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<title>Sign Manipulation Functions</title>
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<a name="math_toolkit.sign_functions"></a><a class="link" href="sign_functions.html" title="Sign Manipulation Functions">Sign Manipulation Functions</a>
</h2></div></div></div>
<h5>
<a name="math_toolkit.sign_functions.h0"></a>
<span class="phrase"><a name="math_toolkit.sign_functions.synopsis"></a></span><a class="link" href="sign_functions.html#math_toolkit.sign_functions.synopsis">Synopsis</a>
</h5>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">special_functions</span><span class="special">/</span><span class="identifier">sign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">signbit</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">sign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">z</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span>
<span class="identifier">T</span> <span class="identifier">copysign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<a class="link" href="result_type.html" title="Calculation of the Type of the Result"><span class="emphasis"><em>calculated-result-type</em></span></a> <span class="identifier">changesign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">z</span><span class="special">);</span>
<span class="special">}}</span> <span class="comment">// namespaces</span>
</pre>
<h5>
<a name="math_toolkit.sign_functions.h1"></a>
<span class="phrase"><a name="math_toolkit.sign_functions.description"></a></span><a class="link" href="sign_functions.html#math_toolkit.sign_functions.description">Description</a>
</h5>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">signbit</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
Returns a non-zero value if the sign bit is set in variable <span class="emphasis"><em>x</em></span>,
otherwise <code class="computeroutput"><span class="number">0</span></code>.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The return value from this function is zero or <span class="emphasis"><em>not-zero</em></span>
and <span class="bold"><strong>not</strong></span> zero or one.
</p></td></tr>
</table></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">sign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">z</span><span class="special">);</span>
</pre>
<p>
Returns <code class="computeroutput"><span class="number">1</span></code> if <span class="emphasis"><em>x</em></span>
<code class="computeroutput"><span class="special">&gt;</span> <span class="number">0</span></code>,
<code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
if <span class="emphasis"><em>x</em></span> <code class="computeroutput"><span class="special">&lt;</span> <span class="number">0</span></code>, and <code class="computeroutput"><span class="number">0</span></code>
if <span class="emphasis"><em>x</em></span> is zero.
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span>
<a class="link" href="result_type.html" title="Calculation of the Type of the Result"><span class="emphasis"><em>calculated-result-type</em></span></a> <span class="identifier">copysign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">);</span>
</pre>
<p>
Sets the sign of <span class="emphasis"><em>x</em></span> to be the same as the sign of <span class="emphasis"><em>y</em></span>.
</p>
<p>
See <a href="http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf" target="_top">C99
7.12.11.1 The copysign functions</a> for more detail.
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="identifier">T</span> <span class="identifier">changesign</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">z</span><span class="special">);</span>
</pre>
<p>
Returns a floating-point number with a binary representation where the signbit
is the opposite of the sign bit in <span class="emphasis"><em>x</em></span>, and where the other
bits are the same as in <span class="emphasis"><em>x</em></span>.
</p>
<p>
This function is widely available, but not specified in any standards.
</p>
<p>
Rationale: Not specified by TR1, but <code class="computeroutput"><span class="identifier">changesign</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>
is both easier to read and more efficient than
</p>
<pre class="programlisting"><span class="identifier">copysign</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">signbit</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="number">1.0</span> <span class="special">:</span> <span class="special">-</span><span class="number">1.0</span><span class="special">);</span>
</pre>
<p>
For finite values, this function has the same effect as simple negation, the
assignment z = -z, but for nonfinite values, <a href="http://en.wikipedia.org/wiki/Infinity#Computing" target="_top">infinities</a>
and <a href="http://en.wikipedia.org/wiki/NaN" target="_top">NaNs</a>, the <code class="computeroutput"><span class="identifier">changesign</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code> function
may be the only portable way to ensure that the sign bit is changed.
</p>
<h6>
<a name="math_toolkit.sign_functions.h2"></a>
<span class="phrase"><a name="math_toolkit.sign_functions.sign_bits"></a></span><a class="link" href="sign_functions.html#math_toolkit.sign_functions.sign_bits">Sign
bits</a>
</h6>
<p>
One of the bits in the binary representation of a floating-point number gives
the sign, and the remaining bits give the absolute value. That bit is known
as the sign bit. The sign bit is set = 1 for negative numbers, and is not set
= 0 for positive numbers. (This is true for all binary representations of floating-point
numbers that are used by modern microprocessors.)
</p>
<p>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf" target="_top">C++
TR1</a> specifies <code class="computeroutput"><span class="identifier">copysign</span></code>
functions and function templates for accessing the sign bit.
</p>
<p>
For user-defined types (UDT), the sign may be stored in some other way. They
may also not provide infinity or NaNs. To use these functions with a UDT, it
may be necessary to explicitly specialize them for UDT type T.
