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<title>Mixing Data Types Can Create Subtle Errors</title>
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<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td>
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<td><h2>Safe Numerics</h2></td>
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<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="5.html"><img src="../images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="safe_numerics.tutorial.10"></a>Mixing Data Types Can Create Subtle Errors</h3></div></div></div>
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<p>C++ contains signed and unsigned integer types. In spite of their
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names, they function differently which often produces surprising results
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for some operands. Program errors from this behavior can be exceedingly
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difficult to find. This has lead to recommendations of various ad hoc
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"rules" to avoid these problems. It's not always easy to apply these
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"rules" to existing code without creating even more bugs. Here is a
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typical example of this problem:</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstdint</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="string">"../include/safe_integer.hpp"</span>
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<span class="preprocessor">#include</span> <span class="string">"../include/cpp.hpp"</span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">std</span><span class="special">;</span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">;</span>
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<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>
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<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>
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<span class="special">}</span>
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<span class="keyword">void</span> <span class="identifier">safe_f</span><span class="special">(</span>
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<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>
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<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>
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<span class="special">)</span><span class="special">{</span>
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<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>
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<span class="special">}</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span>
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<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"example 10: "</span><span class="special">;</span>
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<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>
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<span class="keyword">try</span> <span class="special">{</span>
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<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>
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<span class="comment">// problem: arithmetic operations can yield incorrect results.</span>
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<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>
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<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>
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<span class="special">}</span>
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<span class="keyword">catch</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">)</span><span class="special">{</span>
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<span class="comment">// never arrive here</span>
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<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>
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<span class="special">}</span>
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<span class="keyword">try</span> <span class="special">{</span>
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<span class="comment">// solution: use safe types</span>
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<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>
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<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>
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<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>
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<span class="special">}</span>
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<span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">exception</span> <span class="special">&</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
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<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"detected error:"</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>
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<span class="special">}</span>
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<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>Here
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is the output of the above program:</p>
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<pre class="screen">example 10: mixing types produces surprising results
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Not using safe numerics
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10000
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4294957296
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Using safe numerics
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10000
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detected error:converted negative value to unsigned
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</pre>
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<p>This solution is simple, Just replace instances of the
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<code class="computeroutput">int</code>with <code class="computeroutput">safe<int></code>.</p>
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</div>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2012 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
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Software License</a></p>
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