std::shared_mutex Usage and Characteristics

std::shared_mutex is a reader contention aware lock.

When exclusive access is pending due to existing shared access, No more shared access can be acquired. This is the “draining” behavior.

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

using namespace std;

int main()
{
	std::shared_mutex mutex;
	cout << "Hello World" << endl;
	int counter = 0;

	std::thread t1([&]() {
		while (true) {
			cout << "BEGIN T1 acquire shared" << endl;
			mutex.lock_shared();
			cout << "DONE T1 acquire shared" << endl;

			this_thread::sleep_for(chrono::seconds(5));

			cout << "BEGIN T1 unlock shared" << endl;
			mutex.unlock_shared();
			cout << "DONE T1 unlock shared" << endl;

			this_thread::sleep_for(chrono::seconds(5));
		}
		});
	std::thread t2([&]() {
		this_thread::sleep_for(chrono::milliseconds(2500));
		while (true) {
			cout << "BEGIN T2 acquire shared" << endl;
			mutex.lock_shared();
			cout << "DONE T2 acquire shared" << endl;

			this_thread::sleep_for(chrono::seconds(5));

			cout << "BEGIN T2 unlock shared" << endl;
			mutex.unlock_shared();
			cout << "DONE T2 unlock shared" << endl;

			this_thread::sleep_for(chrono::seconds(5));
		}
		});

	string input;
	while (cin >> input) {
		if (input == "1") {
			cout << "BEGIN MAIN acquire exclusive" << endl;
			mutex.lock();
			cout << "END MAIN acquire exclusive" << endl;
		}
		else if (input == "2") {
			cout << "BEGIN MAIN release exclusive" << endl;
			mutex.unlock();
			cout << "END MAIN release exclusive" << endl;
		}
	}

	t1.join();
	t2.join();
	return 0;
}

Output:

Hello World
BEGIN T1 acquire shared
DONE T1 acquire shared
1
BEGIN MAIN acquire exclusive
BEGIN T2 acquire shared
BEGIN T1 unlock shared
DONE T1 unlock shared
END MAIN acquire exclusive
BEGIN T1 acquire shared