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| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</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">></span></code> | |
| 31 | is designed to utilize <code class="computeroutput"><span class="special"><</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">></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"><</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">></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"><>::</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"><</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">></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"><>::</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"><</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">></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"><</span><span class="identifier">stdfloat</span><span class="special">.</span><span class="identifier">h</span><span class="special">></span></code> and | |
| 54 | <code class="computeroutput"><span class="special"><</span><span class="identifier">cstdfloat</span><span class="special">></span></code>, then <code class="computeroutput"><span class="special"><</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">></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"><</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">></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"><</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">></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"><</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>>. | |
| 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"><</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">></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"><></span></code> will be available for these types. | |
| 89 | Typically, specializations of <code class="computeroutput"><span class="identifier">numeric_limits</span><span class="special"><></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"><</span><span class="keyword">float</span><span class="special">.</span><span class="identifier">h</span><span class="special">></span></code> may possibly be visible to users of | |
| 103 | <code class="computeroutput"><span class="special"><</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">></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"><></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"><</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">></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 <cmath>. | |
| 120 | </p> | |
| 121 | <p> | |
| 122 | This is because <code class="computeroutput"><span class="special"><</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">></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"><</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">></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 © 2006-2010, 2012-2014 Nikhar Agrawal, | |
| 142 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
| 143 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
| 144 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
| 145 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 146 | 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>) | |
| 147 | </p> | |
| 148 | </div></td> | |
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