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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_multiprecision.ref.internals"></a><a class="link" href="internals.html" title="Internal Support Code">Internal Support
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Code</a>
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</h3></div></div></div>
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<p>
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There are some traits classes which authors of new backends should be aware
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of:
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</p>
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<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">multiprecision</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">detail</span><span class="special">{</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">></span>
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<span class="keyword">struct</span> <span class="identifier">is_explicitly_convertible</span><span class="special">;</span>
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<span class="special">}}}</span>
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</pre>
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<p>
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Inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">true</span><span class="special">></span></code> if type <code class="computeroutput"><span class="identifier">From</span></code>
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has an explicit conversion from <code class="computeroutput"><span class="identifier">To</span></code>.
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">To</span><span class="special">></span>
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<span class="keyword">struct</span> <span class="identifier">is_lossy_conversion</span>
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<span class="special">{</span>
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<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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Member <code class="computeroutput"><span class="identifier">value</span></code> is true if the
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conversion from <code class="computeroutput"><span class="identifier">From</span></code> to
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<code class="computeroutput"><span class="identifier">To</span></code> would result in a loss
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of precision, and <code class="computeroutput"><span class="keyword">false</span></code> otherwise.
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</p>
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<p>
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The default version of this trait simply checks whether the <span class="emphasis"><em>kind</em></span>
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of conversion (for example from a floating-point to an integer type) is inherently
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lossy. Note that if either of the types <code class="computeroutput"><span class="identifier">From</span></code>
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or <code class="computeroutput"><span class="identifier">To</span></code> are of an unknown number
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category (because <code class="computeroutput"><span class="identifier">number_category</span></code>
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is not specialised for that type) then this trait will be <code class="computeroutput"><span class="keyword">true</span></code>.
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">></span>
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<span class="keyword">struct</span> <span class="identifier">is_restricted_conversion</span>
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<span class="special">{</span>
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<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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Member <code class="computeroutput"><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code> if <code class="computeroutput"><span class="identifier">From</span></code>
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is only explicitly convertible to <code class="computeroutput"><span class="identifier">To</span></code>
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and not implicitly convertible, or if <code class="computeroutput"><span class="identifier">is_lossy_conversion</span><span class="special"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">>::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
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Otherwise <code class="computeroutput"><span class="keyword">false</span></code>.
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</p>
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<p>
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Note that while this trait is the ultimate arbiter of which constructors
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are marked as <code class="computeroutput"><span class="keyword">explicit</span></code> in class
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<code class="computeroutput"><span class="identifier">number</span></code>, authors of backend
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types should generally specialise one of the traits above, rather than this
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one directly.
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
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<span class="identifier">is_signed_number</span><span class="special">;</span>
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<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
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<span class="identifier">is_unsigned_number</span><span class="special">;</span>
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</pre>
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<p>
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These two traits inherit from either <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,</span>
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<span class="keyword">true</span><span class="special">></span></code>
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or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,</span> <span class="keyword">false</span><span class="special">></span></code>, by default types are assumed to be signed
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unless <code class="computeroutput"><span class="identifier">is_unsigned_number</span></code>
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is specialized for that type.
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</p>
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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 © 2002-2020 John
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Maddock and Christopher Kormanyos<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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</div></td>
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</tr></table>
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