1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Cannot Recover From Arithmetic Errors</title> 5<link rel="stylesheet" href="../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Safe Numerics"> 8<link rel="up" href="../tutorial.html" title="Tutorial and Motivating Examples"> 9<link rel="prev" href="7.html" title="Checking of Input Values Can Be Easily Overlooked"> 10<link rel="next" href="9.html" title="Compile Time Arithmetic is Not Always Correct"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td> 15<td><h2>Safe Numerics</h2></td> 16</tr></table> 17<div class="spirit-nav"> 18<a accesskey="p" href="7.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="9.html"><img src="../images/next.png" alt="Next"></a> 19</div> 20<div class="section"> 21<div class="titlepage"><div><div><h3 class="title"> 22<a name="safe_numerics.tutorial.8"></a>Cannot Recover From Arithmetic Errors</h3></div></div></div> 23<p>If a divide by zero error occurs in a program, it's detected by 24 hardware. The way this manifests itself to the program can and will depend 25 upon</p> 26<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 27<li class="listitem"><p>data type - int, float, etc</p></li> 28<li class="listitem"><p>setting of compile time command line switches</p></li> 29<li class="listitem"><p>invocation of some configuration functions which convert these 30 hardware events into C++ exceptions</p></li> 31</ul></div> 32<p>It's not all that clear how one would detect and recover 33 from a divide by zero error in a simple portable way. Usually, users just 34 ignore the issue which usually results in immediate program termination 35 when this situation occurs.</p> 36<p>This library will detect divide by zero errors before the operation 37 is invoked. Any errors of this nature are handled according to the <a class="link" href="exception_policies.html" title="exception_policy<AE, IDB, UB, UV>">exception_policy</a> selected 38 by the library user.</p> 39<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> 40<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 41 42<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">safe_integer</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 43 44<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="special">[</span><span class="special">]</span><span class="special">)</span><span class="special">{</span> 45 <span class="comment">// problem: cannot recover from arithmetic errors</span> 46 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"example 7: "</span><span class="special">;</span> 47 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"cannot recover from arithmetic errors"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 48 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Not using safe numerics"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 49 50 <span class="keyword">try</span><span class="special">{</span> 51 <span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> 52 <span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 53 <span class="comment">// can't do this as it will crash the program with no</span> 54 <span class="comment">// opportunity for recovery - comment out for example</span> 55 <span class="comment">// std::cout << x / y;</span> 56 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error cannot be handled at runtime!"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 57 <span class="special">}</span> 58 <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">&</span><span class="special">)</span><span class="special">{</span> 59 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error handled at runtime!"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 60 <span class="special">}</span> 61 <span class="comment">// solution: replace int with safe<int></span> 62 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Using safe numerics"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 63 <span class="keyword">try</span><span class="special">{</span> 64 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">safe_numerics</span><span class="special">;</span> 65 <span class="keyword">const</span> <span class="identifier">safe</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> 66 <span class="keyword">const</span> <span class="identifier">safe</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 67 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">x</span> <span class="special">/</span> <span class="identifier">y</span><span class="special">;</span> 68 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error NOT detected!"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 69 <span class="special">}</span> 70 <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">&</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span> 71 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error handled at runtime!"</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">(</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 72 <span class="special">}</span> 73 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 74<span class="special">}</span> 75</pre> 76<pre class="screen">example 7: cannot recover from arithmetic errors 77Not using safe numerics 78error NOT detectable! 79Using safe numerics 80error detected:divide by zero: domain error 81</pre> 82</div> 83<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 84<td align="left"></td> 85<td align="right"><div class="copyright-footer">Copyright © 2012-2018 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost 86 Software License</a></p> 87</div></td> 88</tr></table> 89<hr> 90<div class="spirit-nav"> 91<a accesskey="p" href="7.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="9.html"><img src="../images/next.png" alt="Next"></a> 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