常量变量在标题中不起作用

或者你也可以只将非定义声明放入头文件中，然后将定义放入一个(并且只有一个)实现文件中

Or you can put mere non-defining declarations into the header file and put the definitions into one (and only one) implementation file

header 文件中的声明必须包含显式的 extern 并且没有初始化器

The declarations in the header file must include an explicit extern and no initializer

extern const double PI; 
extern const double PI_under_180; 
extern const double PI_over_180; 

和一个实现文件中的定义应如下所示

and definitions in one implementation file should look as follows

const double PI = 3.1415926535; 
const double PI_under_180 = 180.0f / PI; 
const double PI_over_180 = PI/180.0f; 

(定义中的显式 extern 是可选的，如果上述声明位于同一翻译单元中的定义之前).

(explicit extern in the definitions is optional, if the above declarations precede the definitions in the same translation unit).

您将选择哪种方法取决于您的意图.

Which method you will choose depends on your intent.

第一种方法使编译器更容易优化代码，因为它可以看到每个翻译单元中常量的实际值.但同时在概念上你会在每个翻译单元中获得单独的、独立的常量对象.例如，&PI 将评估为每个翻译单元中的不同地址.

The first method makes it easier for the compiler to optimize the code, since it can see the actual value of the constant in each translation unit. But at the same time conceptually you get separate, independent constant objects in every translation unit. For example, &PI will evaluate to a different address in each translation unit.

第二种方法创建真正的全局常量，即整个程序共享的唯一常量对象.例如，&PI 将评估为每个翻译单元中的相同地址.但在这种情况下，编译器只能看到一个且只有一个翻译单元中的实际值，这可能会妨碍优化.

The second method creates truly global constants, i.e. unique constant objects that are shared by the entire program. For example, &PI will evaluate to the same address in each translation unit. But in this case the compiler can only see the actual values in one and only one translation unit, which might impede optimizations.

从 C++17 开始，您将获得第三个选项，它结合了两全其美":内联变量.尽管有外部链接，但内联变量可以安全地定义在头文件中

Starting from C++17 you get the third option, which sort of combines "the best of both worlds": inline variables. Inline variables can be safely defined in header files despite having external linkage

inline extern const double PI = 3.1415926535; 
inline extern const double PI_under_180 = 180.0f / PI; 
inline extern const double PI_over_180 = PI/180.0f; 

在这种情况下，您将获得一个命名常量对象，其初始值设定项在所有翻译单元中都是可见的.同时对象具有外部链接，即它具有全局地址标识(&PI 在所有翻译单元中都相同).

In this case you get a named constant object whose initializer value is visible in all translation units. And at the same time the object has external linkage, i.e. it has a global address identity (&PI is the same in all translation units).

当然，类似的东西可能只在某些特殊目的下才需要(C++ 中的大多数用例都调用第一个变体)，但该功能就在那里.

Granted, something like that might only be necessary for some exotic purposes (most use cases in C++ call for the first variant), but the feature is there.

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