lock
unique_lock provide a way to defer lock and lock can lock two mutex at the same time to avoid dead-lock.
#include <mutex>
#include <thread>
#include <chrono>
struct Box {
explicit Box(int num) : num_things{num} {}
int num_things;
std::mutex m;
};
void transfer(Box &from, Box &to, int num)
{
// don't actually take the locks yet
std::unique_lock<std::mutex> lock1(from.m, std::defer_lock);
std::unique_lock<std::mutex> lock2(to.m, std::defer_lock);
// lock both unique_locks without deadlock
std::lock(lock1, lock2);
from.num_things -= num;
to.num_things += num;
// 'from.m' and 'to.m' mutexes unlocked in 'unique_lock' dtors
}
int main()
{
Box acc1(100);
Box acc2(50);
std::thread t1(transfer, std::ref(acc1), std::ref(acc2), 10);
std::thread t2(transfer, std::ref(acc2), std::ref(acc1), 5);
t1.join();
t2.join();
}
And it can transfer lock ownership to the caller since it can be movable.
std::unique_lock<std::mutex> get_lock()
{
extern std::mutex some_mutex;
std::unique_lock<std::mutex> lk(some_mutex);
prepare_data();
return lk;
}
void process_data()
{
std::unique_lock<std::mutex> lk(get_lock());
do_something();
}
And unique_lock support lock, unlock method, you can release lock earlier if you don't need lock protection before unique_lock object destroy.
void get_and_process_data()
{
std::unique_lock<std::mutex> my_lock(the_mutex);
some_class data_to_process=get_next_data_chunk();
my_lock.unlock();
result_type result=process(data_to_process);
my_lock.lock();
write_result(data_to_process,result);
}
From unique_lock ref
page revision: 6, last edited: 03 Nov 2013 12:57