# std::shared_lock

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Defined in header `<shared_mutex>` |  |
template< class Mutex > class shared_lock; |  | (since C++14)

The class `shared_lock` is a general-purpose shared mutex ownership wrapper allowing deferred locking, timed locking and transfer of lock ownership. Locking a `shared_lock` locks the associated shared mutex in shared mode (to lock it in exclusive mode, std::unique_lock can be used)

The `shared_lock` class is movable, but not copyable -- it meets the requirements of MoveConstructible and MoveAssignable but not of CopyConstructible or CopyAssignable.

In order to wait in a shared mutex in shared ownership mode, std::condition_variable_any can be used (std::condition_variable requires std::unique_lock and so can only wait in unique ownership mode)

### Template parameters

Mutex | - | the type of the shared mutex to lock. The type must meet the SharedMutex requirements

### Member types

Type | Definition
`mutex_type` | Mutex

### Member functions

(constructor) | constructs a `shared_lock`, optionally locking the supplied mutex (public member function)
(destructor) | unlocks the associated mutex (public member function)
operator= | unlocks the mutex, if owned, and acquires ownership of another (public member function)

##### Shared locking

lock | locks the associated mutex (public member function)
try_lock | tries to lock the associated mutex (public member function)
try_lock_for | tries to lock the associated mutex, for the specified duration (public member function)
try_lock_until | tries to lock the associated mutex, until a specified time point (public member function)
unlock | unlocks the associated mutex (public member function)

##### Modifiers

swap | swaps the data members with another shared_lock (public member function)
release | disassociates the mutex without unlocking (public member function)

##### Observers

mutex | returns a pointer to the associated mutex (public member function)
owns_lock | tests whether the lock owns its associated mutex (public member function)
operator bool | tests whether the lock owns its associated mutex (public member function)

### Non-member functions

std::swap(std::shared_lock) | specialization of std::swap for `shared_lock` (function template)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2981 | C++17 | redundant deduction guide from `shared_lock<Mutex>` was provided | removed

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