147 lines
6.6 KiB
HTML
147 lines
6.6 KiB
HTML
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<HTML>
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<HEAD>
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<TITLE>Example Program: An Inventory System</TITLE>
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<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
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<BODY BGCOLOR=#FFFFFF>
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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>
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<H2>6.3 Example Program: An Inventory System</H2>
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<A NAME="idx119"><!></A>
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<A NAME="idx120"><!></A>
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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>
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<BLOCKQUOTE><HR><B>
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NOTE -- The complete version of the widget works program is in file widwork.cpp.
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</B><HR></BLOCKQUOTE>
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<P>Widgets are simple devices, distinguished by different identification numbers:</P>
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<UL><PRE>
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class Widget {
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public:
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Widget(int a = 0) : id(a) { }
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void operator=(const Widget& rhs) { id = rhs.id; }
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int id;
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friend std::ostream& operator<<
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(std::ostream& out, const Widget& w)
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{ return out << "Widget " << w.id; }
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};
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bool operator== (const Widget& lhs, const Widget& rhs) {
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return lhs.id == rhs.id;
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}
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bool operator< (const Widget& lhs, const Widget& rhs) {
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return lhs.id < rhs.id;
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}
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</PRE></UL>
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<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>
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<UL><PRE>
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typedef std::list<Widget, std::allocator<Widget> > widgetList;
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typedef std::list<int, std::allocator<int> > idList;
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</PRE></UL>
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<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>
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<UL><PRE>
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class inventory {
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public:
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void order (int wid); // process order for widget type wid
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void receive (int wid); // receive widget of type wid
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// in shipment
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private:
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widgetList on_hand;
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idList on_order;
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};
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</PRE></UL>
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<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>
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<UL><PRE>
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void inventory::receive (int wid) {
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std::cout << "Received shipment of widget type "
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<< wid << std::endl;
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idList::iterator weneed =
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find(on_order.begin(), on_order.end(), wid);
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if (weneed != on_order.end()) {
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std::cout << "Ship " << Widget(wid)
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<< " to fill back order" << std::endl;
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on_order.erase(weneed);
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}
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else
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on_hand.push_front(Widget(wid));
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}
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</PRE></UL>
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<A NAME="idx121"><!></A>
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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>
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<UL><PRE>
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struct WidgetTester : std::binary_function<Widget, int, bool> {
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public:
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bool operator()(const Widget& wid, int testid) const
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{
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return wid.id == testid;
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}
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};
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</PRE></UL>
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<P>The widget order function is then written as follows:</P>
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<UL><PRE>
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void inventory::order(int wid) {
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std::cout << "Received order for widget type " << wid
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<< std::endl;
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widgetList::iterator wehave =
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std::find_if(on_hand.begin(), on_hand.end(),
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std::bind2nd(WidgetTester(), wid));
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if (wehave != on_hand.end()) {
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std::cout << "Ship " << *wehave << std::endl;
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on_hand.erase(wehave);
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}
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else {
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std::cout << "Back order widget of type " << wid
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<< std::endl;
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on_order.push_front(wid);
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}
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}
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</PRE></UL>
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<BR>
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<HR>
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