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

constructs a lock_guard, optionally locking the given mutex
(public member function)
destructs the lock_guard object, unlocks the underlying mutex
(public member function)
not copy-assignable
(public member function)


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)


#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);
    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);
    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

See also

implements movable mutex ownership wrapper
(class template)
deadlock-avoiding RAII wrapper for multiple mutexes
(class template)

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