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<H2>6.3 Example Program: An Inventory System</H2>
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<P>Let's use a simple inventory management system to illustrate the use of several <B><I><A HREF="../stdlibref/list.html">list</A></I></B> operations. Assume a business, named <I>WorldWideWidgetWorks</I>, requires a software system to manage their supply of widgets. </P>
<BLOCKQUOTE><HR><B>
NOTE -- The complete version of the widget works program is in file widwork.cpp.
</B><HR></BLOCKQUOTE>
<P>Widgets are simple devices, distinguished by different identification numbers:</P>

<UL><PRE>
class  Widget {
public:
  Widget(int a = 0) : id(a) { }
  void operator=(const Widget&amp; rhs) { id = rhs.id; }
  int id;
  friend std::ostream&amp; operator&lt;&lt;
    (std::ostream&amp; out, const Widget&amp; w)
    { return out &lt;&lt; "Widget " &lt;&lt; w.id; }
};

bool operator== (const Widget&amp; lhs, const Widget&amp; rhs) {
  return lhs.id == rhs.id;
}

bool operator&lt; (const Widget&amp; lhs, const Widget&amp; rhs) {
  return lhs.id &lt; rhs.id;
}
</PRE></UL>
<P>The state of the inventory is represented by two <B><I><A HREF="../stdlibref/list.html">list</A></I></B>s: a <B><I>list</I></B> of widgets represents the stock of widgets on hand, and a <B><I>list</I></B> of widget identification types represents the type of widgets that customers have backordered. </P>

<UL><PRE>
typedef std::list&lt;Widget, std::allocator&lt;Widget&gt; &gt; widgetList;
typedef std::list&lt;int, std::allocator&lt;int&gt; &gt;       idList;
</PRE></UL>
<P>To handle our inventory we have two commands: <SAMP>order()</SAMP> processes orders, and <SAMP>receive()</SAMP> processes the shipment of a new widget.</P>

<UL><PRE>
class inventory {
public:
  void order (int wid);   // process order for widget type wid
  void receive (int wid); // receive widget of type wid 
                          // in shipment
private:
  widgetList on_hand;
  idList     on_order;
};
</PRE></UL>
<P>When a new widget arrives in shipment, we compare the widget identification number with the list of widget types on backorder. We use <SAMP>find()</SAMP> to search the backorder list, immediately shipping the widget if necessary. Otherwise it is added to the stock on hand:</P>

<UL><PRE>
void inventory::receive (int wid) {
  std::cout &lt;&lt; "Received shipment of widget type "
            &lt;&lt; wid &lt;&lt; std::endl;

  idList::iterator weneed = 
    find(on_order.begin(), on_order.end(), wid); 

  if (weneed != on_order.end()) {
    std::cout &lt;&lt; "Ship " &lt;&lt; Widget(wid) 
              &lt;&lt; " to fill back order" &lt;&lt; std::endl;
    on_order.erase(weneed);
  }
  else
    on_hand.push_front(Widget(wid));
}
</PRE></UL>
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<P>When a customer orders a new widget, we scan the list of widgets in stock to determine if the order can be processed immediately. We can use the standard algorithm <SAMP>std::find_if()</SAMP> to search the list. To do so, we need a binary function that takes as its argument a widget and determines whether the widget matches the type requested. We create this function by taking a general binary widget-testing function, and binding the second argument to the specific widget type. To use the function <SAMP>std::bind2nd()</SAMP>, however, requires that the binary function be an instance of the class <B><I><A HREF="../stdlibref/binary-function.html">binary_function</A></I></B>. The general widget-testing function is written as follows:</P>

<UL><PRE>
struct WidgetTester : std::binary_function&lt;Widget, int, bool&gt; {

public:
  bool operator()(const Widget&amp; wid, int testid) const
  {
    return wid.id == testid;
  }

};
</PRE></UL>
<P>The widget order function is then written as follows:</P>

<UL><PRE>
void inventory::order(int wid) {

  std::cout &lt;&lt; "Received order for widget type " &lt;&lt; wid
            &lt;&lt; std::endl;

  widgetList::iterator wehave = 
    std::find_if(on_hand.begin(), on_hand.end(),
                 std::bind2nd(WidgetTester(), wid));

    if (wehave != on_hand.end()) {
      std::cout &lt;&lt; "Ship " &lt;&lt; *wehave &lt;&lt; std::endl;
      on_hand.erase(wehave);
   }
   else {
     std::cout &lt;&lt; "Back order widget of type "  &lt;&lt; wid
               &lt;&lt; std::endl;
     on_order.push_front(wid);
   }

}
</PRE></UL>

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