1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Basic Usage</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../boost_typeerasure.html" title="Chapter 40. Boost.TypeErasure"> 10<link rel="prev" href="reading.html" title="How to read this documentation"> 11<link rel="next" href="composing_concepts.html" title="Composing Concepts"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="reading.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.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="composing_concepts.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="boost_typeerasure.basic"></a><a class="link" href="basic.html" title="Basic Usage">Basic Usage</a> 29</h2></div></div></div> 30<p> 31 (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/basic.cpp" target="_top">basic.cpp</a>) 32 </p> 33<p> 34 The main class in the library is <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>. 35 An <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> can store objects 36 that meet whatever requirements we specify. These requirements are passed to 37 <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> as an MPL sequence. 38 </p> 39<div class="note"><table border="0" summary="Note"> 40<tr> 41<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 42<th align="left">Note</th> 43</tr> 44<tr><td align="left" valign="top"><p> 45 The MPL sequence combines multiple concepts. In the rare case when we only 46 want a single concept, it doesn't need to be wrapped in an MPL sequence. 47 </p></td></tr> 48</table></div> 49<p> 50</p> 51<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">copy_constructible</span><span class="special"><>,</span> <span class="identifier">typeid_</span><span class="special"><>,</span> <span class="identifier">relaxed</span><span class="special">></span> <span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span> 52<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">any_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// i == 10</span> 53</pre> 54<p> 55 </p> 56<p> 57 <code class="computeroutput"><a class="link" href="../boost/type_erasure/copy_constructible.html" title="Struct template copy_constructible">copy_constructible</a></code> 58 is a builtin concept that allows us to copy and destroy the object. <code class="computeroutput"><a class="link" href="../boost/type_erasure/typeid_.html" title="Struct template typeid_">typeid_</a></code> provides run-time type 59 information so that we can use <code class="computeroutput"><a class="link" href="../boost/type_erasure/any_cast.html" title="Function any_cast">any_cast</a></code>. 60 <code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code> enables various 61 useful defaults. Without <code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code>, 62 <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> supports <span class="emphasis"><em>exactly</em></span> 63 what you specify and nothing else. In particular, it allows default construction 64 and assignment of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>. 65 </p> 66<p> 67 Now, this example doesn't do very much. <code class="computeroutput"><span class="identifier">x</span></code> 68 is approximately equivalent to a <a href="../../../libs/any/index.html" target="_top">boost::any</a>. 69 We can make it more interesting by adding some operators, such as <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> and 70 <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<</span></code>. 71 </p> 72<p> 73</p> 74<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span> 75 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> 76 <span class="identifier">copy_constructible</span><span class="special"><>,</span> 77 <span class="identifier">typeid_</span><span class="special"><>,</span> 78 <span class="identifier">incrementable</span><span class="special"><>,</span> 79 <span class="identifier">ostreamable</span><span class="special"><></span> 80 <span class="special">></span> 81<span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span> 82<span class="special">++</span><span class="identifier">x</span><span class="special">;</span> 83<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">x</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 11</span> 84</pre> 85<p> 86 </p> 87<p> 88 The library provides concepts for most C++ operators, but this obviously won't 89 cover all use cases; we often need to define our own requirements. Let's take 90 the <code class="computeroutput"><span class="identifier">push_back</span></code> member, defined 91 by several STL containers. 92 </p> 93<p> 94</p> 95<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_MEMBER</span><span class="special">(</span><span class="identifier">push_back</span><span class="special">)</span> 96 97<span class="keyword">void</span> <span class="identifier">append_many</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="keyword">void</span><span class="special">(</span><span class="keyword">int</span><span class="special">)>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">container</span><span class="special">)</span> <span class="special">{</span> 98 <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">10</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> 99 <span class="identifier">container</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 100<span class="special">}</span> 101</pre> 102<p> 103 </p> 104<p> 105 We use the macro <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_MEMBER.html" title="Macro BOOST_TYPE_ERASURE_MEMBER">BOOST_TYPE_ERASURE_MEMBER</a></code> 106 to define a concept called <code class="computeroutput"><span class="identifier">has_push_back</span></code>. 107 When we use <code class="computeroutput"><span class="identifier">has_push_back</span></code>, 108 we have to tell it the signature of the function, <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span></code>. 