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Difference between revisions of "cpp/thread/unique lock"

From cppreference.com
< cpp‎ | thread
m (fmt, headers sorted, Box & -> Box&)
(c -> tt.)
 
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}}
 
}}
  
−
The class {{c|unique_lock}} is a general-purpose mutex ownership wrapper allowing deferred locking, time-constrained attempts at locking, recursive locking, transfer of lock ownership, and use with condition variables.  
+
The class {{tt|unique_lock}} is a general-purpose mutex ownership wrapper allowing deferred locking, time-constrained attempts at locking, recursive locking, transfer of lock ownership, and use with condition variables.  
  
−
The class {{c|unique_lock}} is movable, but not copyable -- it meets the requirements of {{named req|MoveConstructible}} and {{named req|MoveAssignable}} but not of {{named req|CopyConstructible}} or {{named req|CopyAssignable}}.
+
The class {{tt|unique_lock}} is movable, but not copyable -- it meets the requirements of {{named req|MoveConstructible}} and {{named req|MoveAssignable}} but not of {{named req|CopyConstructible}} or {{named req|CopyAssignable}}.
  
−
The class {{c|unique_lock}} meets the {{named req|BasicLockable}} requirements. If '''Mutex''' meets the {{named req|Lockable}} requirements, {{c|unique_lock}} also meets the {{named req|Lockable}} requirements (ex.: can be used in {{lc|std::lock}}); if '''Mutex''' meets the {{named req|TimedLockable}} requirements, {{c|unique_lock}} also meets the {{named req|TimedLockable}} requirements.
+
The class {{tt|unique_lock}} meets the {{named req|BasicLockable}} requirements. If {{tt|Mutex}} meets the {{named req|Lockable}} requirements, {{tt|unique_lock}} also meets the {{named req|Lockable}} requirements (ex.: can be used in {{lc|std::lock}}); if {{tt|Mutex}} meets the {{named req|TimedLockable}} requirements, {{tt|unique_lock}} also meets the {{named req|TimedLockable}} requirements.
  
 
===Template parameters===
 
===Template parameters===
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{{par end}}
 
{{par end}}
  
−
===Member types===
+
===Nested types===
 
{{dsc begin}}
 
{{dsc begin}}
 
{{dsc hitem|Type|Definition}}
 
{{dsc hitem|Type|Definition}}
−
{{dsc|{{tt|mutex_type}}|{{c|Mutex}}}}
+
{{dsc|{{tt|mutex_type}}|{{tt|Mutex}}}}
 
{{dsc end}}
 
{{dsc end}}
  
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{
 
{
 
     explicit Box(int num) : num_things{num} {}
 
     explicit Box(int num) : num_things{num} {}
−
 
+
   
 
     int num_things;
 
     int num_things;
 
     std::mutex m;
 
     std::mutex m;
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     std::unique_lock lock1{from.m, std::defer_lock};
 
     std::unique_lock lock1{from.m, std::defer_lock};
 
     std::unique_lock lock2{to.m, std::defer_lock};
 
     std::unique_lock lock2{to.m, std::defer_lock};
−
 
+
   
 
     // lock both unique_locks without deadlock
 
     // lock both unique_locks without deadlock
 
     std::lock(lock1, lock2);
 
     std::lock(lock1, lock2);
−
 
+
   
 
     from.num_things -= num;
 
     from.num_things -= num;
 
     to.num_things += num;
 
     to.num_things += num;
−
 
+
   
−
     // 'from.m' and 'to.m' mutexes unlocked in 'unique_lock' dtors
+
     // “from.m” and “to.m” mutexes unlocked in unique_lock dtors
 
}
 
}
  
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     Box acc1{100};
 
     Box acc1{100};
 
     Box acc2{50};
 
     Box acc2{50};
−
 
+
   
 
     std::thread t1{transfer, std::ref(acc1), std::ref(acc2), 10};
 
     std::thread t1{transfer, std::ref(acc1), std::ref(acc2), 10};
 
     std::thread t2{transfer, std::ref(acc2), std::ref(acc1), 5};
 
     std::thread t2{transfer, std::ref(acc2), std::ref(acc1), 5};
−
 
+
   
 
     t1.join();
 
     t1.join();
 
     t2.join();
 
     t2.join();
−
 
+
   
 
     std::cout << "acc1: " << acc1.num_things << "\n"
 
     std::cout << "acc1: " << acc1.num_things << "\n"
 
                 "acc2: " << acc2.num_things << '\n';
 
                 "acc2: " << acc2.num_things << '\n';

