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<HTML>
<HEAD>
<TITLE>Example Program: An Inventory System</TITLE>
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<A HREF="6-2.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="7.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
<H2>6.3 Example Program: An Inventory System</H2>
<A NAME="idx119"><!></A>
<A NAME="idx120"><!></A>
<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>
<A NAME="idx121"><!></A>
<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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