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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="math_toolkit.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
  28 </h2></div></div></div>
  29 <p>
  30       The implementation of <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  31       is designed to utilize <code class="computeroutput"><span class="special">&lt;</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span></code>,
  32       defined in the 1989 C standard. The preprocessor is used to query certain preprocessor
  33       definitions in <code class="computeroutput"><span class="special">&lt;</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span></code>
  34       such as FLT_MAX, DBL_MAX, etc. Based on the results of these queries, an attempt
  35       is made to automatically detect the presence of built-in floating-point types
  36       having specified widths. An unequivocal test requiring conformance with <a href="http://en.wikipedia.org/wiki/IEEE_floating_point" target="_top">IEEE_floating_point</a>
  37       (IEC599) based on <a href="http://en.cppreference.com/w/cpp/types/numeric_limits/is_iec559" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;::</span><span class="identifier">is_iec559</span></code></a>
  38       is performed with <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>.
  39     </p>
  40 <p>
  41       In addition, this Boost implementation <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  42       supports an 80-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
  43       if it can be detected, and a 128-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
  44       if it can be detected, provided that the underlying types conform with <a href="http://en.wikipedia.org/wiki/Extended_precision" target="_top">IEEE-754 precision extension</a>
  45       (provided <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;::</span><span class="identifier">is_iec559</span> <span class="special">==</span> <span class="keyword">true</span></code> for this type).
  46     </p>
  47 <p>
  48       The header <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  49       makes the standardized floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s
  50       safely available in <code class="computeroutput"><span class="keyword">namespace</span> <span class="identifier">boost</span></code> without placing any names in <code class="computeroutput"><span class="keyword">namespace</span> <span class="identifier">std</span></code>.
  51       The intention is to complement rather than compete with a potential future
  52       C/C++ Standard Library that may contain these <code class="computeroutput"><span class="keyword">typedef</span></code>s.
  53       Should some future C/C++ standard include <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">stdfloat</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span></code> and
  54       <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">cstdfloat</span><span class="special">&gt;</span></code>, then <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  55       will continue to function, but will become redundant and may be safely deprecated.
  56     </p>
  57 <p>
  58       Because <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  59       is a Boost header, its name conforms to the boost header naming conventions,
  60       not the C++ Standard Library header naming conventions.
  61     </p>
  62 <div class="note"><table border="0" summary="Note">
  63 <tr>
  64 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
  65 <th align="left">Note</th>
  66 </tr>
  67 <tr><td align="left" valign="top"><p>
  68         <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  69         <span class="bold"><strong>cannot synthesize or create a <code class="computeroutput"><span class="keyword">typedef</span></code>
  70         if the underlying type is not provided by the compiler</strong></span>. For example,
  71         if a compiler does not have an underlying floating-point type with 128 bits
  72         (highly sought-after in scientific and numeric programming), then <code class="computeroutput"><span class="identifier">float128_t</span></code> and its corresponding least
  73         and fast types are <span class="bold"><strong>not</strong></span> provided by <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span></code>&gt;.
  74       </p></td></tr>
  75 </table></div>
  76 <div class="warning"><table border="0" summary="Warning">
  77 <tr>
  78 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
  79 <th align="left">Warning</th>
  80 </tr>
  81 <tr><td align="left" valign="top"><p>
  82         If <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  83         uses a compiler-specific non-standardized type (<span class="bold"><strong>not</strong></span>
  84         derived from <code class="computeroutput"><span class="keyword">float</span><span class="special">,</span>
  85         <span class="keyword">double</span><span class="special">,</span></code>
  86         or <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>)
  87         for one or more of its floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s,
  88         then there is no guarantee that specializations of <code class="computeroutput"><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;</span></code> will be available for these types.
  89         Typically, specializations of <code class="computeroutput"><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;</span></code> will only be available for these
  90         types if the compiler itself supports corresponding specializations for the
  91         underlying type(s), exceptions are GCC's <code class="computeroutput"><span class="identifier">__float128</span></code>
  92         type and Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> type
  93         which are explicitly supported via our own code.
  94       </p></td></tr>
  95 </table></div>
  96 <div class="warning"><table border="0" summary="Warning">
  97 <tr>
  98 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
  99 <th align="left">Warning</th>
 100 </tr>
 101 <tr><td align="left" valign="top"><p>
 102         As an implementation artifact, certain C macro names from <code class="computeroutput"><span class="special">&lt;</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span></code> may possibly be visible to users of
 103         <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
 104         Don't rely on using these macros; they are not part of any Boost-specified
 105         interface. Use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;&gt;</span></code>
 106         for floating-point ranges, etc. instead.
 107       </p></td></tr>
 108 </table></div>
 109 <div class="tip"><table border="0" summary="Tip">
 110 <tr>
 111 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../doc/src/images/tip.png"></td>
 112 <th align="left">Tip</th>
 113 </tr>
 114 <tr><td align="left" valign="top">
 115 <p>
 116         For best results, <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 117         should be <code class="computeroutput"><span class="preprocessor">#include</span></code>d before
 118         other headers that define generic code making use of standard library functions
 119         defined in &lt;cmath&gt;.
 120       </p>
 121 <p>
 122         This is because <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 123         may define overloads of standard library functions where a non-standard type
 124         (i.e. other than <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code>, or <code class="computeroutput"><span class="keyword">long</span>
 125         <span class="keyword">double</span></code>) is used for one of the specified
 126         width types. If generic code (for example in another Boost.Math header) calls
 127         a standard library function, then the correct overload will only be found
 128         if these overloads are defined prior to the point of use. See <a class="link" href="float128/overloading.html" title="Overloading template functions with float128_t">overloading
 129         template functions with float128_t</a> and the implementation of <code class="computeroutput"><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span></code> for more details.
 130       </p>
 131 <p>
 132         For this reason, making <code class="computeroutput"><span class="preprocessor">#include</span>
 133         <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 134         the <span class="bold"><strong>first include</strong></span> is usually best.
 135       </p>
 136 </td></tr>
 137 </table></div>
 138 </div>
 139 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 140 <td align="left"></td>
 141 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2019 Nikhar
 142       Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
 143       Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
 144       R&#229;de, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
 145       Daryle Walker and Xiaogang Zhang<p>
 146         Distributed under the Boost Software License, Version 1.0. (See accompanying
 147         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>)
 148       </p>
 149 </div></td>
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