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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_925</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_924.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_925</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_926.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 925</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">When a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> object is created, the constructor for its </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">timeStamp</font><font face="Times New Roman, Times, Serif" size="3"> member is </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">first</font><font face="Times New Roman, Times, Serif" size="3"> invoked. After the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">timeStamp</font><font face="Times New Roman, Times, Serif" size="3"> object is constructed, the body of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard's</font><font face="Times New Roman, Times, Serif" size="3"> constructor is executed, setting the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">id</font><font face="Times New Roman, Times, Serif" size="3"> member equal to </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">idNum</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 second constructor shown in the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> class declarationthe default constructorhas no parameters and could be implemented as follows:</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="Courier New, Courier, Mono New, Courier, Mono" size="2">TimeCard::TimeCard()<br />{<br />聽聽聽聽id聽=聽0;<br />}</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">In this case, what happened to construction of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">timeStamp</font><font face="Times New Roman, Times, Serif" size="3"> member object? We didn't include a constructor initializer, so the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">timeStamp</font><font face="Times New Roman, Times, Serif" size="3"> object is first constructed using the <i>default</i> constructor of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> class, after which the body of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeCard</font><font face="Times New Roman, Times, Serif" size="3"> constructor is executed. The result is a time card having a time stamp of 0:0:0 and an ID number of 0.</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="17"></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">Dynamic Binding and Virtual Functions</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">Early in the chapter, we said that object-oriented programming languages provide language features that support three concepts: data abstraction, inheritance, and dynamic binding. The phrase <i>dynamic binding</i> means, more specifically, <i>dynamic binding of an operation to an object.</i> To explain this concept, let's begin with an example.</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">Given the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ExtTime</font><font face="Times New Roman, Times, Serif" size="3"> classes of this chapter, the following code creates two class objects and outputs the time represented by each.</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="Courier New, Courier, Mono New, Courier, Mono" size="2">Time聽聽聽聽startTime(8,聽30,聽0);<br />ExtTime聽endTime(10,聽45,聽0,聽CST);<br /><br />startTime.Write();<br />cout聽<<聽endl;<br />endTime.Write();<br />cout聽<<聽endl;</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">This code fragment invokes two different </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> functions, even though the functions have the same name. The first function call invokes the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> function of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> class, printing out three values: hours, minutes, and seconds. The second call invokes the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> function of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ExtTime</font><font face="Times New Roman, Times, Serif" size="3"> class, printing out four values: hours, minutes, seconds, and time zone. In this code fragment, the compiler uses static (compile-time) binding of the operation </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">(Write)</font><font face="Times New Roman, Times, Serif" size="3"> to the appropriate object. The compiler can easily determine</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_924.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_925</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_926.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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