////   This file contains the C++ code from Program 4.18 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C++"//   by Bruno R. Preiss.////   Copyright (c) 1998 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.pads.uwaterloo.ca/Bruno.Preiss/books/opus4/programs/pgm04_18.cpp//template <class T>void LinkedList<T>::Extract (T const& item){    ListElement<T>* ptr = head;    ListElement<T>* prevPtr = 0;    while (ptr != 0 && ptr->datum != item)    {	prevPtr = ptr;	ptr = ptr->next;    }    if (ptr == 0)	throw invalid_argument ("item not found");    if (ptr == head)	head = ptr->next;    else	prevPtr->next = ptr->next;    if (ptr == tail)	tail = prevPtr;    delete ptr;}