< cpp‎ | thread
Thread support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Defined in header <mutex>
class mutex;
(since C++11)

The mutex class is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads.

mutex offers exclusive, non-recursive ownership semantics:

  • A calling thread owns a mutex from the time that it successfully calls either lock or try_lock until it calls unlock.
  • When a thread owns a mutex, all other threads will block (for calls to lock) or receive a false return value (for try_lock) if they attempt to claim ownership of the mutex.
  • A calling thread must not own the mutex prior to calling lock or try_lock.

The behavior of a program is undefined if a mutex is destroyed while still owned by any threads, or a thread terminates while owning a mutex. The mutex class satisfies all requirements of Mutex and StandardLayoutType.

std::mutex is neither copyable nor movable.


[edit] Member types

Member type Definition
native_handle_type(optional) implementation-defined

[edit] Member functions

constructs the mutex
(public member function) [edit]
destroys the mutex
(public member function) [edit]
not copy-assignable
(public member function) [edit]
locks the mutex, blocks if the mutex is not available
(public member function) [edit]
tries to lock the mutex, returns if the mutex is not available
(public member function) [edit]
unlocks the mutex
(public member function) [edit]
Native handle
returns the underlying implementation-defined native handle object
(public member function) [edit]

[edit] Notes

std::mutex is usually not accessed directly: std::unique_lock, std::lock_guard, or std::scoped_lock (since C++17) manage locking in a more exception-safe manner.

[edit] Example

This example shows how a mutex can be used to protect a std::map shared between two threads.

#include <iostream>
#include <map>
#include <string>
#include <chrono>
#include <thread>
#include <mutex>
std::map<std::string, std::string> g_pages;
std::mutex g_pages_mutex;
void save_page(const std::string &url)
    // simulate a long page fetch
    std::string result = "fake content";
    std::lock_guard<std::mutex> guard(g_pages_mutex);
    g_pages[url] = result;
int main() 
    std::thread t1(save_page, "http://foo");
    std::thread t2(save_page, "http://bar");
    // safe to access g_pages without lock now, as the threads are joined
    for (const auto &pair : g_pages) {
        std::cout << pair.first << " => " << pair.second << '\n';


http://bar => fake content
http://foo => fake content