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| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</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> | |
| 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"><</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> | |
| 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"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">>::</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"><</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> | |
| 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"><</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> | |
| 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"><</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>. | |
| 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"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | |
| 90 | <span class="identifier">is_signed_number</span><span class="special">;</span> | |
| 91 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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> | |
| 100 | </div> | |
| 101 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 102 | <td align="left"></td> | |
| 103 | <td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<p> | |
| 104 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 105 | 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>) | |
| 106 | </p> | |
| 107 | </div></td> | |
| 108 | </tr></table> | |
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