在VC ++中的1个原子操作中，将值设置为POD（不具有构造函数/结构体）

问题描述

我试图找到一种方法来设置没有构造函数/析构函数（int，byte，long等）作为原子操作的POD。

我需要的原因是因为我想使用__declspec（线程）为了有一个TLSint作为线程本地存储。事情是，目前的VC ++编译器不支持具有构造函数/析构函数的对象，所以我声明：

  static __declspec ）int _thread_val; 
  

接下来，我要设置 _thread_val

使用std :: atomic不工作，因为它有构造函数/析构函数，因此不支持：

  static __declspec（thread）std :: atomic< int> _thread_val; //不在VC ++中编译
  

我也在VC内部函数中查找，

有没有人知道如何做？

解决方案

不幸的是，Microsoft的实现 std :: atomic< T> 是不符合通过没有一个琐碎的默认构造函数。你可以通过使用包装类来解决这个问题。因为你知道 std :: atomic< T> 在没有被构造时应该是可用的（因为你知道它应该是一个POD类型） ，也不需要破坏，你可以使用 std :: aligned_storage 来保存对象。这是一个起点，它可能需要轻微调整取决于你会遇到的编译器特定的怪癖。

  template< typename T> 
 class atomic_wrapper 
 {
 std :: aligned_storage< sizeof（std :: atomic< T>），alignof（std :: atomic&数据; 
 public：
 atomic_wrapper（）= default; 
 atomic_wrapper（const atomic_wrapper< T>&）= delete; 
 std :: atomic< T> & operator *（）{return reinterpret_cast< std :: atomic< T> &>（data）; } 
 std :: atomic< T> * operator  - >（）{return& ** this; } 
}; 
  

现在，不是将变量声明为 std :: atomic< int> a; ，声明为 atomic_wrapper< int> a; ，并更新您的访问以使用 * a 而不是 a p>

I am trying to find a way to set a POD that doesn't have constructor/destructor (int, byte, long etc.) as atomic operation.

The reason I need that is because I want to use __declspec(thread) in order to have a TLS "int" as a thread local storage. The thing is that current VC++ compiler doesn't support objects that have constructor/destructor, so I declared:

static __declspec(thread) int _thread_val;

Next, I want to set that _thread_val atomically.

Using std::atomic doesn't work because it has constructor/destructor, therefore it is not supported:

static __declspec(thread) std::atomic<int> _thread_val; //doesn't compile in VC++

I also looked in VC intrinsic functions, but didn't find exactly what I wanted (guess due to lack of documentation).

Does anyone know how to do it?

解决方案

It is unfortunate that Microsoft's implementation of std::atomic<T> is non-conforming by not having a trivial default constructor. You may be able to work around that by using a wrapper class. Since you know that std::atomic<T> is supposed to be usable when it hasn't been constructed (since you know that it is supposed to be a POD type), and doesn't require destruction either, you can use std::aligned_storage to hold the object. Here's a starting point, it may need slight tweaking depending on the compiler-specific quirks you'll encounter.

template <typename T>
class atomic_wrapper
{
    std::aligned_storage<sizeof(std::atomic<T>), alignof(std::atomic<T>)> data;
public:
    atomic_wrapper() = default;
    atomic_wrapper(const atomic_wrapper<T> &) = delete;
    std::atomic<T> &operator *() { return reinterpret_cast<std::atomic<T> &>(data); }
    std::atomic<T> *operator ->() { return &**this; }
};

Now, instead of declaring a variable as std::atomic<int> a;, declare it as atomic_wrapper<int> a;, and update your accesses to use *a instead of a.

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