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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="math_toolkit.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
</h2></div></div></div>
<p>
      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>
      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>,
      defined in the 1989 C standard. The preprocessor is used to query certain preprocessor
      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>
      such as FLT_MAX, DBL_MAX, etc. Based on the results of these queries, an attempt
      is made to automatically detect the presence of built-in floating-point types
      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>
      (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>
      is performed with <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>.
    </p>
<p>
      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>
      supports an 80-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
      if it can be detected, and a 128-bit floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>
      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>
      (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).
    </p>
<p>
      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>
      makes the standardized floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s
      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>.
      The intention is to complement rather than compete with a potential future
      C/C++ Standard Library that may contain these <code class="computeroutput"><span class="keyword">typedef</span></code>s.
      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
      <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>
      will continue to function, but will become redundant and may be safely deprecated.
    </p>
<p>
      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>
      is a Boost header, its name conforms to the boost header naming conventions,
      not the C++ Standard Library header naming conventions.
    </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
        <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>
        <span class="bold"><strong>cannot synthesize or create a <code class="computeroutput"><span class="keyword">typedef</span></code>
        if the underlying type is not provided by the compiler</strong></span>. For example,
        if a compiler does not have an underlying floating-point type with 128 bits
        (highly sought-after in scientific and numeric programming), then <code class="computeroutput"><span class="identifier">float128_t</span></code> and its corresponding least
        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;.
      </p></td></tr>
</table></div>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
        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>
        uses a compiler-specific non-standardized type (<span class="bold"><strong>not</strong></span>
        derived from <code class="computeroutput"><span class="keyword">float</span><span class="special">,</span>
        <span class="keyword">double</span><span class="special">,</span></code>
        or <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>)
        for one or more of its floating-point <code class="computeroutput"><span class="keyword">typedef</span></code>s,
        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.
        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
        types if the compiler itself supports corresponding specializations for the
        underlying type(s), exceptions are GCC's <code class="computeroutput"><span class="identifier">__float128</span></code>
        type and Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> type
        which are explicitly supported via our own code.
      </p></td></tr>
</table></div>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
        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
        <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>.
        Don't rely on using these macros; they are not part of any Boost-specified
        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>
        for floating-point ranges, etc. instead.
      </p></td></tr>
</table></div>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top">
<p>
        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>
        should be <code class="computeroutput"><span class="preprocessor">#include</span></code>d before
        other headers that define generic code making use of standard library functions
        defined in &lt;cmath&gt;.
      </p>
<p>
        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>
        may define overloads of standard library functions where a non-standard type
        (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>
        <span class="keyword">double</span></code>) is used for one of the specified
        width types. If generic code (for example in another Boost.Math header) calls
        a standard library function, then the correct overload will only be found
        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
        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.
      </p>
<p>
        For this reason, making <code class="computeroutput"><span class="preprocessor">#include</span>
        <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>
        the <span class="bold"><strong>first include</strong></span> is usually best.
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2006-2019 Nikhar
      Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
      Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
      R&#229;de, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
      Daryle Walker and Xiaogang Zhang<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        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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