为何C++17标准库中的辅助变量模板要标记为inline？

Great question! It’s totally reasonable to wonder why those inline specifiers are there—after all, constexpr variables have internal linkage by default, so they wouldn’t trigger One-Definition Rule (ODR) violations even without it. Let’s break down the key reasons the standard includes them:

• Guarantee consistent identity across translation units
  Without inline, every translation unit (TU) that uses is_same_v<T,U> would get its own distinct instance of the variable (thanks to internal linkage). If someone ever takes the address of one of these variables (e.g., &is_same_v<int, int>), the address would differ between TUs. Marking them inline ensures all TUs reference the same global instance, so their addresses are identical—this matches the intuitive expectation that these are single, global constant values.

• Design consistency and future-proofing
  inline variables were a new feature in C17, and using them for these type trait variables aligns the standard library with broader language patterns for shared global entities. It also acts as a safeguard: if future revisions to C ever adjust the default linkage rules for constexpr variables, the inline specifier ensures the behavior of these type traits remains stable and predictable.

• Compatibility with pre-standard implementations
  Before C++17 standardized these _v variable templates, many third-party libraries (like Boost) provided their own equivalent utilities, often using inline to enable cross-TU consistency. By including inline in the standard, code migrating from these libraries to the standard library avoids unexpected changes to linkage or instance identity behavior.

A quick side note: While constexpr gives internal linkage by default, inline overrides this to provide external linkage while still allowing multiple identical definitions across TUs—this is exactly the same behavior as inline functions.

内容的提问来源于stack exchange，提问作者Lingxi