33 Thread support library [thread]

33.3 Threads [thread.threads]

33.3.2 Class thread [thread.thread.class]

33.3.2.6 thread static members [thread.thread.static]

unsigned hardware_concurrency() noexcept;
Returns: The number of hardware thread contexts.
[Note
:
This value should only be considered to be a hint.
end note
]
If this value is not computable or well-defined, an implementation should return 0.