Latest revision as of 23:31, 27 February 2024

 
C++
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++17)(C++17)
this_thread namespace
(C++11)
(C++11)
(C++11)
(C++11)
Cooperative cancellation
Mutual exclusion
(C++11)
(C++11)
(C++17)
Generic lock management
(C++11)
(C++11)
(C++17)
unique_lock
(C++11)
(C++14)
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
Condition variables
(C++11)
(C++11)
(C++11)
(C++11)
Semaphores
(C++20)(C++20)
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
(C++26)
Hazard Pointers
(C++26)
(C++26)
(C++26)
Atomic types
(C++11)
(C++20)
(C++11)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
(C++11)
Memory ordering
(C++11)
(C++11)
(C++11)
(C++11)
Free functions for atomic operations
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++26)(C++26)
(C++26)(C++26)
(C++11)
(C++20)(C++20)
(C++20)
(C++20)
Free functions for atomic flags
(C++11)(C++11)
(C++11)(C++11)
(C++20)(C++20)
(C++20)(C++20)
(C++20)
(C++20)
 
std::unique_lock
 
Defined in header <mutex>
template< class Mutex >
class unique_lock;
 (since C++11)

The class unique_lock is a general-purpose mutex ownership wrapper allowing deferred locking, time-constrained attempts at locking, recursive locking, transfer of lock ownership, and use with condition variables.

The class unique_lock is movable, but not copyable -- it meets the requirements of MoveConstructible and MoveAssignable but not of CopyConstructible or CopyAssignable.

The class unique_lock meets the BasicLockable requirements. If Mutex meets the Lockable requirements, unique_lock also meets the Lockable requirements (ex.: can be used in std::lock); if Mutex meets the TimedLockable requirements, unique_lock also meets the TimedLockable requirements.

Contents

[edit] Template parameters

Mutex - the type of the mutex to lock. The type must meet the BasicLockable requirements

[edit] Nested types

Type Definition
mutex_type Mutex

[edit] Member functions

 constructs a unique_lock, optionally locking (i.e., taking ownership of) the supplied mutex
(public member function) [edit]
unlocks (i.e., releases ownership of) the associated mutex, if owned
(public member function) [edit]
unlocks (i.e., releases ownership of) the mutex, if owned, and acquires ownership of another
(public member function) [edit]
Locking
locks (i.e., takes ownership of) the associated mutex
(public member function) [edit]
tries to lock (i.e., takes ownership of) the associated mutex without blocking
(public member function) [edit]
attempts to lock (i.e., takes ownership of) the associated TimedLockable mutex, returns if the mutex has been unavailable for the specified time duration
(public member function) [edit]
tries to lock (i.e., takes ownership of) the associated TimedLockable mutex, returns if the mutex has been unavailable until specified time point has been reached
(public member function) [edit]
unlocks (i.e., releases ownership of) the associated mutex
(public member function) [edit]
Modifiers
swaps state with another std::unique_lock
(public member function) [edit]
disassociates the associated mutex without unlocking (i.e., releasing ownership of) it
(public member function) [edit]
Observers
returns a pointer to the associated mutex
(public member function) [edit]
tests whether the lock owns (i.e., has locked) its associated mutex
(public member function) [edit]
tests whether the lock owns (i.e., has locked) its associated mutex
(public member function) [edit]

[edit] Non-member functions

(C++11)
 specializes the std::swap algorithm
(function template) [edit]

[edit] Example

Run this code
#include <iostream>
#include <mutex>
#include <thread>
 
struct Box
{
    explicit Box(int num) : num_things{num} {}
 
    int num_things;
    std::mutex m;
};
 
void transfer(Box& from, Box& to, int num)
{
    // don't actually take the locks yet
    std::unique_lock lock1{from.m, std::defer_lock};
    std::unique_lock lock2{to.m, std::defer_lock};
 
    // lock both unique_locks without deadlock
    std::lock(lock1, lock2);
 
    from.num_things -= num;
    to.num_things += num;
 
    // “from.m” and “to.m” mutexes unlocked in unique_lock dtors
}
 
int main()
{
    Box acc1{100};
    Box acc2{50};
 
    std::thread t1{transfer, std::ref(acc1), std::ref(acc2), 10};
    std::thread t2{transfer, std::ref(acc2), std::ref(acc1), 5};
 
    t1.join();
    t2.join();
 
    std::cout << "acc1: " << acc1.num_things << "\n"
                 "acc2: " << acc2.num_things << '\n';
}

Output: 

acc1: 95
acc2: 55

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2981 C++17 redundant deduction guide from unique_lock<Mutex> was provided removed

[edit] See also

(C++11)
 locks specified mutexes, blocks if any are unavailable
(function template) [edit]
(C++11)
 implements a strictly scope-based mutex ownership wrapper
(class template) [edit]
(C++17)
 deadlock-avoiding RAII wrapper for multiple mutexes
(class template) [edit]
(C++11)
 provides basic mutual exclusion facility
(class) [edit]
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