</p>
<h6>
<a name="math_toolkit.sign_functions.h3"></a>
<span class="phrase"><a name="math_toolkit.sign_functions.examples"></a></span><a class="link" href="sign_functions.html#math_toolkit.sign_functions.examples">Examples</a>
</h6>
<pre class="programlisting"><span class="identifier">signbit</span><span class="special">(</span><span class="number">3.5</span><span class="special">)</span> <span class="identifier">is</span> <span class="identifier">zero</span> <span class="special">(</span><span class="keyword">or</span> <span class="keyword">false</span><span class="special">)</span>
<span class="identifier">signbit</span><span class="special">(-</span><span class="number">7.1</span><span class="special">)</span> <span class="identifier">is</span> <span class="number">1</span> <span class="special">(</span><span class="keyword">or</span> <span class="keyword">true</span><span class="special">)</span>
<span class="identifier">copysign</span><span class="special">(</span><span class="number">4.2</span><span class="special">,</span> <span class="number">7.9</span><span class="special">)</span> <span class="identifier">is</span> <span class="number">4.2</span>
<span class="identifier">copysign</span><span class="special">(</span><span class="number">3.5</span> <span class="special">-</span><span class="number">1.4</span><span class="special">)</span> <span class="identifier">is</span> <span class="special">-</span><span class="number">3.5</span>
<span class="identifier">copysign</span><span class="special">(-</span><span class="number">4.2</span><span class="special">,</span> <span class="number">1.0</span><span class="special">)</span> <span class="identifier">is</span> <span class="number">4.2</span>
<span class="identifier">copysign</span><span class="special">(-</span><span class="number">8.6</span><span class="special">,</span> <span class="special">-</span><span class="number">3.3</span><span class="special">)</span> <span class="identifier">is</span> <span class="special">-</span><span class="number">8.6</span>
<span class="identifier">changesign</span><span class="special">(</span><span class="number">6.9</span><span class="special">)</span> <span class="identifier">is</span> <span class="special">-</span><span class="number">6.9</span>
<span class="identifier">changesign</span><span class="special">(-</span><span class="number">1.8</span><span class="special">)</span> <span class="identifier">is</span> <span class="number">1.8</span>
</pre>
<h6>
<a name="math_toolkit.sign_functions.h4"></a>
<span class="phrase"><a name="math_toolkit.sign_functions.portability"></a></span><a class="link" href="sign_functions.html#math_toolkit.sign_functions.portability">Portability</a>
</h6>
<p>
The library supports the following binary floating-point formats:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
IEEE 754 single precision
</li>
<li class="listitem">
IEEE 754 double precision
</li>
<li class="listitem">
IEEE 754 extended double precision with 15 exponent bits
</li>
<li class="listitem">
Intel extended double precision
</li>
<li class="listitem">
PowerPC extended double precision
</li>
<li class="listitem">
Motorola 68K extended double precision
</li>
</ul></div>
<p>
The library does not support the VAX floating-point formats. (These are available
on VMS, but the default on VMS is the IEEE 754 floating-point format.)
</p>
<p>
The main portability issues are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Unsupported floating-point formats.
</li>
<li class="listitem">
The library depends on the header <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">endian</span><span class="special">.</span><span class="identifier">hpp</span></code>
to detemine endianness.
</li>
<li class="listitem">
Code such as <code class="computeroutput"><span class="preprocessor">#if</span> <span class="identifier">defined</span><span class="special">(</span><span class="identifier">__ia64</span><span class="special">)</span> <span class="special">||</span> <span class="identifier">defined</span><span class="special">(</span><span class="identifier">__ia64__</span><span class="special">)</span> <span class="special">||</span> <span class="identifier">defined</span><span class="special">(</span><span class="identifier">_M_IA64</span><span class="special">)</span></code> is used to determine the processor type.
</li>
</ul></div>
<p>
The library has passed all tests on the following platforms:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Win32 / MSVC 7.1 / 10.0 / x86 32 and 64-bit, and later Win32
</li>
<li class="listitem">
Win32 / Intel C++ 7.1, 8.1, 9.1 / x86
</li>
<li class="listitem">
Mac OS X / GCC 3.3, 4.0 / ppc
</li>
<li class="listitem">
Linux / Intel C++ 9.1 / x86, ia64
</li>
<li class="listitem">
Linux / GCC 3.3 / x86, x64, ia64, ppc, hppa, mips, m68k
</li>
<li class="listitem">
Linux / GCC 3.4 / x64
</li>
<li class="listitem">
HP-UX / aCC, GCC 4.1 / ia64
</li>
<li class="listitem">
HP-UX / aCC / hppa
</li>
<li class="listitem">
Tru64 / Compaq C++ 7.1 / alpha
</li>
<li class="listitem">
VMS / HP C++ 7.1 / alpha (in IEEE floating-point mode)
</li>
<li class="listitem">
VMS / HP C++ 7.2 / ia64 (in IEEE floating-point mode)
</li>
</ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<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>)
</p>
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