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std::lock_guard

Defined in header `<mutex>`
template< class Mutex > class lock_guard;		(since C++11)

The class lock_guard is a mutex wrapper that provides a convenient RAII-style mechanism for owning a mutex for the duration of a scoped block.

When a lock_guard object is created, it attempts to take ownership of the mutex it is given. When control leaves the scope in which the lock_guard object was created, the lock_guard is destructed and the mutex is released.

The lock_guard class is non-copyable.

Template parameters

Mutex

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

Member types

Member type	Definition
`mutex_type`	Mutex

Member functions

(constructor)	constructs a lock_guard, optionally locking the given mutex (public member function)
(destructor)	destructs the lock_guard object, unlocks the underlying mutex (public member function)
operator= [deleted]	not copy-assignable (public member function)

Notes

std::scoped_lock offers a replacement for lock_guard that provides the ability to lock multiple mutexes using a deadlock avoidance algorithm.

(since C++17)

Example

#include <thread>
#include <mutex>
#include <iostream>
 
int g_i = 0;
std::mutex g_i_mutex;  // protects g_i
 
void safe_increment()
{
    const std::lock_guard<std::mutex> lock(g_i_mutex);
    ++g_i;
 
    std::cout << "g_i: " << g_i << "; in thread #"
              << std::this_thread::get_id() << '\n';
 
    // g_i_mutex is automatically released when lock
    // goes out of scope
}
 
int main()
{
    std::cout << "g_i: " << g_i << "; in main()\n";
 
    std::thread t1(safe_increment);
    std::thread t2(safe_increment);
 
    t1.join();
    t2.join();
 
    std::cout << "g_i: " << g_i << "; in main()\n";
}

Possible output:

g_i: 0; in main()
g_i: 1; in thread #140487981209344
g_i: 2; in thread #140487972816640
g_i: 2; in main()

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 `lock_guard<Mutex>` was provided	removed

unique_lock (C++11)	implements movable mutex ownership wrapper (class template)
scoped_lock (C++17)	deadlock-avoiding RAII wrapper for multiple mutexes (class template)