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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_1003</title> <link rel="stylesheet" href="reset.css" type="text/css" media="all"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> </head> <body> <table summary="top nav" border="0" width="100%"> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1002.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1003</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1004.html">next page ></a></td> </tr> <tr> <td align="left" colspan="3" style="background: #ffffff; padding: 20px;"> <table border="0" width="100%" cellpadding="0"><tr><td align="center"> <table border="0" cellpadding="2" cellspacing="0" width="100%"><tr><td align="left"></td> <td align="right"></td> </tr></table></td></tr><tr><td align="left"><p></p><table border="0" cellspacing="0" cellpadding="0" width="100%"><tr><td align="right"><font face="Times New Roman, Times, Serif" size="2" color="#FF0000">Page 1003</font></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Notice that the record list object is an implementation-level object. This object is not readily apparent in the problem domain, yet we need it in order for the operation of sorting to make any sense. (Sort is not an operation on an individual personnel record; it is an operation on a <i>list</i> of records.)</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The second major step in OOD is to determine if there are any inheritance or composition relationships among the objects. Using <i>is-a</i> as a guide, we do not find any inheritance relationships. However, the record list object and the personnel record object clearly are related by composition. That is, a record list <i>has-a</i> personnel record within it (in fact, it probably contains many personnel records). Discovery of this relationship helps us as we proceed to design and implement each object.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">masterFile</font><font face="Times New Roman, Times, Serif" size="3"> Object:</font><font face="Times New Roman, Times, Serif" size="3"> For the concrete representation of this object, we can use the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ifstream</font><font face="Times New Roman, Times, Serif" size="3"> class supplied by the standard library. Then the abstract operations of opening a file and reading data are naturally implemented by using the operations already provided by the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ifstream</font><font face="Times New Roman, Times, Serif" size="3"> class.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The Personnel Record Object: We could implement this object by using a C++ class, hiding the data members as private data and supplying public operations for reading and writing the members. (Also, we might need to provide observer and transformer operations that retrieve and store the values of individual members.) Instead, let's treat this particular object as passive data (in the form of a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> with all members public) rather than as an active object with associated operations. We'll use the struct declarations shown earlier (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">AddressType</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData</font><font face="Times New Roman, Times, Serif" size="3">), and we can implement the reading and writing operations by using </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">>></font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3"><<</font><font face="Times New Roman, Times, Serif" size="3">operators to input and output individual members of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3">.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The Record List Object: This object represents a list of personnel records. Thinking of a list as an ADT, we can use a C++ class to conceal the private data representation and provide public operations to read records into the list, sort the list, and print the list.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">To choose a concrete data representation for the list, we remember the composition relationship we proposednamely, that a record list is composed of one or more personnel record objects. Therefore, we could use a 1000-element array of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData聽structs</font><font face="Times New Roman, Times, Serif" size="3">, along with an integer variable to keep track of the length of the list. But there are two disadvantages to using an array of structs in this particular problem.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">First, the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData聽structs</font><font face="Times New Roman, Times, Serif" size="3"> are quite large (600 bytes each). If we declare a 1000-element array of these </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">, the compiler reserves 600,000 bytes of memory even though the input file may contain only a few records!</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Second, the act of sorting an array of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> can take a lot of time. Consider the selection sort we introduced in Chapter 12. In the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">SelSort</font><font face="Times New Roman, Times, Serif" size="3"> function, the contents of two variables are swapped during each iteration. Swapping two simple variables is a fast operation. If large </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> are being</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr></table><p><font size="0"></font></p>聽 </td> </tr> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1002.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1003</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1004.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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