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122 lines
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<title>automatic</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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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="native.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../promotion_policies.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="cpp.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><h4 class="title">
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<a name="safe_numerics.promotion_policies.automatic"></a>automatic</h4></div></div></div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm130202722656"></a>Description</h5></div></div></div>
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<p>This type contains the meta-functions to return a type with
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sufficient capacity to hold the result of a given binary arithmetic
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operation.</p>
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<p>The standard C/C++ procedure for executing arithmetic operations on
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operands of different types is:</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p>Convert operands to some common type using a somewhat
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elaborate elaborate rules defined in the C++ standard.</p></li>
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<li class="listitem"><p>Execute the operation.</p></li>
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<li class="listitem"><p>If the result of the operation cannot fit in the common type
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of the operands, discard the high order bits.</p></li>
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</ul></div>
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<p>The automatic promotion policy replaces the standard C/C++ procedure
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for the following one:</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<p>Convert operands to some common type using to the following
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rules.</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
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<li class="listitem"><p>For addition. If the operands are both unsigned the
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common type will be unsigned. Otherwise it will be
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signed.</p></li>
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<li class="listitem"><p>For subtraction, the common type will be signed.</p></li>
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<li class="listitem"><p>For left/right shift, the sign of the result will be the
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sign of the left operand.</p></li>
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<li class="listitem"><p>For all other types of operants, if both operands are
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unsigned the common type will be unsigned. Otherwise, it will
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be signed.</p></li>
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</ul></div>
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</li>
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<li class="listitem"><p>Determine the smallest size of the signed or unsigned type
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which can be guaranteed hold the result.</p></li>
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<li class="listitem"><p>If this size exceeds the maximum size supported by the
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compiler, use the maximum size supported by the compiler.</p></li>
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<li class="listitem">
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<p>Execute the operation.</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
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<li class="listitem"><p>Convert each operand to the common type.</p></li>
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<li class="listitem"><p>If the result cannot be contained in the result type as
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above, invoke an error procedure.</p></li>
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<li class="listitem"><p>Otherwise, return the result in the common type</p></li>
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</ul></div>
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</li>
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</ul></div>
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<p>This type promotion policy is applicable only to safe types whose
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base type is an <a class="link" href="integer.html" title="Integer<T>">Integer</a>
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type.</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm130202706000"></a>Model of</h5></div></div></div>
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<p><a class="link" href="promotion_policy.html" title="PromotionPolicy<PP>">PromotionPolicy</a></p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm130202704320"></a>Example of use</h5></div></div></div>
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<p>The following example illustrates the <code class="computeroutput">automatic</code> type
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being passed as a template parameter for the type
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<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">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="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">automatic</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="special">)</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="comment">// use automatic promotion policy where C++ standard arithmetic </span>
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<span class="comment">// might lead to incorrect results</span>
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<span class="keyword">using</span> <span class="identifier">safe_t</span> <span class="special">=</span> <span class="identifier">safe</span><span class="special"><</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">></span><span class="special">;</span>
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<span class="comment">// In such cases, there is no runtime overhead from using safe types.</span>
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<span class="identifier">safe_t</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
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<span class="identifier">safe_t</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
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<span class="comment">// z is guaranteed correct without any runtime overhead or exception.</span>
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<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>
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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></pre>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm130202662272"></a>Header</h5></div></div></div>
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<p><code class="computeroutput"><a href="../../include/automatic.hpp" target="_top"><code class="computeroutput">#include
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<boost/numeric/safe_numerics/automatic.hpp>
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</code></a></code></p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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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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</div></td>
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</tr></table>
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