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<td><h2>Safe Numerics</h2></td>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="safe_numerics.tutorial.5"></a>Array Index Value Can Exceed Array Limits</h3></div></div></div>
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<p>Using an intrinsic C++ array, it's very easy to exceed array limits.
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This can fail to be detected when it occurs and create bugs which are hard
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to find. There are several ways to address this, but one of the simplest
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would be to use safe_unsigned_range;</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</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_range.hpp"</span>
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<span class="keyword">void</span> <span class="identifier">detected_msg</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">detected</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="special">(</span><span class="identifier">detected</span> <span class="special">?</span> <span class="string">"error detected!"</span> <span class="special">:</span> <span class="string">"error NOT detected! "</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="special">}</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="comment">// problem: array index values can exceed array bounds</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 5: "</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">"array index values can exceed array bounds"</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="identifier">std</span><span class="special">::</span><span class="identifier">array</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="number">37</span><span class="special">></span> <span class="identifier">i_array</span><span class="special">;</span>
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<span class="comment">// unsigned int i_index = 43;</span>
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<span class="comment">// the following corrupts memory.</span>
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<span class="comment">// This may or may not be detected at run time.</span>
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<span class="comment">// i_array[i_index] = 84; // comment this out so it can be tested!</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="comment">// solution: replace unsigned array index with safe_unsigned_range</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="keyword">using</span> <span class="identifier">i_index_t</span> <span class="special">=</span> <span class="identifier">safe_unsigned_range</span><span class="special"><</span><span class="number">0</span><span class="special">,</span> <span class="identifier">i_array</span><span class="special">.</span><span class="identifier">size</span><span class="special">(</span><span class="special">)</span> <span class="special">-</span> <span class="number">1</span><span class="special">></span><span class="special">;</span>
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<span class="identifier">i_index_t</span> <span class="identifier">i_index</span><span class="special">;</span>
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<span class="identifier">i_index</span> <span class="special">=</span> <span class="number">36</span><span class="special">;</span> <span class="comment">// this works fine</span>
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<span class="identifier">i_array</span><span class="special">[</span><span class="identifier">i_index</span><span class="special">]</span> <span class="special">=</span> <span class="number">84</span><span class="special">;</span>
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<span class="identifier">i_index</span> <span class="special">=</span> <span class="number">43</span><span class="special">;</span> <span class="comment">// throw exception 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 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> <span class="comment">// so we never arrive here</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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<p>Collections
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like standard arrays, vectors do array index checking in some function
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calls and not in others so this may not be the best example. However it
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does illustrate the usage of <code class="computeroutput">safe_range<T></code> for
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assigning legal range to variables. This will guarantee that under no
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circumstances will the variable contain a value outside of the specified
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range.</p>
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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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