std::shared_lock::lock

C++

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Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Thread support library

Threads
thread (C++11)
hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

std::shared_lock

Member functions

shared_lock::shared_lock

shared_lock::~shared_lock

shared_lock::operator=

Shared locking

shared_lock::lock

shared_lock::try_lock

shared_lock::try_lock_for

shared_lock::try_lock_until

shared_lock::unlock

Modifiers

shared_lock::swap

shared_lock::release

Observers

shared_lock::mutex

shared_lock::owns_lock

shared_lock::operator bool

Non-member functions

std::swap(shared_lock)

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()

• If there is no associated mutex, std::system_error with an error code of std::errc::operation_not_permitted

• If the associated mutex is already locked by this shared_lock (that is, owns_lock returns true), std::system_error with an error code of std::errc::resource_deadlock_would_occur

Example

Run this code

#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

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