一个静态constexpr变量的自初始化，它是否形式？ [英] Self-initialization of a static constexpr variable, is it well-formed?

问题描述

给定全局命名空间中的以下声明：

Given the following declaration in the global namespace:

constexpr int x = x;

这是否格式良好？

草稿C ++ 14标准版面 3.6.2 [basic.start.init] 说：

The draft C++14 standard section 3.6.2 [basic.start.init] says:

在任何其他初始化发生之前，具有静态存储持续时间（3.7.1）或线程存储持续时间（3.7.2）的变量应该是零初始化（8.5）
。 [...]

Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place. [...]

看起来好像定义的例子是 x 在常数初始化期间用其自己的值初始化，由于零初始化，它将 0 。

What seems to make the example well defined is that x is initialized with its own value during constant initialization which will be 0 due to zero initialization.

真的是这样吗？ clang 接受此代码，而 gcc产生诊断：

Is this really the case? clang accepts this code while gcc produces a diagnostic:

error: the value of 'x' is not usable in a constant expression
constexpr int x = x;
                  ^

推荐答案

由缺陷报告2026：Zero-initialization and constexpr 其中询问：

根据3.6.2 [basic.start.init]第2段，

According to 3.6.2 [basic.start.init] paragraph 2,

具有静态存储持续时间的变量（3.7.1 [basic.stc.static]）或
线程存储持续时间（3.7.2 [basic.stc.thread ]）在任何其他初始化
发生之前应为
零初始化（8.5 [dcl.init]）。

Variables with static storage duration (3.7.1 [basic.stc.static]) or thread storage duration (3.7.2 [basic.stc.thread]) shall be zero-initialized (8.5 [dcl.init]) before any other initialization takes place.

这是否也适用于常量初始化？例如，
应该根据在常数初始化之前假定的
零初始化，以下形式良好？

Does this apply to constant initialization as well? For example, should the following be well-formed, relying on the presumed zero-initialization preceding the constant initialization?

constexpr int i = i;
struct s {
  constexpr s() : v(v) { }
  int v;
};
constexpr s s1;

建议的解决方案前面的注释说：

The note before the proposed resolution says:

CWG同意在这些情况下应将常量初始化视为发生，而不是零初始化，使声明不成形。

CWG agreed that constant initialization should be considered as happening instead of zero initialization in these cases, making the declarations ill-formed.

并且建议的解决方案澄清并在许多更改中删除以下措辞：

and the proposed resolution clarifies and amongst many changes, removes the following wording:

在任何其他初始化发生之前，具有静态存储持续时间（3.7.1）或线程存储持续时间（3.7.2）的变量应该被初始化为（8.5）
。 [...]

Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place. [...]

并添加以下措辞：

如果不执行常量初始化，则具有静态存储持续时间（3.7.1 [basic.stc.static]）或线程存储持续时间（3.7.2 [basic.stc.thread]）的变量为零，初始化（8.5 [dcl.init]）。 [...]

If constant initialization is not performed, a variable with static storage duration (3.7.1 [basic.stc.static]) or thread storage duration (3.7.2 [basic.stc.thread]) is zero-initialized (8.5 [dcl.init]). [...]

这是一个很大的变化，它重命名 [basic.start.init]到 [basic.start.static] 并创建了一个新的 [basic.start.dynamic] 部分并修改 [stmt.dcl]

It is a large change, it renames [basic.start.init] to [basic.start.static] and created a new section [basic.start.dynamic] and modifies [stmt.dcl]

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