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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_973</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_972.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_973</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_974.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 973</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">expression </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr</font><font face="Times New Roman, Times, Serif" size="3"> denotes a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> variable of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PatientRec</font><font face="Times New Roman, Times, Serif" size="3">. Furthermore, the expressions (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr).idNum</font><font face="Times New Roman, Times, Serif" size="3">, (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr).height</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">(*patientPtr).weight</font><font face="Times New Roman, Times, Serif" size="3"> denote the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">idNum</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">height</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">weight</font><font face="Times New Roman, Times, Serif" size="3"> members of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr</font><font face="Times New Roman, Times, Serif" size="3">. Notice how the accessing expression is built.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table cellpadding="0" cellspacing="0" border="0" width="100%"><tr><td height="12"></td></tr><tr><td><table cellspacing="0" width="614" cellpadding="7"><tr><td valign="top"><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="3">patientPtr</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td><td valign="top"><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">A pointer variable of type pointer to </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PatientRec</font><font face="Times New Roman, Times, Serif" size="3">.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr><tr><td valign="top"><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="3">*patientPtr</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td><td valign="top"><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">A </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> variable of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PatientRed</font><font face="Times New Roman, Times, Serif" size="3">.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr><tr><td valign="top"><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="3">(*patientPtr).weight</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td><td valign="top"><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 </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">weight</font><font face="Times New Roman, Times, Serif" size="3"> member of a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> variable of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PatientRec</font><font face="Times New Roman, Times, Serif" size="3">.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr></table></td></tr></table><br /><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 expression (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr).weight</font><font face="Times New Roman, Times, Serif" size="3"> is a mixture of pointer dereferencing and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> member selection. The parentheses are necessary because the dot operator has higher precedence than the dereference operator (see Appendix B for C++ operator precedence). Without the parentheses, the expression </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr.weight</font><font face="Times New Roman, Times, Serif" size="3"> would be interpreted wrongly as </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*(patientPtr.weight)</font><font face="Times New Roman, Times, Serif" size="3">.</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">When a pointer points to a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3"> (or a class or a union) variable, enclosing the pointer dereference within parentheses can become tedious. In addition to the dot operator, C++ provides another member selection operator: </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">-></font><font face="Times New Roman, Times, Serif" size="3">. This <i>arrow operator</i> consists of two consecutive symbols: a hyphen and a greater-than symbol. By definition,</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">PointerExpression </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">-></font><font face="Times New Roman, Times, Serif" size="3"> MemberName</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 equivalent to</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">(*PointerExpression).MemberName</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">Therefore, we can write (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">*patientPtr).weight</font><font face="Times New Roman, Times, Serif" size="3"> as </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">patientPtr->weight</font><font face="Times New Roman, Times, Serif" size="3">. The general guideline for choosing between the two member selection operators (dot and arrow) is the following:</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">Use the dot operator if the first operand denotes a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3">, class, or union variable; use the arrow operator if the first operand denotes a <i>pointer</i> to a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">struct</font><font face="Times New Roman, Times, Serif" size="3">, class, or union variable.</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">If we want to increment and print the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">TimeType</font><font face="Times New Roman, Times, Serif" size="3"> class object pointed to by </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">timePtr</font><font face="Times New Roman, Times, Serif" size="3">, we could use either 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">(*timePtr).Increment();<br />(*timePtr).Write();</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">or 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">timePtr->Increment();<br />timePtr->Write();</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">And if we had declared an array of pointers</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_972.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_973</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_974.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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