std::shared_lock<Mutex>:: lock

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

#include <iostream>
#include <mutex>
#include <shared_mutex>
#include <string>
#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();
}

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