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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_926</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_925.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_926</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_927.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 926</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">which </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Write</font><font face="Times New Roman, Times, Serif" size="3"> function to call by checking the data type of the associated 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" 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">Static Binding The compile-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">In some situations, the compiler cannot determine the type of an object, and the binding of an operation to an object must occur at run time. One situation, which we look at now, involves parameter passage of class objects.</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 basic C++ rule for passing class objects as parameters is that the actual parameter and its corresponding formal parameter must be of identical type. With inheritance, though, C++ relaxes the rule. You may pass an object of a child class <i>C</i> to an object of its parent class <i>P,</i> but not the other way aroundthat is, you cannot pass an object of type <i>P</i> to an object of type <i>C.</i> More generally, you can pass an object of a descendant class to an object of any of its ancestor classes. This rule has a tremendous benefitit allows us to write a single function that applies to any descendant class instead of writing a different function for each. For example, we could write a fancy </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Print</font><font face="Times New Roman, Times, Serif" size="3"> function that takes as a parameter an object of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> or any class descended from </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time:</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cout聽<<聽*************************聽<聽endl;<br />聽聽聽聽cout聽<<聽**聽The聽time聽is聽;<br />聽聽聽聽someTime.Write();<br />聽聽聽聽cout聽<<聽endl;<br />聽聽聽聽cout聽<<聽*************************聽<聽endl;<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">Given the code fragment</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 />Print(startTime);<br />Print(endTime);</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 compiler lets us pass either a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Time</font><font face="Times New Roman, Times, Serif" size="3"> object or an </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ExtTime</font><font face="Times New Roman, Times, Serif" size="3"> object to the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Print</font><font face="Times New Roman, Times, Serif" size="3"> function. Unfortunately, the output is not what we would like. When </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">endTime</font><font face="Times New Roman, Times, Serif" size="3"> is printed, the time zone </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">CST</font><font face="Times New Roman, Times, Serif" size="3"> is missing from the output. Let's see why.</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_925.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_926</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_927.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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