Any specific reason for variable templates in the standard library be inline?

Asked Apr 03 '18 at 13:36

Active Apr 03 '18 at 13:37

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C++17 introduced various helper variable templates, e.g.,

template< class T, class U >
inline constexpr bool is_same_v = is_same<T, U>::value;

template< class T >
inline constexpr bool is_aggregate_v = is_aggregate<T>::value;

They are all marked inline. Since constexpr implies internal linkage by default [ref], even without inline, the one definition rule will not be violated. So I don't see inline necessary here. Neither do I see any benefit it brings. So why inline?

edited Apr 03 '18 at 13:37

asked Apr 03 '18 at 13:36

Lingxi

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Do note I marked it as a duplicate because of the example in [this answer](https://stackoverflow.com/a/48041749/817643). Oliv describes how an ODR violation can happen in a hurry. – StoryTeller - Unslander Monica Apr 03 '18 at 13:41
@StoryTeller Finally, got to understood the example the second time I read it. What a tricky one! – Lingxi Apr 03 '18 at 13:47

Any specific reason for variable templates in the standard library be inline?

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