void withdrawer() {
    unique_lock<mutex> lock(m);
    //  m.lock();  done as part of above constructor
    //  note: you can specify additional parameter and "deferred_lock" to avoid
    //        this
    while (balance < 100) {
        cond.wait(lock);
    }
    withdraw(100);
    m.unlock();
}

void depositer() {
    m.lock();
    deposit(100);
    m.unlock();
    cond.notify_one();
}