ceph/src/boost/libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html

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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_numericconversion.improved_numeric_cast__"></a><a class="link" href="improved_numeric_cast__.html" title="Improved numeric_cast&lt;&gt;">Improved
  28     numeric_cast&lt;&gt;</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.introduction">Introduction</a></span></dt>
  32 <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast">numeric_cast</a></span></dt>
  33 <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast_traits">numeric_cast_traits</a></span></dt>
  34 <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.examples">Examples</a></span></dt>
  35 </dl></div>
  36 <div class="section">
  37 <div class="titlepage"><div><div><h3 class="title">
  38 <a name="boost_numericconversion.improved_numeric_cast__.introduction"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.introduction" title="Introduction">Introduction</a>
  39 </h3></div></div></div>
  40 <p>
  41         The lack of preservation of range makes conversions between numeric types
  42         error prone. This is true for both implicit conversions and explicit conversions
  43         (through <code class="computeroutput"><span class="keyword">static_cast</span></code>). <a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast" title="numeric_cast"><code class="computeroutput"><span class="identifier">numeric_cast</span></code></a> detects loss of range
  44         when a numeric type is converted, and throws an exception if the range cannot
  45         be preserved.
  46       </p>
  47 <p>
  48         There are several situations where conversions are unsafe:
  49       </p>
  50 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  51 <li class="listitem">
  52             Conversions from an integral type with a wider range than the target
  53             integral type.
  54           </li>
  55 <li class="listitem">
  56             Conversions from unsigned to signed (and vice versa) integral types.
  57           </li>
  58 <li class="listitem">
  59             Conversions from floating point types to integral types.
  60           </li>
  61 </ul></div>
  62 <p>
  63         The C++ Standard does not specify the behavior when a numeric type is assigned
  64         a value that cannot be represented by the type, except for unsigned integral
  65         types [3.9.1.4], which must obey the laws of arithmetic modulo 2n (this implies
  66         that the result will be reduced modulo the number that is one greater than
  67         the largest value that can be represented). The fact that the behavior for
  68         overflow is undefined for all conversions (except the aforementioned unsigned
  69         to unsigned) makes any code that may produce positive or negative overflows
  70         exposed to portability issues.
  71       </p>
  72 <p>
  73         By default <code class="computeroutput"><span class="identifier">numeric_cast</span></code> adheres
  74         to the rules for implicit conversions mandated by the C++ Standard, such
  75         as truncating floating point types when converting to integral types. The
  76         implementation must guarantee that for a conversion to a type that can hold
  77         all possible values of the source type, there will be no runtime overhead.
  78       </p>
  79 </div>
  80 <div class="section">
  81 <div class="titlepage"><div><div><h3 class="title">
  82 <a name="boost_numericconversion.improved_numeric_cast__.numeric_cast"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast" title="numeric_cast">numeric_cast</a>
  83 </h3></div></div></div>
  84 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Target</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Source</span><span class="special">&gt;</span> <span class="keyword">inline</span>
  85 <span class="identifier">Target</span> <span class="identifier">numeric_cast</span><span class="special">(</span> <span class="identifier">Source</span> <span class="identifier">arg</span> <span class="special">)</span>
  86 <span class="special">{</span>
  87     <span class="keyword">typedef</span> <span class="identifier">conversion_traits</span><span class="special">&lt;</span><span class="identifier">Target</span><span class="special">,</span> <span class="identifier">Source</span><span class="special">&gt;</span>   <span class="identifier">conv_traits</span><span class="special">;</span>
  88     <span class="keyword">typedef</span> <span class="identifier">numeric_cast_traits</span><span class="special">&lt;</span><span class="identifier">Target</span><span class="special">,</span> <span class="identifier">Source</span><span class="special">&gt;</span> <span class="identifier">cast_traits</span><span class="special">;</span>
  89     <span class="keyword">typedef</span> <span class="identifier">converter</span>
  90         <span class="special">&lt;</span>
  91             <span class="identifier">Target</span><span class="special">,</span>
  92             <span class="identifier">Source</span><span class="special">,</span>
  93             <span class="identifier">conv_traits</span><span class="special">,</span>
  94             <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">overflow_policy</span><span class="special">,</span>
  95             <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">rounding_policy</span><span class="special">,</span>
  96             <span class="identifier">raw_converter</span><span class="special">&lt;</span><span class="identifier">conv_traits</span><span class="special">&gt;,</span>
  97             <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">range_checking_policy</span>
  98         <span class="special">&gt;</span> <span class="identifier">converter</span><span class="special">;</span>
  99     <span class="keyword">return</span> <span class="identifier">converter</span><span class="special">::</span><span class="identifier">convert</span><span class="special">(</span><span class="identifier">arg</span><span class="special">);</span>
 100 <span class="special">}</span>
 101 </pre>
 102 <p>
 103         <code class="computeroutput"><span class="identifier">numeric_cast</span></code> returns the
 104         result of converting a value of type Source to a value of type Target. If
 105         out-of-range is detected, an overflow policy is executed whose default behavior
 106         is to throw an an exception (see <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_bad_numeric_cast">bad_numeric_cast</a>,
 107         <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_negative_overflow">negative_overflow</a>
 108         and <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_possitive_overflow">positive_overflow</a>
 109         ).
