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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.automatic"></a>automatic</h4></div></div></div>
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<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202722656"></a>Description</h5></div></div></div>
<p>This type contains the meta-functions to return a type with
    sufficient capacity to hold the result of a given binary arithmetic
    operation.</p>
<p>The standard C/C++ procedure for executing arithmetic operations on
    operands of different types is:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Convert operands to some common type using a somewhat
          elaborate elaborate rules defined in the C++ standard.</p></li>
<li class="listitem"><p>Execute the operation.</p></li>
<li class="listitem"><p>If the result of the operation cannot fit in the common type
          of the operands, discard the high order bits.</p></li>
</ul></div>
<p>The automatic promotion policy replaces the standard C/C++ procedure
    for the following one:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>Convert operands to some common type using to the following
          rules.</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p>For addition. If the operands are both unsigned the
                common type will be unsigned. Otherwise it will be
                signed.</p></li>
<li class="listitem"><p>For subtraction, the common type will be signed.</p></li>
<li class="listitem"><p>For left/right shift, the sign of the result will be the
                sign of the left operand.</p></li>
<li class="listitem"><p>For all other types of operants, if both operands are
                unsigned the common type will be unsigned. Otherwise, it will
                be signed.</p></li>
</ul></div>
</li>
<li class="listitem"><p>Determine the smallest size of the signed or unsigned type
          which can be guaranteed hold the result.</p></li>
<li class="listitem"><p>If this size exceeds the maximum size supported by the
          compiler, use the maximum size supported by the compiler.</p></li>
<li class="listitem">
<p>Execute the operation.</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p>Convert each operand to the common type.</p></li>
<li class="listitem"><p>If the result cannot be contained in the result type as
                above, invoke an error procedure.</p></li>
<li class="listitem"><p>Otherwise, return the result in the common type</p></li>
</ul></div>
</li>
</ul></div>
<p>This type promotion policy is applicable only to safe types whose
    base type is an <a class="link" href="integer.html" title="Integer&lt;T&gt;">Integer</a>
    type.</p>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202706000"></a>Model of</h5></div></div></div>
<p><a class="link" href="promotion_policy.html" title="PromotionPolicy&lt;PP&gt;">PromotionPolicy</a></p>
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<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202704320"></a>Example of use</h5></div></div></div>
<p>The following example illustrates the <code class="computeroutput">automatic</code> type
    being passed as a template parameter for the type
    <code class="computeroutput">safe&lt;int&gt;</code>.</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span> 
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">automatic</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 automatic 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_t</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">automatic</span><span class="special">&gt;</span><span class="special">;</span>

    <span class="comment">// In such cases, there is no runtime overhead from using safe types.</span>
    <span class="identifier">safe_t</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
    <span class="identifier">safe_t</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
    <span class="comment">// z is guaranteed correct without any runtime overhead or exception.</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="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span></pre>
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<div class="titlepage"><div><div><h5 class="title">
<a name="idm130202662272"></a>Header</h5></div></div></div>
<p><code class="computeroutput"><a href="../../include/automatic.hpp" target="_top"><code class="computeroutput">#include
    &lt;boost/numeric/safe_numerics/automatic.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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