1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Types</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.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. The Variadic Macro Data Library 1.9"> 8<link rel="up" href="../vmd_specific.html" title="Specific macros for working with data types"> 9<link rel="prev" href="vmd_number.html" title="Numbers"> 10<link rel="next" href="vmd_pp_data_types.html" title="VMD and Boost PP data types"> 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="vmd_number.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_specific.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="vmd_pp_data_types.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="variadic_macro_data.vmd_specific.vmd_type"></a><a class="link" href="vmd_type.html" title="Types">Types</a> 28</h3></div></div></div> 29<p> 30 A subset of identifiers is VMD types, called a 'v-type'. These are identifiers 31 which represent all of the preprocessor data types which VMD can parse. This 32 subset of identifiers is automatically registered and pre-detected by VMD. 33 Each identifier type begins with the unique prefix 'BOOST_VMD_TYPE_'. 34 </p> 35<p> 36 The actual types are: 37 </p> 38<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 39<li class="listitem"> 40 BOOST_VMD_TYPE_EMPTY, represents emptiness, ie. "empty data" 41 </li> 42<li class="listitem"> 43 BOOST_VMD_TYPE_ARRAY, a Boost PP array 44 </li> 45<li class="listitem"> 46 BOOST_VMD_TYPE_LIST, a Boost PP list 47 </li> 48<li class="listitem"> 49 BOOST_VMD_TYPE_SEQ, a Boost PP seq 50 </li> 51<li class="listitem"> 52 BOOST_VMD_TYPE_TUPLE, a Boost PP tuple 53 </li> 54<li class="listitem"> 55 BOOST_VMD_TYPE_IDENTIFIER, identifier 56 </li> 57<li class="listitem"> 58 BOOST_BMD_TYPE_NUMBER, a number 59 </li> 60<li class="listitem"> 61 BOOST_VMD_TYPE_TYPE, a type itself 62 </li> 63<li class="listitem"> 64 BOOST_VMD_TYPE_SEQUENCE, a sequence 65 </li> 66<li class="listitem"> 67 BOOST_VMD_TYPE_UNKNOWN, an unknown type 68 </li> 69</ul></div> 70<p> 71 Since a v-type is itself an identifier the particular constraint on the input 72 to test is exactly the same as for identifiers. 73 </p> 74<p> 75 The constraint is that the beginning input character, ignoring any whitespace, 76 passed as the input to test must be either: 77 </p> 78<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 79<li class="listitem"> 80 an identifier character, ie. an alphanumeric or an underscore 81 </li> 82<li class="listitem"> 83 the left parenthesis of a tuple 84 </li> 85</ul></div> 86<p> 87 and if the first character is not the left parenthesis of a tuple the remaining 88 characters must be alphanumeric or an underscore until a space character 89 or end of input occurs. 90 </p> 91<p> 92 If this is not the case the behavior is undefined, and most likely a preprocessing 93 error will occur. 94 </p> 95<p> 96 The macro used to test for a particular type in VMD is called BOOST_VMD_IS_TYPE. 97 The macro takes a single variadic parameter, the input to test against. 98 </p> 99<p> 100 The macro returns 1 if the parameter is a v-type, otherwise the macro returns 101 0. 102 </p> 103<p> 104 A v-type is also an identifier, which has been registered and pre-detected, 105 so you can also use the VMD functionality which works with identifiers to 106 work with a v-type as an identifier if you like. 107 </p> 108<h5> 109<a name="variadic_macro_data.vmd_specific.vmd_type.h0"></a> 110 <span class="phrase"><a name="variadic_macro_data.vmd_specific.vmd_type.example"></a></span><a class="link" href="vmd_type.html#variadic_macro_data.vmd_specific.vmd_type.example">Example</a> 111 </h5> 112<p> 113 Let us look at an example of how to use BOOST_VMD_IS_TYPE. 114 </p> 115<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 116 117<span class="identifier">BOOST_VMD_IS_TYPE</span><span class="special">(</span><span class="identifier">input</span><span class="special">)</span> 118 119<span class="identifier">returns</span><span class="special">:</span> 120 121<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span><span class="special">,</span> <span class="number">1</span> 122<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_NUMBER</span><span class="special">,</span> <span class="number">1</span> 123<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">SQUARE</span><span class="special">,</span> <span class="number">0</span> 124<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span> <span class="identifier">DATA</span><span class="special">,</span> <span class="number">0</span> <span class="identifier">since</span> <span class="identifier">there</span> <span class="identifier">are</span> <span class="identifier">tokens</span> <span class="identifier">after</span> <span class="identifier">the</span> <span class="identifier">type</span> 125<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="special">%</span><span class="number">44</span><span class="special">,</span> <span class="identifier">does</span> <span class="keyword">not</span> <span class="identifier">meet</span> <span class="identifier">the</span> <span class="identifier">constraint</span> <span class="identifier">therefore</span> <span class="identifier">undefined</span> <span class="identifier">behavior</span> 126<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">BOOST_VMD_TYPE_EMPTY</span> <span class="special">),</span> <span class="number">0</span> <span class="identifier">since</span> <span class="identifier">the</span> <span class="identifier">macro</span> <span class="identifier">begins</span> <span class="identifier">with</span> <span class="identifier">a</span> <span class="identifier">tuple</span> <span class="keyword">and</span> <span class="keyword">this</span> <span class="identifier">can</span> <span class="identifier">be</span> <span class="identifier">tested</span> <span class="keyword">for</span> 127</pre> 128<h5> 129<a name="variadic_macro_data.vmd_specific.vmd_type.h1"></a> 130 <span class="phrase"><a name="variadic_macro_data.vmd_specific.vmd_type.usage"></a></span><a class="link" href="vmd_type.html#variadic_macro_data.vmd_specific.vmd_type.usage">Usage</a> 131 </h5> 132<p> 133 To use the BOOST_VMD_IS_TYPE macro either include the general header: 134 </p> 135<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 136</pre> 137<p> 138 or include the specific header: 139 </p> 140<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 141</pre> 142</div> 143<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 144<td align="left"></td> 145<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 146 East Inc</div></td> 147</tr></table> 148<hr> 149<div class="spirit-nav"> 150<a accesskey="p" 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