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)


A common beginner error is to "forget" to give a lock_guard variable a name, e.g. std::lock_guard(mtx); (which default constructs a lock_guard variable named mtx) or std::lock_guard{mtx}; (which constructs a prvalue object that is immediately destroyed), thereby not actually constructing a lock that holds a mutex for the rest of the scope.

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

(since C++17)


Demonstrates safe and unsafe increments of a volatile variable by two threads.

#include <iostream>
#include <mutex>
#include <string_view>
#include <syncstream>
#include <thread>
volatile int g_i = 0;
std::mutex g_i_mutex;  // protects g_i
void safe_increment(int iterations)
    const std::lock_guard<std::mutex> lock(g_i_mutex);
    while (iterations-- > 0)
        g_i = g_i + 1;
    std::cout << "thread #" << std::this_thread::get_id() << ", g_i: " << g_i << '\n';
    // g_i_mutex is automatically released when lock goes out of scope
void unsafe_increment(int iterations)
    while (iterations-- > 0)
        g_i = g_i + 1;
    std::osyncstream(std::cout) << "thread #" << std::this_thread::get_id()
                                << ", g_i: " << g_i << '\n';
int main()
    auto test = [](std::string_view fun_name, auto fun)
        g_i = 0;
        std::cout << fun_name << ":\nbefore, g_i: " << g_i << '\n';
            std::jthread t1(fun, 1'000'000);
            std::jthread t2(fun, 1'000'000);
        std::cout << "after, g_i: " << g_i << "\n\n";
    test("safe_increment", safe_increment);
    test("unsafe_increment", unsafe_increment);

Possible output:

before, g_i: 0
thread #140121493231360, g_i: 1000000
thread #140121484838656, g_i: 2000000
after, g_i: 2000000
before, g_i: 0
thread #140121484838656, g_i: 1028945
thread #140121493231360, g_i: 1034337
after, g_i: 1034337

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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