NAME¶

std::shared_timed_mutex - std::shared_timed_mutex

Synopsis¶

Defined in header <shared_mutex>
class shared_timed_mutex; (since C++14)

The shared_timed_mutex class is a synchronization primitive that can be used to
protect shared data from being simultaneously accessed by multiple threads. In
contrast to other mutex types which facilitate exclusive access, a
shared_timed_mutex has two levels of access:

* exclusive - only one thread can own the mutex.

* shared - several threads can share ownership of the same mutex.

Shared mutexes are usually used in situations when multiple readers can access the
same resource at the same time without causing data races, but only one writer can
do so.

In a manner similar to timed_mutex, shared_timed_mutex provides the ability to
attempt to claim ownership of a shared_timed_mutex with a timeout via the
try_lock_for(), try_lock_until(), try_lock_shared_for(), try_lock_shared_until()
member functions.

The shared_timed_mutex class satisfies all requirements of SharedTimedMutex and
StandardLayoutType.

Member functions¶

constructor constructs the mutex
(public member function)
destructor destroys the mutex
(public member function)
operator= not copy-assignable
[deleted] (public member function)
Exclusive locking
lock locks the mutex, blocks if the mutex is not available
(public member function)
try_lock tries to lock the mutex, returns if the mutex is not available
(public member function)
tries to lock the mutex, returns if the mutex has been
try_lock_for unavailable for the specified timeout duration
(public member function)
tries to lock the mutex, returns if the mutex has been
try_lock_until unavailable until specified time point has been reached
(public member function)
unlock unlocks the mutex
(public member function)

Shared locking¶

locks the mutex for shared ownership, blocks if the mutex is
lock_shared not available
(public member function)
tries to lock the mutex for shared ownership, returns if the
try_lock_shared mutex is not available
(public member function)
tries to lock the mutex for shared ownership, returns if the
try_lock_shared_for mutex has been
unavailable for the specified timeout duration
(public member function)
tries to lock the mutex for shared ownership, returns if the
try_lock_shared_until mutex has been
unavailable until specified time point has been reached
(public member function)
unlock_shared unlocks the mutex (shared ownership)
(public member function)

Notes¶

Feature-test macro: __cpp_lib_shared_timed_mutex

Example¶

This section is incomplete
Reason: build a motivating example

A copy assignment operator for a class that holds resources that can handle multiple
readers, but only one writer

// Run this code

#include <mutex>
#include <shared_mutex>

class R
{
mutable std::shared_timed_mutex mut;
/* data */
public:
R& operator=(const R& other)
{
// requires exclusive ownership to write to *this
std::unique_lock<std::shared_timed_mutex> lhs(mut, std::defer_lock);
// requires shared ownership to read from other
std::shared_lock<std::shared_timed_mutex> rhs(other.mut, std::defer_lock);
std::lock(lhs, rhs);
/* assign data */
return *this;
}
};

int main() {
R r;
}