📄 page_999.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_999</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_998.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_999</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_1000.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 999</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">For example, our </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Date</font><font face="Times New Roman, Times, Serif" size="3"> class declaration in Figure 17-10 shows the prototype of the copy-constructor to be</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const聽Date&聽otherDate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">If a copy-constructor is present, the default method of initialization (member-by-member copying) is inhibited. Instead, the copy-constructor is implicitly invoked whenever one class object is initialized by another. The following implementation 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 copy-constructor shows the steps that are involved:</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::Date(聽const聽Date&聽otherDate聽)<br /><br />//聽Copy-constructor<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'聽s聽message聽string<br />//聽聽聽聽聽on聽the聽free聽store<br /><br />{<br />聽聽聽聽mo聽=聽otherDate.mo;<br />聽聽聽聽day聽=聽otherDate.day;<br />聽聽聽聽yr聽=聽otherDate.yr;<br />聽聽聽聽msg聽=聽new聽char[strlen(otherDate.msg)聽+聽1];<br />聽聽聽聽strcpy(msg,聽otherDate.msg);<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">The body of the copy-constructor function differs from the body of 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 in only one line of code: 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 executes</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">delete聽[]聽msg;</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">before allocating a new array. The difference between these two functions is that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">CopyFrom</font><font face="Times New Roman, Times, Serif" size="3"> is copying to an <i>existing</i> class object (which is already pointing to a dynamic array that must be deallocated), whereas the copyconstructor is creating a new class object that doesn't already exist.</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 the use of the reserved word </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">const</font><font face="Times New Roman, Times, Serif" size="3"> in the parameter list of the copy-constructor. The word </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">const</font><font face="Times New Roman, Times, Serif" size="3"> ensures that the function cannot alter </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">otherDate</font><font face="Times New Roman, Times, Serif" size="3">, even though </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">otherDate</font><font face="Times New Roman, Times, Serif" size="3"> is passed by reference.</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_998.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_999</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_1000.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -