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<a name="math_toolkit.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
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<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">
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<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>
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<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>
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<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; 2006-2010, 2012-2014 Nikhar Agrawal,
Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan R&#229;de, Gautam Sewani,
Benjamin Sobotta, 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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