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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_980</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_979.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_980</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_981.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 980</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 entire program. In contrast, an automatic variablea local variable not declared as </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">static</font><font face="Times New Roman, Times, Serif" size="3">is allocated (created) when control reaches its declaration and deallocated (destroyed) when control exits the block in which the variable is declared.</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">With the aid of pointers, C++ provides a third category of program data: dynamic data. Dynamic variables are not declared with ordinary variable declarations; they are explicitly allocated and deallocated at execution time by means of two special operators, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">delete</font><font face="Times New Roman, Times, Serif" size="3">. When a program requires an additional variable, it uses </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> to allocate the variable. When the program no longer needs the variable, it uses </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">delete</font><font face="Times New Roman, Times, Serif" size="3"> to deallocate it. The lifetime of a dynamic variable is therefore the time between the execution of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> and the execution of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">delete</font><font face="Times New Roman, Times, Serif" size="3">. The advantage of being able to create new variables at execution time is that we don't need to create any more of them than we need.</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"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td> <td colspan="3" height="12"></td> <td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="2">Dynamic Data Variables created during execution of a program by means of special operations. In C++, these operations are </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="2"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">delete</font><font face="Times New Roman, Times, Serif" size="2">.</font></td><td></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></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 </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> operation has two forms, one for allocating a single variable and one for allocating an array. Here is the syntax template:</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"><img src="d5bcb7a964b45d18c0069e4bffc16fcc.gif" border="0" alt="0980-01.gif" width="243" height="91" /></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 first form is used for creating a single variable of type DataType. The second form creates an array whose elements are of type DataType; the desired number of array elements is given by IntExpression. Here is an example that demonstrates both forms of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">new</font><font face="Times New Roman, Times, Serif" size="3"> operation:</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="576" 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">int*聽intPtr;</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td><td valign="top"></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">char*聽nameStr;</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td><td valign="top"></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">intPtr聽=聽new聽int;</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">Creates a variable of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">int</font><font face="Times New Roman, Times, Serif" size="3"> and stores its address into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">intPtr</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">nameStr聽=聽new聽char[6];</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">Creates a 6-element </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> array and stores the base address of the array into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">nameStr</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">Normally, 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 does two things: it creates an uninitialized variable (or array) of the designated type, and it returns a pointer to this variable (or the base address of an array). However, if the computer system has run</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_979.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_980</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_981.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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