C++

Reference

<mutex>

unique_lock

class template
<mutex>

std::unique_lock

template <class Mutex> class unique_lock;
Unique lock
A unique lock is an object that manages a mutex object with unique ownership in both states: locked and unlocked.

On construction (or by move-assigning to it), the object acquires a mutex object, for whose locking and unlocking operations becomes responsible.

The object supports both states: locked and unlocked.

This class guarantees an unlocked status on destruction (even if not called explicitly). Therefore it is especially useful as an object with automatic duration, as it guarantees the mutex object is properly unlocked in case an exception is thrown.

Note though, that the unique_lock object does not manage the lifetime of the mutex object in any way: the duration of the mutex object shall extend at least until the destruction of the unique_lock that manages it.

Template parameters

Mutex
A mutex-like type.
It shall be a basic lockable type, such as mutex (see BasicLockable for requirements).

Member types

member typedefinitiondescription
mutex_typeThe template parameter (Mutex)The managed mutex object type

Member functions

(constructor)
Construct unique_lock (public member function)
(destructor)
Destroy unique_lock (public member function)

Locking/unlocking

lock
Lock mutex (public member function)
try_lock
Lock mutex if not locked (public member function)
try_lock_for
Try to lock mutex during time span (public member function)
try_lock_until
Try to lock mutex until time point (public member function)
unlock
Unlock mutex (public member function)

Modifiers

operator=
Move-assign unique_lock (public member function)
swap
Swap unique locks (public member function)
release
Release mutex (public member function)

Observers

owns_lock
Owns lock (public member function)
operator bool
Return whether it owns a lock (public member function)
mutex
Get mutex (public member function)

Non-member overloads

swap (unique_lock)
Swap locks (function)

Example

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// unique_lock example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock

std::mutex mtx;           // mutex for critical section

void print_block (int n, char c) {
  // critical section (exclusive access to std::cout signaled by lifetime of lck):
  std::unique_lock<std::mutex> lck (mtx);
  for (int i=0; i<n; ++i) { std::cout << c; }
  std::cout << '\n';
}

int main ()
{
  std::thread th1 (print_block,50,'*');
  std::thread th2 (print_block,50,'$');

  th1.join();
  th2.join();

  return 0;
}

Possible output (order of lines may vary, but characters are never mixed):

**************************************************
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$