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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="math_toolkit.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
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</h2></div></div></div>
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<p>
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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>
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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>,
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defined in the 1989 C standard. The preprocessor is used to query certain preprocessor
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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>
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such as FLT_MAX, DBL_MAX, etc. Based on the results of these queries, an attempt
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is made to automatically detect the presence of built-in floating-point types
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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>
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(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>
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is performed with <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>.
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</p>
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<p>
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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>
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supports an 80-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
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if it can be detected, and a 128-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
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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>
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(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).
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</p>
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<p>
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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>
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makes the standardized floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s
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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>.
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The intention is to complement rather than compete with a potential future
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C/C++ Standard Library that may contain these <code class="computeroutput"><span class="keyword">typedef</span></code>s.
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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
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<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>
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will continue to function, but will become redundant and may be safely deprecated.
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</p>
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<p>
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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>
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is a Boost header, its name conforms to the boost header naming conventions,
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not the C++ Standard Library header naming conventions.
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</p>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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<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>
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<span class="bold"><strong>cannot synthesize or create a <code class="computeroutput"><span class="keyword">typedef</span></code>
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if the underlying type is not provided by the compiler</strong></span>. For example,
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if a compiler does not have an underlying floating-point type with 128 bits
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(highly sought-after in scientific and numeric programming), then <code class="computeroutput"><span class="identifier">float128_t</span></code> and its corresponding least
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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>>.
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</p></td></tr>
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</table></div>
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<div class="warning"><table border="0" summary="Warning">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
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<th align="left">Warning</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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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>
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uses a compiler-specific non-standardized type (<span class="bold"><strong>not</strong></span>
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derived from <code class="computeroutput"><span class="keyword">float</span><span class="special">,</span>
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<span class="keyword">double</span><span class="special">,</span></code>
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or <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>)
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for one or more of its floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s,
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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.
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Typically, specializations of <code class="computeroutput"><span class="identifier">numeric_limits</span><span class="special"><></span></code> will only be available for these
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types if the compiler itself supports corresponding specializations for the
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underlying type(s), exceptions are GCC's <code class="computeroutput"><span class="identifier">__float128</span></code>
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type and Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> type
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which are explicitly supported via our own code.
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</p></td></tr>
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</table></div>
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<div class="warning"><table border="0" summary="Warning">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
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<th align="left">Warning</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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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
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<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>.
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Don't rely on using these macros; they are not part of any Boost-specified
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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>
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for floating-point ranges, etc. instead.
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</p></td></tr>
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</table></div>
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<div class="tip"><table border="0" summary="Tip">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../doc/src/images/tip.png"></td>
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<th align="left">Tip</th>
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</tr>
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<tr><td align="left" valign="top">
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<p>
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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>
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should be <code class="computeroutput"><span class="preprocessor">#include</span></code>d before
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other headers that define generic code making use of standard library functions
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defined in <cmath>.
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</p>
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<p>
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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>
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may define overloads of standard library functions where a non-standard type
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(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>
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<span class="keyword">double</span></code>) is used for one of the specified
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width types. If generic code (for example in another Boost.Math header) calls
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a standard library function, then the correct overload will only be found
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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
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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.
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</p>
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<p>
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For this reason, making <code class="computeroutput"><span class="preprocessor">#include</span>
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<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>
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the <span class="bold"><strong>first include</strong></span> is usually best.
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</p>
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</td></tr>
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</table></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 © 2006-2019 Nikhar
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Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
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Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
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Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
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Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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