Template:cpp/thread/example lock tag

#include <iostream>
#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)
{
    if (&from == &to) // avoid deadlock in case of self transfer
        return;

    // lock both mutexes without deadlock
    std::lock(from.m, to.m);
    // make sure both already-locked mutexes are unlocked at the end of scope
    std::lock_guard lock1{from.m, std::adopt_lock};
    std::lock_guard lock2{to.m, std::adopt_lock};

// equivalent approach:
//  std::unique_lock<std::mutex> lock1{from.m, std::defer_lock};
//  std::unique_lock<std::mutex> lock2{to.m, std::defer_lock};
//  std::lock(lock1, lock2);

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

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

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

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

    std::cout << "my_account.balance = " << my_account.balance << "\n"
                 "your_account.balance = " << your_account.balance << '\n';
}