替换常量：什么时候使用静态constexpr和内联constexpr？ [英] Replacing constants: when to use static constexpr and inline constexpr?

问题描述

此问题是对 C ++ 17的后续问题：仍使用枚举作为常量？。

旧式常量有几种形式，尤其是：

Legacy constants come in several forms, notably:

• #define CONSTANT x


• enum {CONSTANT = x};


• const / * int / unsigned / whatever * / CONSTANT = x;

• #define CONSTANT x
• enum { CONSTANT = x };
• const /*int/unsigned/whatever*/ CONSTANT = x;

关于静态constexpr 和 inline constexpr 常量的评论作为替换得到了我在考虑更新许多旧式常量（尤其是 #define 常量）的主题。

A comment about static constexpr and inline constexpr constants as a replacement got me thinking on the subject of updating our many, many legacy constants (particularly #define constants).

可以理解，内联constexpr 值基本上只是被替换，就像内联函数一样（我被证明是错误的）。相反，静态constexpr 值作为二进制文件的一部分存储在单独的区域中。假设我理解正确，什么时候应该优先选择另一个？我的直觉是，对于整数常量，通常首选内联constexpr 。

As I understand, an inline constexpr value is basically just substituted in place, like an inlined function (which I've been shown to be wrong about). Conversely, a static constexpr value is stored as part of the binary in a separate area. Assuming I understand correctly, when should one be preferred over the other? My hunch is that, for integral constants, inline constexpr will generally be preferred.

推荐答案

在C ++ 17中，替换名称空间范围标头中的那些旧习惯用法（例如 #define ）的正确方法是使用 constexpr内联变量-和 not static （这意味着：它们已经具有内部链接）了。

In C++17, the proper way to replace those old idioms (e.g. #define) in headers in namespace scope is to use constexpr inline variables -- and not static (which is implied: they already have internal linkage).

虽然通常不会遇到ODR问题（因为您所描述的整数编译时常数很少使用ODR，并且在 inline 函数），现在最好将它们标记为 inline ，因为我们已经使用了该功能并避免了所有问题。

While typically you won't encounter ODR issues (because integer compile-time constants such as those you describe are rarely ODR-used and there is a provision for their typical usage within inline functions), it is best to mark them as inline now that we have the feature in the language and avoid all problems.

请参见标头中的const和constexpr变量应内联`以防止违反ODR？以获得有关它的技术详细信息。

See Should `const` and `constexpr` variables in headers be `inline` to prevent ODR violations? for the technical details about it.

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