NAME

std::shared_lock::lock - std::shared_lock::lock

Synopsis

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

This section is incomplete
Reason: show a meaningful use of shared_lock::lock

// 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_content(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_content()
{
{
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_content, 1};
std::thread reader2{read_content, 2};
std::thread writer{write_content};
reader1.join();
reader2.join();
writer.join();
std::cout << "The first few operations to file are done.\n";
reader1 = std::thread{read_content, 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