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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

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)

Example

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:

safe_increment:
before, g_i: 0
thread #140121493231360, g_i: 1000000
thread #140121484838656, g_i: 2000000
after, g_i: 2000000
 
unsafe_increment:
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

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