33 Thread support library [thread]

33.2 Requirements [thread.req]

33.2.5 Requirements for Lockable types [thread.req.lockable]

33.2.5.1 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
:
Implementations or users may 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
:
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 BasicLockable requirements, the Lockable requirements, and the TimedLockable 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
:
The nature of any lock ownership and any synchronization it may entail are not part of these requirements.
end note
]

33.2.5.2 BasicLockable requirements [thread.req.lockable.basic]

A type L meets the BasicLockable requirements if the following expressions are well-formed and have the specified semantics (m denotes a value of type L).
m.lock()
Effects: Blocks until a lock can be acquired for the current execution agent.
If an exception is thrown then a lock shall not have been acquired for the current execution agent.
m.unlock()
Requires: The current execution agent shall hold a lock on m.
Effects: Releases a lock on m held by the current execution agent.
Throws: Nothing.

33.2.5.3 Lockable requirements [thread.req.lockable.req]

A type L meets the Lockable requirements if it meets the BasicLockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L).
m.try_lock()
Effects: Attempts to acquire a lock for the current execution agent without blocking.
If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.

33.2.5.4 TimedLockable requirements [thread.req.lockable.timed]

A type L meets the TimedLockable requirements if it meets the Lockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L, rel_­time denotes a value of an instantiation of duration, and abs_­time denotes a value of an instantiation of time_­point).
m.try_lock_for(rel_time)
Effects: Attempts to acquire a lock for the current execution agent within the relative timeout ([thread.req.timing]) specified by rel_­time.
The function shall not return within the timeout specified by rel_­time unless it has obtained a lock on m for the current execution agent.
If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.
m.try_lock_until(abs_time)
Effects: Attempts to acquire a lock for the current execution agent before the absolute timeout ([thread.req.timing]) specified by abs_­time.
The function shall not return before the timeout specified by abs_­time unless it has obtained a lock on m for the current execution agent.
If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.