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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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<div class="titlepage"><div><div><h3 class="title">
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<a name="safe_numerics.composition_with_other_libraries"></a>Composition with Other Libraries</h3></div></div></div>
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<p>For many years, Boost has included a library to represent and operate
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on <a href="http://www.boost.org/doc/libs/1_64_0/libs/rational/" target="_top">rational
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numbers</a>. Its well crafted, has good documentation and is well
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maintained. Using the rational library is as easy as construction an
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instance with the expression <code class="computeroutput">rational r(n, d)</code> where n and d are
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integers of the same type. From then on it can be used pretty much as any
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other number. Reading the documentation with safe integers in mind one
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finds</p>
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<div class="blockquote"><blockquote class="blockquote"><p>Limited-precision integer types [such as <code class="computeroutput">int</code>] may
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raise issues with the range sizes of their allowable negative values and
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positive values. If the negative range is larger, then the
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extremely-negative numbers will not have an additive inverse in the
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positive range, making them unusable as denominator values since they
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cannot be normalized to positive values (unless the user is lucky enough
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that the input components are not relatively prime
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pre-normalization).</p></blockquote></div>
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<p>Which hints of trouble. Examination of the code reveals which
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suggest that care has been taken implement operations so as to avoid
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overflows, catch divide by zero, etc. But the code itself doesn't seem to
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consistently implement this idea. So we make a small demo program:
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</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">limits</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">rational</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<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>
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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="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>
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<span class="comment">// simple demo of rational library</span>
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<span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">r</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span> <span class="number">2</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">"r = "</span> <span class="special"><<</span> <span class="identifier">r</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">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">q</span> <span class="special">{</span><span class="special">-</span><span class="number">2</span><span class="special">,</span> <span class="number">4</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">"q = "</span> <span class="special"><<</span> <span class="identifier">q</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">// display the product</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">"r * q = "</span> <span class="special"><<</span> <span class="identifier">r</span> <span class="special">*</span> <span class="identifier">q</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: rational doesn't handle integer overflow well</span>
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<span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">c</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span> <span class="identifier">INT_MAX</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">"c = "</span> <span class="special"><<</span> <span class="identifier">c</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">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">d</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span> <span class="number">2</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">"d = "</span> <span class="special"><<</span> <span class="identifier">d</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">// display the product - wrong answer</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">"c * d = "</span> <span class="special"><<</span> <span class="identifier">c</span> <span class="special">*</span> <span class="identifier">d</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: use safe integer in rational definition</span>
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<span class="keyword">using</span> <span class="identifier">safe_rational</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">rational</span><span class="special"><</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">safe_numerics</span><span class="special">::</span><span class="identifier">safe</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span>
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<span class="special">></span><span class="special">;</span>
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<span class="comment">// use rationals created with safe_t</span>
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<span class="keyword">const</span> <span class="identifier">safe_rational</span> <span class="identifier">sc</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span> <span class="identifier">INT_MAX</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">"c = "</span> <span class="special"><<</span> <span class="identifier">sc</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">const</span> <span class="identifier">safe_rational</span> <span class="identifier">sd</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span> <span class="number">2</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">"d = "</span> <span class="special"><<</span> <span class="identifier">sd</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">"c * d = "</span><span class="special">;</span>
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<span class="keyword">try</span> <span class="special">{</span>
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<span class="comment">// multiply them. This will overflow</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">sc</span> <span class="special">*</span> <span class="identifier">sd</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="keyword">const</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
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<span class="comment">// catch exception due to multiplication overflow</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="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>which
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produces the following output</p>
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<pre class="screen">r = 1/2
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q = -1/2
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r * q = -1/4
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c = 1/2147483647
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d = 1/2
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c * d = 1/-2
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c = 1/2147483647
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d = 1/2
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c * d = multiplication overflow: positive overflow error
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</pre>
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<p>The <a href="http://www.boost.org/doc/libs/1_64_0/libs/rational/" target="_top">rational library
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documentation</a> is quite specific as to the <a href="http://www.boost.org/doc/libs/1_64_0/libs/rational/rational.html#Integer%20Type%20Requirements" target="_top">type
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requirements</a> placed on the underlying type. Turns out the our <a class="link" href="integer.html" title="Integer<T>">own definition of an integer type</a>
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fulfills (actually surpasses) these requirements. So we have reason to hope
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that we can use <code class="computeroutput">safe<int></code> as the underlying type to
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create what we might call a "<code class="computeroutput">safe_rational</code>". This we have done
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in the above example where we demonstrate how to compose the rational
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library with the safe numerics library in order to create what amounts to a
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safe_rational library - all without writing a line of code! Still, things
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are not perfect. Since the <a href="http://www.boost.org/doc/libs/1_64_0/libs/rational/" target="_top">rational numbers
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library</a> implements its own checking for divide by zero by throwing
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an exception, the safe numerics code for handling this - included exception
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policy will not be respected. To summarize:</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p>In some cases safe types can be used as template parameters to
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other types to inject the concept of "no erroneous results" into the
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target type.</p></li>
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<li class="listitem"><p>Such composition is not guaranteed to work. The target type must
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be reviewed in some detail.</p></li>
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</ul></div>
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</div>
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<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
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Software License</a></p>
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