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<title>Implicit Conversions Change Data Values</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="2.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>
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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.4"></a>Implicit Conversions Change Data Values</h3></div></div></div>
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<p>A simple assignment or arithmetic expression will convert all the
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terms to the same type. Sometimes this can silently change values. For
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example, when a signed data variable contains a negative type, assigning
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to a unsigned type will be permitted by any C/C++ compiler but will be
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treated as large unsigned value. Most modern compilers will emit a compile
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time warning when this conversion is performed. The user may then decide
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to change some data types or apply a <code class="computeroutput">static_cast</code>. This is
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less than satisfactory for two reasons:</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem"><p>It may be unwieldy to change all the types to signed or
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unsigned.</p></li>
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<li class="listitem"><p>We may believe that our signed type will never contain a
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negative value. <code class="computeroutput">static_cast</code> changes the data type - not
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the data value. If we ignore the any compiler warnings or use a
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<code class="computeroutput">static_cast</code> to suppress them, we'll fail to detect a
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program error when it is committed. This is aways a risk with
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casts.</p></li>
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</ul></div>
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<p>This solution is simple, Just replace instances of the <code class="computeroutput">int
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</code>with <code class="computeroutput">safe<int></code>.</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">exception</span><span class="special">></span>
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<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="string">"../include/safe_integer.hpp"</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span> <span class="identifier">argv</span><span class="special">[</span><span class="special">]</span><span class="special">)</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">"example 4: "</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">"implicit conversions change data values"</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">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: implicit conversions change data values</span>
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<span class="keyword">try</span><span class="special">{</span>
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<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="special">-</span><span class="number">1000</span><span class="special">;</span>
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<span class="comment">// the following silently produces an incorrect result</span>
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<span class="keyword">char</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">x</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="identifier">x</span> <span class="special"><<</span> <span class="string">" != "</span> <span class="special"><<</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="identifier">std</span><span class="special">::</span><span class="identifier">endl</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">"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>
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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="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> <span class="comment">// never arrive here</span>
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<span class="special">}</span>
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<span class="comment">// solution: replace int with safe<int> and char with safe<char></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="keyword">try</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="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="special">-</span><span class="number">1000</span><span class="special">;</span>
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<span class="comment">// throws exception when conversion change data value</span>
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<span class="identifier">safe</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">y1</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span><span class="special">;</span>
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<span class="identifier">safe</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">y2</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">;</span>
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<span class="identifier">safe</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">y3</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">x</span><span class="special">}</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">"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>
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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="identifier">e</span><span class="special">)</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="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>
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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">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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<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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