Lock-free pop-operation

template <class T>
bool CASStack<T>::pop(T& ret) {
    node<T>* old_head = head.load(std::memory_order_relaxed);

    if(old_head == nullptr) {
        return false;
    }

    // from here on we can assume that there is an element to pop
    node<T>* new_head;

    do {
        new_head = old_head->next;
    } while(!head.compare_exchange_weak(old_head, new_head, std::memory_order_acquire, std::memory_order_relaxed));

    ret = old_head->data;

    return true;
}