1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Mixing Data Types Can Create Subtle 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="4.html" title="Implicit Conversions Can Lead to Erroneous Results"> 10<link rel="next" href="6.html" title="Array Index Value Can Exceed Array Limits"> 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="4.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="6.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.5"></a>Mixing Data Types Can Create Subtle Errors</h3></div></div></div> 23<p>C++ contains signed and unsigned integer types. In spite of their 24 names, they function differently which often produces surprising results 25 for some operands. Program errors from this behavior can be exceedingly 26 difficult to find. This has lead to recommendations of various ad hoc 27 "rules" to avoid these problems. It's not always easy to apply these 28 "rules" to existing code without creating even more bugs. Here is a 29 typical example of this problem:</p> 30<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 31<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstdint</span><span class="special">></span> 32 33<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> 34 35<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">std</span><span class="special">;</span> 36<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> 37 38<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">int8_t</span> <span class="special">&</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span> 39 <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="identifier">endl</span><span class="special">;</span> 40<span class="special">}</span> 41<span class="keyword">void</span> <span class="identifier">safe_f</span><span class="special">(</span> 42 <span class="keyword">const</span> <span class="identifier">safe</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">></span> <span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> 43 <span class="keyword">const</span> <span class="identifier">safe</span><span class="special"><</span><span class="identifier">int8_t</span><span class="special">></span> <span class="special">&</span> <span class="identifier">y</span> 44<span class="special">)</span><span class="special">{</span> 45 <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="identifier">endl</span><span class="special">;</span> 46<span class="special">}</span> 47 48<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span> 49 <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"example 4: "</span><span class="special">;</span> 50 <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"mixing types produces surprising results"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> 51 <span class="keyword">try</span> <span class="special">{</span> 52 <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> 53 <span class="comment">// problem: mixing types produces surprising results.</span> 54 <span class="identifier">f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// works as expected</span> 55 <span class="identifier">f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="special">-</span><span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// wrong result - unnoticed</span> 56 <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error NOT detected!"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</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="identifier">e</span><span class="special">)</span><span class="special">{</span> 59 <span class="comment">// never arrive here</span> 60 <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">endl</span><span class="special">;</span><span class="special">;</span> 61 <span class="special">}</span> 62 <span class="keyword">try</span> <span class="special">{</span> 63 <span class="comment">// solution: use safe types</span> 64 <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> 65 <span class="identifier">safe_f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// works as expected</span> 66 <span class="identifier">safe_f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="special">-</span><span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// throw error</span> 67 <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"error NOT detected!"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span><span class="special">;</span> 68 <span class="special">}</span> 69 <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> 70 <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">endl</span><span class="special">;</span><span class="special">;</span> 71 <span class="special">}</span> 72 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 73<span class="special">}</span> 74 75</pre> 76<p>Here 77 is the output of the above program:</p> 78<pre class="screen">example 4: mixing types produces surprising results 79Not using safe numerics 8010000 814294957296 82error NOT detected! 83Using safe numerics 8410000 85error detected!converted negative value to unsigned: domain error 86</pre> 87<p>This solution is simple, just replace instances of <code class="computeroutput">int</code> 88 with <code class="computeroutput">safe<int></code>.</p> 89</div> 90<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 91<td align="left"></td> 92<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 93 Software License</a></p> 94</div></td> 95</tr></table> 96<hr> 97<div class="spirit-nav"> 98<a accesskey="p" href="4.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="6.html"><img src="../images/next.png" alt="Next"></a> 99</div> 100</body> 101</html> 102
