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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_847</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_846.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_847</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_848.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 847</font></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="73ff1f55a7e52b2b736a56dfb5555adc.gif" border="0" alt="0847-01.gif" width="542" height="355" /></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15-3<br />Conceptual聽View聽of聽Two聽Class聽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">trol exits its surrounding block) or static (created once when control reaches its declaration and destroyed when the program terminates).</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 other ways, C++ treats </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> and classes differently from built-in types. Most of the built-in operations do not apply to </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> or classes. You cannot use the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">+</font><font face="Times New Roman, Times, Serif" size="3"> operator to add two </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeType</font><font face="Times New Roman, Times, Serif" size="3"> objects, nor can you use the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">==</font><font face="Times New Roman, Times, Serif" size="3"> operator to compare two </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeType</font><font face="Times New Roman, Times, Serif" size="3"> objects for equality.</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">Two built-in operations that are valid for struct and class objects are member selection (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">.</font><font face="Times New Roman, Times, Serif" size="3">) and assignment (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">=</font><font face="Times New Roman, Times, Serif" size="3">). As with </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">, you select an individual member of a class by writing the name of the class object, then a dot, then the member name. The 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">time1.Increment();</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">invokes the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Increment</font><font face="Times New Roman, Times, Serif" size="3"> function for the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">time1</font><font face="Times New Roman, Times, Serif" size="3"> object, presumably to add one second to the time stored in </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">time1</font><font face="Times New Roman, Times, Serif" size="3">. The other built-in operation, assignment, performs aggregate assignment of one class object to another with the following semantics: If </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">y</font><font face="Times New Roman, Times, Serif" size="3"> are objects of the same class, then the assignment </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> = </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">y</font><font face="Times New Roman, Times, Serif" size="3"> copies the data members of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">y</font><font face="Times New Roman, Times, Serif" size="3"> into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3">. Below is a fragment of client code that demonstrates member selection and assignment.</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_846.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_847</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_848.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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