1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Internal Support Code</title> 5<link rel="stylesheet" href="../../multiprecision.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.Multiprecision"> 8<link rel="up" href="../ref.html" title="Reference"> 9<link rel="prev" href="cpp_dec_ref.html" title="cpp_dec_float"> 10<link rel="next" href="backendconc.html" title="Backend Requirements"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="cpp_dec_ref.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../ref.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="backendconc.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 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 © 2002-2020 John 104 Maddock 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>) 107 </p> 108</div></td> 109</tr></table> 110<hr> 111<div class="spirit-nav"> 112<a accesskey="p" href="cpp_dec_ref.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../ref.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="backendconc.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 113</div> 114</body> 115</html> 116
