1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>automatic</title> 5<link rel="stylesheet" href="../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Safe Numerics"> 8<link rel="up" href="../promotion_policies.html" title="Promotion Policies"> 9<link rel="prev" href="native.html" title="native"> 10<link rel="next" href="cpp.html" title="cpp<int C, int S, int I, int L, int LL>"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td> 15<td><h2>Safe Numerics</h2></td> 16</tr></table> 17<div class="spirit-nav"> 18<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> 19</div> 20<div class="section"> 21<div class="titlepage"><div><div><h4 class="title"> 22<a name="safe_numerics.promotion_policies.automatic"></a>automatic</h4></div></div></div> 23<div class="section"> 24<div class="titlepage"><div><div><h5 class="title"> 25<a name="idm130202722656"></a>Description</h5></div></div></div> 26<p>This type contains the meta-functions to return a type with 27 sufficient capacity to hold the result of a given binary arithmetic 28 operation.</p> 29<p>The standard C/C++ procedure for executing arithmetic operations on 30 operands of different types is:</p> 31<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 32<li class="listitem"><p>Convert operands to some common type using a somewhat 33 elaborate elaborate rules defined in the C++ standard.</p></li> 34<li class="listitem"><p>Execute the operation.</p></li> 35<li class="listitem"><p>If the result of the operation cannot fit in the common type 36 of the operands, discard the high order bits.</p></li> 37</ul></div> 38<p>The automatic promotion policy replaces the standard C/C++ procedure 39 for the following one:</p> 40<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 41<li class="listitem"> 42<p>Convert operands to some common type using to the following 43 rules.</p> 44<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "> 45<li class="listitem"><p>For addition. If the operands are both unsigned the 46 common type will be unsigned. Otherwise it will be 47 signed.</p></li> 48<li class="listitem"><p>For subtraction, the common type will be signed.</p></li> 49<li class="listitem"><p>For left/right shift, the sign of the result will be the 50 sign of the left operand.</p></li> 51<li class="listitem"><p>For all other types of operants, if both operands are 52 unsigned the common type will be unsigned. Otherwise, it will 53 be signed.</p></li> 54</ul></div> 55</li> 56<li class="listitem"><p>Determine the smallest size of the signed or unsigned type 57 which can be guaranteed hold the result.</p></li> 58<li class="listitem"><p>If this size exceeds the maximum size supported by the 59 compiler, use the maximum size supported by the compiler.</p></li> 60<li class="listitem"> 61<p>Execute the operation.</p> 62<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "> 63<li class="listitem"><p>Convert each operand to the common type.</p></li> 64<li class="listitem"><p>If the result cannot be contained in the result type as 65 above, invoke an error procedure.</p></li> 66<li class="listitem"><p>Otherwise, return the result in the common type</p></li> 67</ul></div> 68</li> 69</ul></div> 70<p>This type promotion policy is applicable only to safe types whose 71 base type is an <a class="link" href="integer.html" title="Integer<T>">Integer</a> 72 type.</p> 73</div> 74<div class="section"> 75<div class="titlepage"><div><div><h5 class="title"> 76<a name="idm130202706000"></a>Model of</h5></div></div></div> 77<p><a class="link" href="promotion_policy.html" title="PromotionPolicy<PP>">PromotionPolicy</a></p> 78</div> 79<div class="section"> 80<div class="titlepage"><div><div><h5 class="title"> 81<a name="idm130202704320"></a>Example of use</h5></div></div></div> 82<p>The following example illustrates the <code class="computeroutput">automatic</code> type 83 being passed as a template parameter for the type 84 <code class="computeroutput">safe<int></code>.</p> 85<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> 86<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> 87 88<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span> 89 <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> 90 <span class="comment">// use automatic promotion policy where C++ standard arithmetic </span> 91 <span class="comment">// might lead to incorrect results</span> 92 <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> 93 94 <span class="comment">// In such cases, there is no runtime overhead from using safe types.</span> 95 <span class="identifier">safe_t</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span> 96 <span class="identifier">safe_t</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 97 <span class="comment">// z is guaranteed correct without any runtime overhead or exception.</span> 98 <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> 99 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 100<span class="special">}</span></pre> 101</div> 102<div class="section"> 103<div class="titlepage"><div><div><h5 class="title"> 104<a name="idm130202662272"></a>Header</h5></div></div></div> 105<p><code class="computeroutput"><a href="../../include/automatic.hpp" target="_top"><code class="computeroutput">#include 106 <boost/numeric/safe_numerics/automatic.hpp> 107 </code></a></code></p> 108</div> 109</div> 110<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 111<td align="left"></td> 112<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 113 Software License</a></p> 114</div></td> 115</tr></table> 116<hr> 117<div class="spirit-nav"> 118<a 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