33 Concurrency support library [thread]

33.2 Requirements [thread.req]

33.2.5 Requirements for Cpp17Lockable types [thread.req.lockable] In general [thread.req.lockable.general]

An execution agent is an entity such as a thread that may perform work in parallel with other execution agents.
[Note 1: 
Implementations or users can introduce other kinds of agents such as processes or thread-pool tasks.
— end note]
The calling agent is determined by context, e.g., the calling thread that contains the call, and so on.
[Note 2: 
Some lockable objects are “agent oblivious” in that they work for any execution agent model because they do not determine or store the agent's ID (e.g., an ordinary spin lock).
— end note]
The standard library templates unique_lock ([thread.lock.unique]), shared_lock ([thread.lock.shared]), scoped_lock ([thread.lock.scoped]), lock_guard ([thread.lock.guard]), lock, try_lock ([thread.lock.algorithm]), and condition_variable_any ([thread.condition.condvarany]) all operate on user-supplied lockable objects.
The Cpp17BasicLockable requirements, the Cpp17Lockable requirements, the Cpp17TimedLockable requirements, the Cpp17SharedLockable requirements, and the Cpp17SharedTimedLockable requirements list the requirements imposed by these library types in order to acquire or release ownership of a lock by a given execution agent.
[Note 3: 
The nature of any lock ownership and any synchronization it entails are not part of these requirements.
— end note]
A lock on an object m is said to be
  • a non-shared lock if it is acquired by a call to lock, try_lock, try_lock_for, or try_lock_until on m, or
  • a shared lock if it is acquired by a call to lock_shared, try_lock_shared, try_lock_shared_for, or try_lock_shared_until on m.
[Note 4: 
Only the method of lock acquisition is considered; the nature of any lock ownership is not part of these definitions.
— end note]