the difference between unique_lock and lock_guard

unique_lock 和 lock_guard 都提供RAII style的编程方式，就是在构造函数中加锁，在析构函数中解锁。
lock_guard 只有构造函数和析构函数，不提供其他成员方法。
unique_lock 提供成员函数形式的lock和unlock
condition_varible的lock参数必须是unique_lock

lock_guard

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

result:

g_i: 0; in main()
g_i: 1; in thread #140487981209344
g_i: 2; in thread #140487972816640
g_i: 2; in main()

unique_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();
}