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<td><h2>Safe Numerics</h2></td>
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<div class="titlepage"><div><div><h4 class="title">
<a name="safe_numerics.promotion_policies.native"></a>native</h4></div></div></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202851568"></a>Description</h5></div></div></div>
<p>This type contains the functions to return a safe type corresponding
to the C++ type resulting from a given arithmetic operation.</p>
<p>Usage of this policy with safe types will produce the exact same
arithmetic results that using normal unsafe integer types will. Hence this
policy is suitable as a drop-in replacement for these unsafe types. Its
main function is to trap incorrect arithmetic results when using C++ for
integer arithmetic.</p>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202849536"></a>Model of</h5></div></div></div>
<p><a class="link" href="promotion_policy.html" title="PromotionPolicy&lt;PP&gt;">PromotionPolicy</a></p>
<p>As an example of how this works consider C++ rules from section 5 of
the standard - "usual arithmetic conversions".</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
<span class="keyword">auto</span> <span class="identifier">z</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="comment">// z will be of type "int"</span>
<span class="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
<span class="special">}</span></pre>
<p>According to these rules, z will be of type int. Depending on the
values of x and y, z may or may not contain the correct arithmetic result
of the operation x + y.</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">safe_int</span> <span class="special">=</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span><span class="special">;</span>
<span class="keyword">void</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">safe_int</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">safe_int</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
<span class="keyword">auto</span> <span class="identifier">z</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="comment">// z will be of type "safe_int"</span>
<span class="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
<span class="special">}</span></pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202813168"></a>Example of use</h5></div></div></div>
<p>The following example illustrates the <code class="computeroutput">native</code> type being
passed as a template parameter for the type <code class="computeroutput">safe&lt;int&gt;</code>.
This example is slightly contrived in that <code class="computeroutput">safe&lt;int&gt;</code>
has <code class="computeroutput">native</code> as its default promotion parameter so explicitly
using <code class="computeroutput">native</code> is not necessary.</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">numeric</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">numeric</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">native</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span>
<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>
<span class="comment">// use native promotion policy where C++ standard arithmetic </span>
<span class="comment">// might lead to incorrect results</span>
<span class="keyword">using</span> <span class="identifier">safe_int8</span> <span class="special">=</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">int8_t</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span><span class="special">;</span>
<span class="keyword">try</span><span class="special">{</span>
<span class="identifier">safe_int8</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
<span class="identifier">safe_int8</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="identifier">safe_int8</span> <span class="identifier">z</span><span class="special">;</span>
<span class="comment">// rather than producing an invalid result an exception is thrown</span>
<span class="identifier">z</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">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">)</span><span class="special">;</span> <span class="comment">// never arrive here</span>
<span class="special">}</span>
<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">&amp;</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
<span class="comment">// which we can catch here</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// When result is an int, C++ promotion rules guarentee </span>
<span class="comment">// that there will be no incorrect result.</span>
<span class="comment">// In such cases, there is no runtime overhead from using safe types.</span>
<span class="identifier">safe_int8</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
<span class="identifier">safe_int8</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="identifier">safe</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span> <span class="identifier">z</span><span class="special">;</span> <span class="comment">// z can now hold the result of the addition of any two 8 bit numbers</span>
<span class="identifier">z</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="comment">// is guaranteed correct without any runtime overhead or exception.</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span></pre>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202727600"></a>Notes</h5></div></div></div>
<p>See Chapter 5, Expressions, C++ Standard</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202726384"></a>Header</h5></div></div></div>
<p><code class="computeroutput"><a href="../../include/native.hpp" target="_top"><code class="computeroutput">#include
&lt;boost/numeric/safe_numerics/native.hpp&gt;
</code></a></code></p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2012-2018 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
Software License</a></p>
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