1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Identifier modifiers</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_modifiers.html" title="Macros with modifiers"> 9<link rel="prev" href="vmd_modifiers_filter.html" title="Filtering modifiers"> 10<link rel="next" href="vmd_modifiers_splitting.html" title="Splitting modifiers"> 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_modifiers_filter.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_modifiers.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_modifiers_splitting.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_modifiers.vmd_modifiers_identifier"></a><a class="link" href="vmd_modifiers_identifier.html" title="Identifier modifiers">Identifier 28 modifiers</a> 29</h3></div></div></div> 30<p> 31 Identifier modifiers are optional parameters which specify a set of identifiers 32 to search in order to look for a particular identifier match rather than 33 just any identifier. 34 </p> 35<h5> 36<a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.h0"></a> 37 <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.usage_with_boost_vmd_is_identifi"></a></span><a class="link" href="vmd_modifiers_identifier.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.usage_with_boost_vmd_is_identifi">Usage 38 with BOOST_VMD_IS_IDENTIFIER</a> 39 </h5> 40<p> 41 Once we have both registered and pre-detected an identifier we can test whether 42 an identifier is a particular identifier using BOOST_VMD_IS_IDENTIFER and 43 identifier modifiers. We do this by passing optional parameter(s) to BOOST_VMD_IS_IDENTIFER. 44 The optional parameter(s) are either a single tuple of possible identifiers 45 we are trying to match, or the individual identifiers themselves as separate 46 parameters. 47 </p> 48<p> 49 Using the optional parameter(s) with BOOST_VMD_IS_IDENTIFER we are asking 50 not only if our input is any of the registered identifiers but also if it 51 is one of a number of pre-detected identifiers. 52 </p> 53<p> 54 As an example: 55 </p> 56<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_identifier</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 57 58<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_yellow</span> <span class="special">(</span><span class="identifier">yellow</span><span class="special">)</span> 59<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_green</span> <span class="special">(</span><span class="identifier">green</span><span class="special">)</span> 60<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_blue</span> <span class="special">(</span><span class="identifier">blue</span><span class="special">)</span> 61<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_red</span> <span class="special">(</span><span class="identifier">red</span><span class="special">)</span> 62 63<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_yellow_yellow</span> 64<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_green_green</span> 65<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_blue_blue</span> 66 67<span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,</span><span class="identifier">yellow</span><span class="special">)</span> <span class="comment">// returns 1 only if 'some_input is 'yellow', else returns 0</span> 68<span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,</span><span class="identifier">yellow</span><span class="special">,</span><span class="identifier">blue</span><span class="special">)</span> <span class="comment">// returns 1 only if 'some_input is 'yellow' or 'blue', else returns 0</span> 69<span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,(</span><span class="identifier">yellow</span><span class="special">,</span><span class="identifier">green</span><span class="special">))</span> <span class="comment">// returns 1 if 'some_input' is 'yellow' or 'green', else returns 0</span> 70 71<span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,</span><span class="identifier">red</span><span class="special">)</span> 72 <span class="comment">// always returns 0, even if 'some_input' is 'red' since 'red' has not been pre-detected</span> 73</pre> 74<p> 75 whereas 76 </p> 77<pre class="programlisting"><span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">)</span> <span class="comment">// returns 1 if 'some_input' is 'red' since 'red' has been registered</span> 78</pre> 79<p> 80 If you invoke BOOST_VMD_IS_IDENTIFIER with the optional parameter(s), the 81 invocation will only return 1 if the input matches one the identifier(s) 82 of the optional parameters and the identifier it matches has been registered 83 and pre-detected. 84 </p> 85<p> 86 Both VMD numbers and v-types are identifier subtypes so you can also use 87 them as identifier modifiers. You do not have to register or pre-detect VMD 88 numbers or v-types since VMD has already done that for you. 89 </p> 90<p> 91 As an example of using VMD numbers or v-types as identifier modifiers with 92 BOOST_VMD_IS_IDENTIFIER: 93 </p> 94<pre class="programlisting"><span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,</span><span class="number">1</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">5</span><span class="special">)</span> <span class="comment">// returns 1 only if 'some_input' is 1 or 3 or 5, else returns 0</span> 95<span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">some_input</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">,</span><span class="number">59</span><span class="special">)</span> 96 <span class="comment">// returns 1 only if 'some_input is the v-type BOOST_VMD_TYPE_TUPLE or the v-type BOOST_VMD_TYPE_LIST or 59, else returns 0</span> 97</pre> 98<h5> 99<a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.h1"></a> 100 <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.usage_with_boost_vmd_elem"></a></span><a class="link" href="vmd_modifiers_identifier.