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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_multiprecision.ref.internals"></a><a class="link" href="internals.html" title="Internal Support Code">Internal Support
  28       Code</a>
  29 </h3></div></div></div>
  30 <p>
  31         There are some traits classes which authors of new backends should be aware
  32         of:
  33       </p>
  34 <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>
  35
  36 <span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  37 <span class="keyword">struct</span> <span class="identifier">is_explicitly_convertible</span><span class="special">;</span>
  38
  39 <span class="special">}}}</span>
  40 </pre>
  41 <p>
  42         Inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">true</span><span class="special">&gt;</span></code> if type <code class="computeroutput"><span class="identifier">From</span></code>
  43         has an explicit conversion from <code class="computeroutput"><span class="identifier">To</span></code>.
  44       </p>
  45 <p>
  46         For compilers that support C++11 SFINAE-expressions this trait should "just
  47         work". Otherwise it inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">&gt;::</span><span class="identifier">type</span></code>, and will need to be specialised for
  48         Backends that have constructors marked as <code class="computeroutput"><span class="keyword">explicit</span></code>.
  49       </p>
  50 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</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">&gt;</span>
  51 <span class="keyword">struct</span> <span class="identifier">is_lossy_conversion</span>
  52 <span class="special">{</span>
  53    <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>
  54 <span class="special">};</span>
  55 </pre>
  56 <p>
  57         Member <code class="computeroutput"><span class="identifier">value</span></code> is true if the
  58         conversion from <code class="computeroutput"><span class="identifier">From</span></code> to
  59         <code class="computeroutput"><span class="identifier">To</span></code> would result in a loss
  60         of precision, and <code class="computeroutput"><span class="keyword">false</span></code> otherwise.
  61       </p>
  62 <p>
  63         The default version of this trait simply checks whether the <span class="emphasis"><em>kind</em></span>
  64         of conversion (for example from a floating-point to an integer type) is inherently
  65         lossy. Note that if either of the types <code class="computeroutput"><span class="identifier">From</span></code>
  66         or <code class="computeroutput"><span class="identifier">To</span></code> are of an unknown number
  67         category (because <code class="computeroutput"><span class="identifier">number_category</span></code>
  68         is not specialised for that type) then this trait will be <code class="computeroutput"><span class="keyword">true</span></code>.
  69       </p>
  70 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  71 <span class="keyword">struct</span> <span class="identifier">is_restricted_conversion</span>
  72 <span class="special">{</span>
  73    <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>
  74 <span class="special">};</span>
  75 </pre>
  76 <p>
  77         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>
  78         is only explicitly convertible to <code class="computeroutput"><span class="identifier">To</span></code>
  79         and not implicitly convertible, or if <code class="computeroutput"><span class="identifier">is_lossy_conversion</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
  80         Otherwise <code class="computeroutput"><span class="keyword">false</span></code>.
  81       </p>
  82 <p>
  83         Note that while this trait is the ultimate arbiter of which constructors
  84         are marked as <code class="computeroutput"><span class="keyword">explicit</span></code> in class
  85         <code class="computeroutput"><span class="identifier">number</span></code>, authors of backend
  86         types should generally specialise one of the traits above, rather than this
  87         one directly.
  88       </p>
  89 <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>
  90 <span class="identifier">is_signed_number</span><span class="special">;</span>
  91 <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>
  92 <span class="identifier">is_unsigned_number</span><span class="special">;</span>
  93 </pre>
  94 <p>
  95         These two traits inherit from either <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
  96         or <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>, by default types are assumed to
  97         be signed unless <code class="computeroutput"><span class="identifier">is_unsigned_number</span></code>
  98         is specialized for that type.
  99       </p>
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 103 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2019 John Maddock
 104       and Christopher Kormanyos<p>
 105         Distributed under the Boost Software License, Version 1.0. (See accompanying
 106         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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