What's the proper way to associate a mutex with its data?

Question

In the classic problem of transferring money from one bank account to another, the accepted solution (I believe) is to associate a mutex with each bank account, then lock both b

Answer 1

There's nothing wrong with having the money "in flight" for a while. Make it like so:

Account src, dst;

dst.deposit(src.withdraw(400));

Now just make each individual method thread-safe, e.g.

int Account::withdraw(int n)
{
    std::lock_guard _(m_);
    balance -= n;
    return n;
}