adoptlock

Adopt_lock is a C++ standard library tag used with locking constructors to indicate that a mutex is already locked by the current thread and that a lock object should adopt that existing ownership rather than performing a new lock operation. It is defined in the mutex facilities as std::adopt_lock_t and is exposed as the tag object std::adopt_lock.

Usage typically involves std::unique_lock, not std::lock_guard. A common pattern is to acquire the mutex through some means, then transfer ownership to a unique_lock without locking again:

std::mutex m;

m.lock();

std::unique_lock<std::mutex> ul(m, std::adopt_lock);

In this case, the unique_lock takes ownership of the already-held lock and will release the mutex when it is destroyed. The current thread must indeed own the mutex when adopting; otherwise the behavior is undefined.

Adopt_lock is often employed to transfer lock ownership between guards or to interface with functions that require a std::unique_lock while the lock has been acquired by other code paths. It is one of several std::unique_lock constructor tag options, alongside defer_lock (no locking) and try_to_lock (attempt to lock).

Key points:

- Adopt_lock signals that the mutex is already locked by the calling thread and should be adopted by the new lock object.

- It does not perform a locking operation.

- The resulting lock object will unlock the mutex upon destruction.

- It is specific to std::unique_lock and to the standard mutex facilities in <mutex>.

- Correct use requires that the current thread owns the mutex at the time of adoption.