NAME¶

std::lock_guard - std::lock_guard

Synopsis¶

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

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= not copy-assignable
[deleted] (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 (since C++17)
algorithm.

Example¶

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

// Run this code

#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 removed
lock_guard<Mutex> was provided

See also¶

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