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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_996</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_995.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_996</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_997.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 996</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"><i>Shallow Versus Deep Copying</i></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">Next, let's look at the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">CopyFrom</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">Date</font><font face="Times New Roman, Times, Serif" size="3"> class (Figure 17-10). This function is designed to copy one class object to another, <i>including the dynamic message array.</i> With the built-in assignment operator(</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">=</font><font face="Times New Roman, Times, Serif" size="3">), assignment of one class object to another copies only the class members; it does not copy any data pointed to by the class members. For example, given the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">date1</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">date2</font><font face="Times New Roman, Times, Serif" size="3"> objects of Figure 17-9, the effect of the assignment 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">date1聽=聽date2;</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">is shown in Figure 17-11. The result is called a shallow copy operation. The pointer is copied, but the pointed-to data are not.</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">Shallow copying is perfectly fine if none of the class members are pointers. But if one or more members are pointers, then shallow copying may be erroneous. In Figure 17-11, the dynamic array originally pointed to by the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">datel</font><font face="Times New Roman, Times, Serif" size="3"> object has been left inaccessible.</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">What we want is a deep copy operationone that duplicates not only the class members but also the pointed-to data. The </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">CopyFrom</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">Date</font><font face="Times New Roman, Times, Serif" size="3"> class performs a deep copy. Here is the function implementation:</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Date::CopyFrom(聽/*聽in聽*/聽Date聽otherDate聽)<br /><br />//聽Postcondition:<br />//聽聽聽聽聽mo聽==聽otherDate.mo<br />//聽聽&&聽day聽==聽otherDate.day<br />//聽聽&&聽yr聽==聽otherDate.yr<br />//聽聽&&聽msg聽points聽to聽a聽duplicate聽of聽otherDate's聽message聽string<br />//聽聽聽聽聽on聽the聽free聽store</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"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td align="center"><font face="Times New Roman, Times, Serif" size="3"><img src="7a9dd3480a18e21579047478dacbde99.gif" border="0" alt="0996-01.gif" width="479" height="217" /></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"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td align="center"><font face="Times New Roman, Times, Serif" size="2">Figure聽17-11<br />A聽Shallow聽Copy聽Caused聽by聽the聽Assignment聽</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">date1聽=聽date2</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_995.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_996</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_997.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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