thread_local int tls_var;

In the above code, tls_var is declared as a thread-local variable. Each thread will have its own tls_var variable, and changes made to tls_var in one thread will not affect the tls_var in another thread.

This can be useful when you want to store thread-specific data, such as thread IDs or thread-local caches.

References:

• C++ thread_local specifier