std::shared_lock::lock

From cppreference.com
< cpplrm; | threadlrm; | shared lock
Thread support library
Threads
(C++11)
this_thread namespace
(C++11)
(C++11)
(C++11)
Mutual exclusion
(C++11)
Generic lock management
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Condition variables
(C++11)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
void lock();
(since C++14)

Locks the associated mutex in shared mode. Effectively calls mutex()->lock_shared().

Parameters

(none)

Return value

(none)

Exceptions

  • Any exceptions thrown by mutex()->lock_shared()

Example

#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();
}

Possible output:

Two readers reading from file.
A writer competes with them.
Contents read by reader #1: Original content.
Contents read by reader #2: Original content.
New content saved.
The first few operations to file are done.
Contents read by reader #3: New content

See also

tries to lock the associated mutex
(public member function)
unlocks the associated mutex
(public member function)