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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_929</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_928.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_929</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_930.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 929</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="Courier New, Courier, Mono New, Courier, Mono" size="2">private:<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽.<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">Declaring a member function to be </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">virtual</font><font face="Times New Roman, Times, Serif" size="3"> instructs the compiler to generate code that guarantees dynamic (run-time) binding of a function to an object. That is, the determination of which function to call is postponed until run time. (Note that to make </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> a virtual function, the word </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">virtual</font><font face="Times New Roman, Times, Serif" size="3"> appears in one place onlythe </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> class declaration. It does not appear in 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 definition that is located in the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">time.cpp</font><font face="Times New Roman, Times, Serif" size="3"> file, nor does it appear in any descendant classsuch as </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ExtTime</font><font face="Times New Roman, Times, Serif" size="3">that overrides 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.)</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" width="100%"><tr><td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td> <td colspan="3" height="12"></td> <td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="2">Dynamic Binding The run-time determination of which function to call for a particular object.</font></td><td></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></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">Virtual functions work in the following way. If a class object is passed <i>by reference</i> to some function, and if the body of that function contains a statement</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">formalParam.MemberFunc(聽聽);</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">then</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">1. If </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">MemberFunc</font><font face="Times New Roman, Times, Serif" size="3"> is not a virtual function, the type of the <i>formal parameter</i> determines which function to call. (Static binding is used.)</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">2. If </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">MemberFunc</font><font face="Times New Roman, Times, Serif" size="3"> is a virtual function, the type of the <i>actual parameter</i> determines which function to call. (Dynamic binding is used.)</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">With just one word</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">virtual</font><font face="Times New Roman, Times, Serif" size="3">the difficulties we encountered with our </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Print</font><font face="Times New Roman, Times, Serif" size="3"> function disappear entirely. If we declare </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> to be a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">virtual</font><font face="Times New Roman, Times, Serif" size="3"> function in 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, the function</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">void聽Print(聽/*聽in聽*/聽Time&聽someTime聽)<br />{<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽someTime.Write();<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽.<br />}</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_928.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_929</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_930.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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