109 This means that the type we store in the any has to have a member that looks 110 like: 111 </p> 112<p> 113</p> 114<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">push_back</span><span class="special">(</span><span class="keyword">int</span><span class="special">);</span> 115</pre> 116<p> 117 </p> 118<p> 119 Thus, we could call <code class="computeroutput"><span class="identifier">append_many</span></code> 120 with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, 121 or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> (because 122 <code class="computeroutput"><span class="keyword">int</span></code> is convertible to <code class="computeroutput"><span class="keyword">long</span></code>), but not <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>. 123 </p> 124<p> 125 Also, note that <code class="computeroutput"><span class="identifier">append_many</span></code> 126 has to operate directly on its argument. It cannot make a copy. To handle this 127 we use <code class="computeroutput"><span class="identifier">_self</span><span class="special">&</span></code> 128 as the second argument of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>. 129 <code class="computeroutput"><span class="identifier">_self</span></code> is a <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>. 130 By using <code class="computeroutput"><span class="identifier">_self</span><span class="special">&</span></code>, 131 we indicate that the <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> 132 stores a reference to an external object instead of allocating its own object. 133 </p> 134<p> 135 Member functions can be const. 136 </p> 137<p> 138</p> 139<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_MEMBER</span><span class="special">(</span><span class="identifier">empty</span><span class="special">)</span> 140<span class="keyword">bool</span> <span class="identifier">is_empty</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_empty</span><span class="special"><</span><span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span><span class="special">>,</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> 141 <span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">empty</span><span class="special">();</span> 142<span class="special">}</span> 143</pre> 144<p> 145 </p> 146<p> 147 For free functions, we can use the macro <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_FREE.html" title="Macro BOOST_TYPE_ERASURE_FREE">BOOST_TYPE_ERASURE_FREE</a></code>. 148 </p> 149<p> 150</p> 151<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_FREE</span><span class="special">(</span><span class="identifier">getline</span><span class="special">)</span> 152<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">read_lines</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_getline</span><span class="special"><</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">_self</span><span class="special">&,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&)>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">stream</span><span class="special">)</span> 153<span class="special">{</span> 154 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">result</span><span class="special">;</span> 155 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">tmp</span><span class="special">;</span> 156 <span class="keyword">while</span><span class="special">(</span><span class="identifier">getline</span><span class="special">(</span><span class="identifier">stream</span><span class="special">,</span> <span class="identifier">tmp</span><span class="special">))</span> 157 <span class="identifier">result</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">tmp</span><span class="special">);</span> 158 <span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span> 159<span class="special">}</span> 160</pre> 161<p> 162 </p> 163<p> 164 The use of <code class="computeroutput"><span class="identifier">has_getline</span></code> is very 165 similar to <code class="computeroutput"><span class="identifier">has_push_back</span></code> above. 166 The difference is that the placeholder <code class="computeroutput"><span class="identifier">_self</span></code> 167 is passed in the function signature instead of as a separate argument. 168 </p> 169<p> 170 The <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code> 171 doesn't have to be the first argument. We could just as easily make it the 172 second argument. 173 </p> 174<p> 175</p> 176<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">read_line</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_getline</span><span class="special"><</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span><span class="special">&,</span> <span class="identifier">_self</span><span class="special">&)>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">str</span><span class="special">)</span> 177<span class="special">{</span> 178 <span class="identifier">getline</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span><span class="special">,</span> <span class="identifier">str</span><span class="special">);</span> 179<span class="special">}</span> 180</pre> 181<p> 182 </p> 183</div> 184<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 185<td align="left"></td> 186<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Steven Watanabe<p> 187 Distributed under the Boost Software License, Version 1.0. (See accompanying 188 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>) 189 </p> 190</div></td> 191</tr></table> 192<hr> 193<div class="spirit-nav"> 194<a accesskey="p" href="reading.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.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="composing_concepts.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 195</div> 196</body> 197</html> 198