 110       </p>
 111 </div>
 112 <div class="section">
 113 <div class="titlepage"><div><div><h3 class="title">
 114 <a name="boost_numericconversion.improved_numeric_cast__.numeric_cast_traits"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast_traits" title="numeric_cast_traits">numeric_cast_traits</a>
 115 </h3></div></div></div>
 116 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Target</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Source</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
 117 <span class="keyword">struct</span> <span class="identifier">numeric_cast_traits</span>
 118 <span class="special">{</span>
 119     <span class="keyword">typedef</span> <span class="identifier">def_overflow_handler</span>    <span class="identifier">overflow_policy</span><span class="special">;</span>
 120     <span class="keyword">typedef</span> <span class="identifier">UseInternalRangeChecker</span> <span class="identifier">range_checking_policy</span><span class="special">;</span>
 121     <span class="keyword">typedef</span> <span class="identifier">Trunc</span><span class="special">&lt;</span><span class="identifier">Source</span><span class="special">&gt;</span>           <span class="identifier">rounding_policy</span><span class="special">;</span>
 122 <span class="special">};</span>
 123 </pre>
 124 <p>
 125         The behavior of <code class="computeroutput"><span class="identifier">numeric_cast</span></code>
 126         may be tailored for custom numeric types through the specialization of <code class="computeroutput"><span class="identifier">numeric_cast_traits</span></code>. (see <a class="link" href="type_requirements_and_user_defined_types_support.html" title="Type Requirements and User-defined-types support">User
 127         Defined Types</a> for details. )
 128       </p>
 129 </div>
 130 <div class="section">
 131 <div class="titlepage"><div><div><h3 class="title">
 132 <a name="boost_numericconversion.improved_numeric_cast__.examples"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.examples" title="Examples">Examples</a>
 133 </h3></div></div></div>
 134 <p>
 135         The following example performs some typical conversions between numeric types:
 136       </p>
 137 <div class="orderedlist"><ol class="orderedlist" type="1">
 138 <li class="listitem">
 139             include &lt;boost/numeric/conversion/cast.hpp&gt;
 140           </li>
 141 <li class="listitem">
 142             include &lt;iostream&gt;
 143           </li>
 144 </ol></div>
 145 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 146 <span class="special">{</span>
 147     <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span><span class="special">;</span>
 148
 149     <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">bad_numeric_cast</span><span class="special">;</span>
 150     <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">positive_overflow</span><span class="special">;</span>
 151     <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">negative_overflow</span><span class="special">;</span>
 152
 153     <span class="keyword">try</span>
 154     <span class="special">{</span>
 155         <span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">42</span><span class="special">;</span>
 156         <span class="keyword">short</span> <span class="identifier">s</span><span class="special">=</span><span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="keyword">short</span><span class="special">&gt;(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// This conversion succeeds (is in range)</span>
 157     <span class="special">}</span>
 158     <span class="keyword">catch</span><span class="special">(</span><span class="identifier">negative_overflow</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
 159         <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>
 160     <span class="special">}</span>
 161     <span class="keyword">catch</span><span class="special">(</span><span class="identifier">positive_overflow</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
 162         <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>
 163     <span class="special">}</span>
 164
 165     <span class="keyword">try</span>
 166     <span class="special">{</span>
 167         <span class="keyword">float</span> <span class="identifier">f</span><span class="special">=-</span><span class="number">42.1234</span><span class="special">;</span>
 168
 169         <span class="comment">// This will cause a boost::numeric::negative_overflow exception to be thrown</span>
 170         <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">f</span><span class="special">);</span>
 171     <span class="special">}</span>
 172     <span class="keyword">catch</span><span class="special">(</span><span class="identifier">bad_numeric_cast</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
 173         <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>
 174     <span class="special">}</span>
 175
 176     <span class="keyword">double</span> <span class="identifier">d</span><span class="special">=</span> <span class="identifier">f</span> <span class="special">+</span> <span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">123</span><span class="special">);</span> <span class="comment">// int -&gt; double</span>
 177
 178     <span class="keyword">unsigned</span> <span class="keyword">long</span> <span class="identifier">l</span><span class="special">=</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">long</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">();</span>
 179
 180     <span class="keyword">try</span>
 181     <span class="special">{</span>
 182         <span class="comment">// This will cause a boost::numeric::positive_overflow exception to be thrown</span>
 183         <span class="comment">// NOTE: *operations* on unsigned integral types cannot cause overflow</span>
 184         <span class="comment">// but *conversions* to a signed type ARE range checked by numeric_cast.</span>
 185
 186         <span class="keyword">unsigned</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">=</span><span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;(</span><span class="identifier">l</span><span class="special">);</span>
 187     <span class="special">}</span>
 188     <span class="keyword">catch</span><span class="special">(</span><span class="identifier">positive_overflow</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
 189         <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>
 190     <span class="special">}</span>
 191
 192
 193     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 194 <span class="special">}</span>
 195 </pre>
 196 </div>
 197 </div>
 198 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 199 <td align="left"></td>
 200 <td align="right"><div class="copyright-footer">Copyright &#169; 2004-2007 Fernando
 201       Luis Cacciola Carballal<p>
 202         Distributed under the Boost Software License, Version 1.0. (See accompanying
 203         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 204       </p>
 205 </div></td>
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