std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t

#include <mutex>
#include <thread>

struct bank_account {
    explicit bank_account(int balance) : balance(balance) {}
    int balance;
    std::mutex m;
};

void transfer(bank_account &from, bank_account &to, int amount)
{
    // attempt to lock both mutexes without deadlock
    std::lock(from.m, to.m);

    // make sure both already-locked mutexes are unlocked when
    // we're done; if we just used the lock_guard without std::lock
    // and std::adopt_lock, we might deadlock with other calls to transfer
    std::lock_guard lock1(from.m, std::adopt_lock);
    std::lock_guard lock2(to.m, std::adopt_lock);

    from.balance -= amount;
    to.balance += amount;
}

int main()
{
    bank_account my_account(100);
    bank_account your_account(50);

    std::thread t1(transfer, my_account, your_account, 10);
    std::thread t2(transfer, your_account, my_account, 5);

    t1.join();
    t2.join();
}