1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Index 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_splitting.html" title="Splitting modifiers"> 10<link rel="next" href="vmd_modifiers_single.html" title="Modifiers and the single-element sequence"> 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_splitting.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_single.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_index"></a><a class="link" href="vmd_modifiers_index.html" title="Index modifiers">Index 28 modifiers</a> 29</h3></div></div></div> 30<p> 31 Index modifiers can be used with the BOOST_VMD_ELEM macro when identifier 32 modifiers are being used. Index modifiers take two values: 33 </p> 34<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 35<li class="listitem"> 36 BOOST_VMD_RETURN_INDEX, return an index as a number, starting with 0, 37 of the particular identifier modifier which matched, as part of the output 38 of the BOOST_VMD_ELEM macro. If no particular identifier modifier matches, 39 return emptiness as part of the output. The index number is determined 40 purely by the order in which identifier modifiers are specified as optional 41 parameters to BOOST_VMD_ELEM, whether singly as individual optional parameters 42 or as a tuple of identifier modifiers. 43 </li> 44<li class="listitem"> 45 BOOST_VMD_RETURN_NO_INDEX, do not return an index as part of the output. 46 This is the default value and need only be used to override the BOOST_VMD_RETURN_INDEX 47 value if it is specified. 48 </li> 49</ul></div> 50<p> 51 The BOOST_VMD_RETURN_INDEX tells the programmer which one of the identifier 52 modifiers matched the element's data as an index. Some macro programmers 53 find this more useful for the purposes of macro branching logic than branching 54 using the actual name of the identifier itself. 55 </p> 56<p> 57 When the index modifier BOOST_VMD_RETURN_INDEX is specified, and identifier 58 modifiers are specified along with the BOOST_VMD_TYPE_IDENTIFIER filter modifier, 59 the output of BOOST_VMD_ELEM becomes a tuple of two elements. The first tuple 60 element is the element matched and the last tuple element is the index, starting 61 with 0, of the identifier modifier which matched. If an element is not matched 62 both tuple elements are empty. 63 </p> 64<p> 65 If the splitting modifier BOOST_VMD_RETURN_AFTER is also specified then the 66 output is a tuple of three elements. The first tuple element is the element 67 matched, the second tuple element is the rest of the sequence after the matching 68 element, and the last tuple element is the numeric index. If an element is 69 not matched then all three tuple elements are empty. 70 </p> 71<p> 72 If identifier modifiers and the BOOST_VMD_TYPE_IDENTIFIER filter modifier 73 are not specified as optional parameters, then if BOOST_VMD_RETURN_INDEX 74 is specified it is ignored. If the splitting modifier BOOST_VMD_RETURN_ONLY_AFTER 75 is specified, if BOOST_VMD_RETURN_INDEX is also specified it is ignored. 76 </p> 77<p> 78 Let's see how this works: 79 </p> 80<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> 81 82<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> 83<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> 84<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> 85 86<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_ANAME_ANAME</span> 87<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_APLACE_APLACE</span> 88 89<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="special">(</span><span class="number">1</span><span class="special">)(</span><span class="number">2</span><span class="special">)</span> <span class="number">46</span> 90 91<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> 92<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> 93<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">BOOST_VMD_RETURN_INDEX</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="special">(,)</span> 94<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">BOOST_VMD_RETURN_INDEX</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,0)'</span> 95<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">BOOST_VMD_RETURN_INDEX</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,2)'</span> 96</pre> 97<p> 98 Used with splitting modifiers: 99 </p> 100<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> 101 102<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">BOOST_VMD_RETURN_INDEX</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">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="special">(,,)</span> 103<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">BOOST_VMD_RETURN_INDEX</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">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,(1)(2) 46,0)'</span> 104<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">BOOST_VMD_RETURN_INDEX</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">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,(1)(2) 46,2)'</span> 105 106<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">BOOST_VMD_RETURN_INDEX</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">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(1)(2) 46'</span> 107</pre> 108</div> 109<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 110<td align="left"></td> 111<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 112 East Inc</div></td> 113</tr></table> 114<hr> 115<div class="spirit-nav"> 116<a accesskey="p" href="vmd_modifiers_splitting.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../vmd_modifiers.html"><img 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