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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_371</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_370.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_371</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_372.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 371</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">stars. When this happens, you might assume that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PrintData</font><font face="Times New Roman, Times, Serif" size="3"> has gone haywire, but it's the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">main</font><font face="Times New Roman, Times, Serif" size="3"> function's fault for not checking the validity of the data. (The program should perform this test in function </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">GetData</font><font face="Times New Roman, Times, Serif" size="3">). Thus, a side effect of one function can multiply and give the appearance of errors elsewhere in a program. We take a closer look at the concept of side effects in the next chapter.</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"><i>The </i></font><i><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">assert</font><font face="Times New Roman, Times, Serif" size="3"> Library Function</font></i></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">We have discussed how function preconditions and postconditions are useful for debugging (by checking that the precondition of each function is true prior to a function call, and by verifying that each function correctly transforms the precondition into the postcondition) and for testing (by pushing the precondition to its limits and even violating it). To state the preconditions and postconditions for our functions, we've been writing the assertions as program comments:</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">//聽Precondition:<br />//聽聽聽聽聽studentCount聽>聽0</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">All comments, of course, are ignored by the compiler. They are not executable statements; they are for humans to examine.</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">On the other hand, the C++ standard library gives us a way in which to write <i>executable assertions.</i> Through the header file </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">assert.h</font><font face="Times New Roman, Times, Serif" size="3">, the library provides a void function named </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">assert</font><font face="Times New Roman, Times, Serif" size="3">. This function takes a logical (Boolean) expression as a parameter and halts the program if the expression is false. Here's an example:</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">#include聽<assert.h><br />.<br />.<br />.<br />assert(studentCount聽>聽0);<br />average聽=聽sumOfScores聽/聽studentCount;</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 parameter to the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">assert</font><font face="Times New Roman, Times, Serif" size="3"> function must be a valid C++ logical expression. If its value is nonzero (TRUE), nothing happens; execution continues on to the next statement. If its value is zero (FALSE), execution of the program terminates immediately with a message stating the assertion as it appears in the parameter list, the name of the file containing the program source code, and the line number in the program. In the example above, if the value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">studentCount</font><font face="Times New Roman, Times, Serif" size="3"> is less than or equal to zero, the program halts after printing a message like this:</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">Assertion聽failed:聽studentCount聽>聽0,聽file聽myprog.cpp,聽line聽48</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_370.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_371</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_372.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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