6 Basics [basic]

6.6 Memory and objects [basic.memobj]

6.6.5 Storage duration [basic.stc] Thread storage duration [basic.stc.thread]

All variables declared with the thread_­local keyword have thread storage duration.
The storage for these entities shall last for the duration of the thread in which they are created.
There is a distinct object or reference per thread, and use of the declared name refers to the entity associated with the current thread.
A variable with thread storage duration shall be initialized before its first odr-use and, if constructed, shall be destroyed on thread exit.