1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Compile Time Arithmetic is Not Always Correct</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="8.html" title="Cannot Recover From Arithmetic Errors"> 10<link rel="next" href="10.html" title="Programming by Contract is Too Slow"> 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="8.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="10.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.9"></a>Compile Time Arithmetic is Not Always Correct</h3></div></div></div> 23<p>If a divide by zero error occurs while a program is being compiled, 24 there is not guarantee that it will be detected. This example shows a real 25 example compiled with a recent version of CLang.</p> 26<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 27<li class="listitem"><p>Source code includes a constant expression containing a simple 28 arithmetic error.</p></li> 29<li class="listitem"><p>The compiler emits a warning but otherwise calculates the wrong 30 result.</p></li> 31<li class="listitem"><p>Replacing int with safe<int> will guarantee that the error 32 is detected at runtime</p></li> 33<li class="listitem"><p>Operations using safe types are marked constexpr. So we can 34 force the operations to occur at runtime by marking the results as 35 constexpr. This will result in an error at compile time if the 36 operations cannot be correctly calculated.</p></li> 37</ul></div> 38<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span> 39<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 40 41<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> 42 43<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> 44 <span class="comment">// problem: cannot recover from arithmetic errors</span> 45 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"example 8: "</span><span class="special">;</span> 46 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"cannot detect compile time 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> 47 <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> 48 49 <span class="keyword">try</span><span class="special">{</span> 50 <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> 51 <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 52 <span class="comment">// will emit warning at compile time</span> 53 <span class="comment">// will leave an invalid result at runtime.</span> 54 <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> <span class="comment">// will display "0"!</span> 55 <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> 56 <span class="special">}</span> 57 <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> 58 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error detected!"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 59 <span class="special">}</span> 60 <span class="comment">// solution: replace int with safe<int></span> 61 <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> 62 <span class="keyword">try</span><span class="special">{</span> 63 <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> 64 <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> 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">y</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 66 <span class="comment">// constexpr const safe<int> z = x / y; // note constexpr here!</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> <span class="comment">// error would be detected at runtime</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 detected:"</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 8: cannot detect compile time arithmetic errors 77Not using safe numerics 780error NOT detected! 79Using safe numerics 80error detected:positive overflow error 81Program ended with exit code: 0</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> 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