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_identifier.usage_with_boost_vmd_elem">Usage 101 with BOOST_VMD_ELEM</a> 102 </h5> 103<p> 104 When we use the specific filter modifier BOOST_VMD_TYPE_IDENTIFIER as an 105 optional parameter of BOOST_VMD_ELEM we are asking for a particular element 106 of a sequence as long as it is a VMD identifier. With that specific filter 107 modifier BOOST_VMD_TYPE_IDENTIFIER we can use identifier modifiers to ask 108 for a particular element of a sequence as long as it matches one of our identifier 109 modifiers. If the specific filter modifier BOOST_VMD_TYPE_IDENTIFIER is not 110 being used then all identifier modifiers are ignored. 111 </p> 112<p> 113 The syntax for specifying identifier modifiers using BOOST_VMD_ELEM is exactly 114 the same as the equivalent feature of the BOOST_VMD_IS_IDENTIFIER macro explained 115 just previously. Optional parameters in the form of identifiers can be specified 116 either singly any number of times or once as part of a tuple. For an identifier 117 found as a sequence element to match against one of these possible identifiers, 118 the possible identifiers must be both registered and pre-detected. 119 </p> 120<p> 121 Since filter modifiers, which are v-types, are also identifiers, if you want 122 to use v-types as identifier modifiers you must use the form which places 123 all identifier modifiers as part of a tuple. Otherwise any v-types specified 124 singly as optional parameters are seen as filter modifiers and never as identifier 125 modifiers. VMD numbers are also identifiers and may be used as identifier 126 modifiers, but in this case VMD numbers as identifier modifiers do not need 127 to be part of a tuple to be detected. 128 </p> 129<p> 130 Let's see how this works: 131 </p> 132<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">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 133 134<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANAME</span> <span class="special">(</span><span class="identifier">ANAME</span><span class="special">)</span> 135<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_APLACE</span> <span class="special">(</span><span class="identifier">APLACE</span><span class="special">)</span> 136<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ACOUNTRY</span> <span class="special">(</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> 137 138<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_ANAME_ANAME</span> 139<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_APLACE_APLACE</span> 140 141<span class="preprocessor">#define</span> <span class="identifier">A_SEQUENCE</span> <span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">)</span> <span class="identifier">ANAME</span> <span class="number">46</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="identifier">ACOUNTRY</span> 142 143<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANAME'</span> 144<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANAME'</span> 145<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span> 146<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANAME'</span> 147<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,(</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">))</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANAME'</span> 148 149<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ACOUNTRY'</span> 150<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ACOUNTRY'</span> 151<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">)</span> 152 <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span> <span class="identifier">since</span> <span class="identifier">ACOUNTRY</span> <span class="identifier">is</span> <span class="keyword">not</span> <span class="identifier">pre</span><span class="special">-</span><span class="identifier">detected</span> 153</pre> 154<p> 155 Let us illustrate the case where VMD identifiers can be represented as either 156 filter modifiers or identifier modifiers. 157 </p> 158<p> 159 Using the sequence above: 160 </p> 161<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">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 162 163<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="identifier">type</span> 164<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">)</span> 165 <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="identifier">type</span> <span class="identifier">since</span> <span class="identifier">a</span> <span class="identifier">type</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">identifier</span> 166<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_SEQ</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span> 167</pre> 168<p> 169 The last use of our macro returns emptiness because if there is more than 170 one type specified as an optional parameter the last type is chosen for filtering. 171 Since the last type for type filtering is BOOST_VMD_TYPE_TUPLE and our fourth 172 element is a v-type and not a tuple, emptiness is returned. The syntax does 173 not specifying filtering with identifiers as might be supposed since BOOST_VMD_TYPE_SEQ 174 and BOOST_VMD_TYPE_TUPLE are not treated as identifier modifiers but rather 175 as additional filter modifiers. 176 </p> 177<p> 178 In order to do filtering with an identifier and do it against various types 179 themselves, since v-types are identifiers, we must use the tuple form to 180 specify our identifier modifiers: 181 </p> 182<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">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 183 184<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,(</span><span class="identifier">BOOST_VMD_TYPE_SEQ</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">))</span> 185 <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> 186</pre> 187<p> 188 Now BOOST_VMD_TYPE_SEQ and BOOST_VMD_TYPE_TUPLE are treated as identifiers 189 modifiers to match against. 190 </p> 191</div> 192<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 193<td align="left"></td> 194<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 195 East Inc</div></td> 196</tr></table> 197<hr> 198<div class="spirit-nav"> 199<a accesskey="p" href="vmd_modifiers_filter.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_modifiers.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_modifiers_splitting.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 200</div> 201</body> 202</html> 203
