NAME

std::shared_mutex - std::shared_mutex

Synopsis

Defined in header <shared_mutex>
class shared_mutex; (since C++17)

The shared_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_mutex has two levels
of access:

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

* exclusive - only one thread can own the mutex.

If one thread has acquired the exclusive lock (through lock, try_lock), no other
threads can acquire the lock (including the shared).

If one thread has acquired the shared lock (through lock_shared, try_lock_shared),
no other thread can acquire the exclusive lock, but can acquire the shared lock.

Only when the exclusive lock has not been acquired by any thread, the shared lock
can be acquired by multiple threads.

Within one thread, only one lock (shared or exclusive) can be acquired at the same
time.

Shared mutexes are especially useful when shared data can be safely read by any
number of threads simultaneously, but a thread may only write the same data when no
other thread is reading or writing at the same time.

The shared_mutex class satisfies all requirements of SharedMutex and
StandardLayoutType.

Member types

Member type Definition
native_handle_type(not always present) implementation-defined

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)
unlock unlocks the mutex
(public member function)

Shared locking

locks the mutex for shared ownership, blocks if the mutex is not
lock_shared available
(public member function)
tries to lock the mutex for shared ownership, returns if the mutex
try_lock_shared is not available
(public member function)
unlock_shared unlocks the mutex (shared ownership)
(public member function)

Native handle

native_handle returns the underlying implementation-defined native handle object
(public member function)

Example

// Run this code

#include <iostream>
#include <mutex>
#include <shared_mutex>
#include <thread>

class ThreadSafeCounter {
public:
ThreadSafeCounter() = default;

// Multiple threads/readers can read the counter's value at the same time.
unsigned int get() const {
std::shared_lock lock(mutex_);
return value_;
}

// Only one thread/writer can increment/write the counter's value.
unsigned int increment() {
std::unique_lock lock(mutex_);
return ++value_;
}

// Only one thread/writer can reset/write the counter's value.
void reset() {
std::unique_lock lock(mutex_);
value_ = 0;
}

private:
mutable std::shared_mutex mutex_;
unsigned int value_ = 0;
};

int main() {
ThreadSafeCounter counter;

auto increment_and_print = [&counter]() {
for (int i = 0; i < 3; i++) {
std::cout << std::this_thread::get_id() << ' ' << counter.increment() << '\n';

// Note: Writing to std::cout actually needs to be synchronized as well
// by another std::mutex. This has been omitted to keep the example small.
}
};

std::thread thread1(increment_and_print);
std::thread thread2(increment_and_print);

thread1.join();
thread2.join();
}

// Explanation: The output below was generated on a single-core machine. When
// thread1 starts, it enters the loop for the first time and calls increment()
// followed by get(). However, before it can print the returned value to
// std::cout, the scheduler puts thread1 to sleep and wakes up thread2, which
// obviously has time enough to run all three loop iterations at once. Back to
// thread1, still in the first loop iteration, it finally prints its local copy
// of the counter's value, which is 1, to std::cout and then runs the remaining
// two loop iterations. On a multi-core machine, none of the threads is put to
// sleep and the output is more likely to be in ascending order.

Possible output:

123084176803584 2
123084176803584 3
123084176803584 4
123084185655040 1
123084185655040 5
123084185655040 6