std :: atomic_flag作为成员变量 [英] std::atomic_flag as member variable

问题描述

在类构造函数中初始化 std :: atomic_flag 的安全方法是什么？

What is a safe way to initialize an std::atomic_flag in a class constructor?

这个问题 似乎在问我同样的问题-除了在此询问者抱怨编译器问题外。

This question seems to be asking the same question I'm asking - except here the asker is complaining about a compiler problem.

我的问题与C ++标准本身有关。根据 此网站 ，初始化<$ c未指定使用构造函数初始化程序语法的$ c> std :: atomic_flag 。

My question relates to the C++ standard itself. According to this site, initializing an std::atomic_flag using constructor initializer syntax is unspecified.

std::atomic_flag static_flag = ATOMIC_FLAG_INIT; // static initialization,
// guaranteed to be available during dynamic initialization of static objects.

int main()
{
    std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT; // guaranteed to work
//    std::atomic_flag another_flag(ATOMIC_FLAG_INIT); // unspecified
}

此信息正确吗？如果是这样，我认为：

Is this information correct? If so, I assume that:

struct Foo
{
  Foo() : flag(ATOMIC_FLAG_INIT)
  { }

  std::atomic_flag flag;
};

...也未指定。因此，这是否意味着我们不能使用 std :: atomic_flag 作为类的成员变量？还是从类构造函数中简单调用 std :: atomic_flag :: clear（）是安全的吗？

...is also unspecified. So, does this mean that we can't use an std::atomic_flag as a member variable of a class? Or is it safe if we simple call std::atomic_flag::clear() from within a class constructor?

推荐答案

从N3337到N3936（当前的C ++ 14草案）以来，与 ATOMIC_FLAG_INIT 的使用有关的措词已更改。前者在示例中显示了 ATOMIC_FLAG_INIT 宏在复制初始化上下文中的可能用法，这是非规范性的，并且未提及有关内部使用的任何内容。其他初始化上下文。

The wording concerning use of ATOMIC_FLAG_INIT has changed since N3337 to N3936 (the current C++14 draft). The former shows a possible usage, in a copy-initialization context, of the ATOMIC_FLAG_INIT macro in an example, which are non-normative, and doesn't mention anything about uses within other initialization contexts.

N3936阐明了用法，不再作为示例列出复制初始化用法，而是作为描述本身的一部分。

N3936 clarifies the usage, and no longer lists the copy-initialization usage as an example, but as part of the description itself.

§29.7/ 4 [atomics.flag]

宏 ATOMIC_FLAG_INIT 的定义方式应使其可用于
初始化 atomic_flag 类型的对象到清晰的状态。该宏可以
的形式使用：

The macro ATOMIC_FLAG_INIT shall be defined in such a way that it can be used to initialize an object of type atomic_flag to the clear state. The macro can be used in the form:

 atomic_flag guard = ATOMIC_FLAG_INIT;

尚不确定该宏是否可以在其他初始化上下文中使用。对于完整的静态持续时间对象，该初始化应为静态。除非使用 ATOMIC_FLAG_INIT 初始化，否则无法确定 atomic_flag 对象的初始状态为set还是clear。

It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static. Unless initialized with ATOMIC_FLAG_INIT, it is unspecified whether an atomic_flag object has an initial state of set or clear.

在此处讨论了这些更改的理由。

The rationale for these changes is discussed here.

所以您说对了，不能依赖在成员初始值设定项列表中使用宏。解决方案是使用非静态数据成员初始化程序或
brace-or-equal-initializer 初始化 atomic_flag 。然后将在副本初始化上下文中对其进行初始化。

So you're right that using the macro in the member initializer list cannot be relied upon. The solution is to use a non-static data member initializer, or brace-or-equal-initializer, to initialize the atomic_flag. Then it will be initialized in a copy initialization context.

struct Foo
{
  std::atomic_flag flag = ATOMIC_FLAG_INIT;
};

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