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.
mutex_type Mutex [edit] Member functions constructs a lock_guard, optionally locking the given mutex
lock_guard object, unlocks the underlying mutex
A common beginner error is to forget to give a lock_guard variable a name, such as by std::lock_guard{mtx}. This 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.
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[edit] 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 fromlock_guard<Mutex> was provided removed [edit] See also implements movable mutex ownership wrapper
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