储到容器

std::mutex as class member, and store class obect to container

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