std::shared_lock<Mutex>::lock

#include <iostream>
#include <mutex>
#include <string>
#include <shared_mutex>
#include <thread>
 
std::string file = "Original content."; // Simulates a file
std::mutex output_mutex; // mutex that protects output operations.
std::shared_mutex file_mutex; // reader/writer mutex
 
void read(int id)
{
    std::string content;
    {
        std::shared_lock lock(file_mutex, std::defer_lock); // Do not lock it first.
        lock.lock(); // Lock it here.
        content = file;
    }
    std::lock_guard lock(output_mutex);
    std::cout << "Contents read by reader #" << id << ": " << content << '\n';
}
 
void write()
{
    {
        std::lock_guard file_lock(file_mutex);
        file = "New content";
    }
    std::lock_guard output_lock(output_mutex);
    std::cout << "New content saved.\n";
}
 
int main()
{
    std::cout << "Two readers reading from file.\n"
              << "A writer competes with them.\n";
    std::thread reader1(read, 1);
    std::thread reader2(read, 2);
    std::thread writer(write);
    reader1.join();
    reader2.join();
    writer.join();
    std::cout << "The first few operations to file are done.\n";
    reader1 = std::thread(read, 3);
    reader1.join();
}