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| <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& rhs) { id = rhs.id; } |
| int id; |
| friend std::ostream& operator<< |
| (std::ostream& out, const Widget& w) |
| { return out << "Widget " << w.id; } |
| }; |
| |
| bool operator== (const Widget& lhs, const Widget& rhs) { |
| return lhs.id == rhs.id; |
| } |
| |
| bool operator< (const Widget& lhs, const Widget& rhs) { |
| return lhs.id < 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<Widget, std::allocator<Widget> > widgetList; |
| typedef std::list<int, std::allocator<int> > 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 << "Received shipment of widget type " |
| << wid << std::endl; |
| |
| idList::iterator weneed = |
| find(on_order.begin(), on_order.end(), wid); |
| |
| if (weneed != on_order.end()) { |
| std::cout << "Ship " << Widget(wid) |
| << " to fill back order" << 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<Widget, int, bool> { |
| |
| public: |
| bool operator()(const Widget& 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 << "Received order for widget type " << wid |
| << 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 << "Ship " << *wehave << std::endl; |
| on_hand.erase(wehave); |
| } |
| else { |
| std::cout << "Back order widget of type " << wid |
| << std::endl; |
| on_order.push_front(wid); |
| } |
| |
| } |
| </PRE></UL> |
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