std::lock_guard

Defined in header `<mutex>`
template< class Mutex > 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= [deleted]	not copy-assignable (public member function)

Example

Run this code

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

Possible output:

main: 0
140641306900224: 1
140641298507520: 2
main: 2

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

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