类型不完整的C ++静态constexpr字段 [英] C++ static constexpr field with incomplete type

问题描述

我正在尝试编译此代码，但是g ++抱怨 0 的类型不完整。这是否意味着在C ++中，结构体不能包含自身的静态constexpr 实例？如果是，为什么？

I'm trying to compile this code, but g++ complains about ZERO having an incomplete type. Does this mean that in C++ a struct cannot contain a static constexpr instance of itself? If so, why?

struct Cursor
{
    size_t row,column;

    static constexpr Cursor ZERO {0,0};
    //error: constexpr const Cursor Cursor::ZERO has incomplete type
};

编辑：我知道 Cursor 不能具有当我声明 0 时为完整类型。我想知道的是：有什么办法可以使 0 属于 Cursor 并仍然是 constexpr ？

I understand that Cursor cannot have a complete type when I declare ZERO. What I'd like to know is: is there any way I can have ZERO belonging to Cursor and still being constexpr?

推荐答案

不幸的是，您根本无法做到这一点！

Unfortunately, you simply cannot do this!

某些静态constexpr 成员可以内联初始化：

Some static constexpr members may be initialised inline:

[C ++ 11 9.4.2 / 3]： [..] A 静态可以使用 constexpr 说明符在类定义中声明文字类型的数据成员；如果是这样，则其声明应指定一个
brace-or-equal-initializer ，其中每个 assignment-expression 的 initializer-clause 常数表达式。 [..]

[C++11 9.4.2/3]: [..] A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [..]

Cursor 是一个

以及 Cursor 本身作为静态变量的使用自己的类型中的数据成员不是问题，只要您在词法命名空间范围内对其进行初始化：

And the use of Cursor itself as a static data member within its own type is not a problem, as long as you initialise it at lexical namespace scope:

[C ++ 11：9.4.2 / 2]： 静态数据成员的声明在其类中的定义不是定义，并且可能是不完整的类型，而不是具有cv限定的void。 静态数据成员的定义应出现在包含该成员的类定义的名称空间范围中。在名称空间范围的定义中， 静态数据成员应使用 :: 运算符通过其类名进行限定。 静态数据成员的定义中的初始化器表达式在其类的范围内（3.3.7） 。

[C++11: 9.4.2/2]: The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member shall appear in a namespace scope enclosing the member’s class definition. In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the :: operator. The initializer expression in the definition of a static data member is in the scope of its class (3.3.7).

但是您不能使用 constexpr 做到这一点：

But you can't do that with constexpr:

[C ++ 11：7.1.5 / 9]： A <$ c $对象声明中使用的c> constexpr 说明符将对象声明为 const 。这样的对象应具有文字类型并应进行初始化。 [..]

[C++11: 7.1.5/9]: A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized. [..]

我认为所有这些措辞都可以得到改善，但与此同时，我认为您将必须使零成为封闭名称空间中的非成员。

I think all of this wording could be improved but, in the meantime, I think you're going to have to make ZERO a non-member in the enclosing namespace.

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