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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_992</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_991.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_992</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_993.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 992</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">The constructor begins by copying the first three incoming parameters into the appropriate private variables. Next, we use the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> operator to allocate a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> array on the free store. (We add 1 to the length of the incoming string to leave room for the terminating </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">\0</font><font face="Times New Roman, Times, Serif" size="3"> character.) Finally, we use </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">strcpy</font><font face="Times New Roman, Times, Serif" size="3"> to copy all the characters from the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">msgStr</font><font face="Times New Roman, Times, Serif" size="3"> array to the new dynamic array. If the client code declares two class objects with the statements</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">Date聽date1(4,聽15,聽1997,聽My聽birthday);<br />Date聽date2(5,聽12,聽1997,聽Meet聽Al);<br />聽聽.<br />聽聽.<br />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 the two class objects point to dynamic </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> arrays as shown in Figure 17-9.</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">Figure 17-9 illustrates an important concept: a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Date</font><font face="Times New Roman, Times, Serif" size="3"> class object does not encapsulate an arrayit only encapsulates <i>access</i> to the array. The array itself is located externally (on the free store), not within the protective abstraction barrier of the class object. This arrangement does not violate the principle of information hiding, however. The only access to the array is through the pointer variable </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">msg</font><font face="Times New Roman, Times, Serif" size="3">, which is a private class member and is therefore inaccessible to clients.</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">Notice that 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 allocates dynamic data, but we have made no provision for deallocating the dynamic data. To deal adequately with class objects that point to dynamic data, we need more than just a class constructor. We need an entire group of class member functions: a class constructor, a <i>class destructor,</i> a <i>deep copy operation,</i> and a <i>class copy-constructor.</i> One by one, we will explain each of these new functions. But first, here is the overall picture of what our new class declaration looks like:</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="55597d4190d18d2907d4ab31fe124c44.gif" border="0" alt="0992-01.gif" width="479" height="216" /></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17-9<br />Class聽Objects聽Pointing聽to聽Dynamically聽Allocated聽Strings</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_991.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_992</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_993.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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