As long as a thread never acquires more than one lock at a time, there is no risk of deadlocks. Sometimes, though, it is necessary to acquire another lock while already holding on to a previously acquired lock. The risk of deadlocks in those situations can be avoided by grabbing both locks at the exact same time. C++ has a way to do this by using the std::lock() function, which takes an arbitrary number of locks, and blocks until all locks have been acquired.
The following is an example of transferring money between accounts. Both accounts need to be protected during the transaction, and therefore we need to acquire two locks at the same time. Here is how it works:
struct Account { Account() {} int balance_ = 0; std::mutex ...
