1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>let</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. Phoenix 3.2.0"> 8<link rel="up" href="../scope.html" title="Scope"> 9<link rel="prev" href="local_variables.html" title="Local Variables"> 10<link rel="next" href="lambda.html" title="lambda"> 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="local_variables.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../scope.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="lambda.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="phoenix.modules.scope.let"></a><a class="link" href="let.html" title="let">let</a> 28</h4></div></div></div> 29<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">scope</span><span class="special">/</span><span class="identifier">let</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 30</pre> 31<p> 32 You declare local variables using the syntax: 33 </p> 34<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">local</span><span class="special">-</span><span class="identifier">declarations</span><span class="special">)</span> 35<span class="special">[</span> 36 <span class="identifier">let</span><span class="special">-</span><span class="identifier">body</span> 37<span class="special">]</span> 38</pre> 39<p> 40 <code class="computeroutput"><span class="identifier">let</span></code> allows 1..N local variable 41 declarations (where N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>). 42 Each declaration follows the form: 43 </p> 44<pre class="programlisting"><span class="identifier">local</span><span class="special">-</span><span class="identifier">id</span> <span class="special">=</span> <span class="identifier">lambda</span><span class="special">-</span><span class="identifier">expression</span> 45</pre> 46<div class="note"><table border="0" summary="Note"> 47<tr> 48<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td> 49<th align="left">Note</th> 50</tr> 51<tr><td align="left" valign="top"><p> 52 You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>, 53 the predefined maximum local variable declarations in a let expression. 54 By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code> 55 is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>. 56 </p></td></tr> 57</table></div> 58<p> 59 Example: 60 </p> 61<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="number">123</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span> 62<span class="special">[</span> 63 <span class="identifier">_a</span> <span class="special">+</span> <span class="identifier">_b</span> 64<span class="special">]</span> 65</pre> 66<p> 67 <span class="bold"><strong>Reference Preservation</strong></span> 68 </p> 69<p> 70 The type of the local variable assumes the type of the lambda- expression. 71 Type deduction is reference preserving. For example: 72 </p> 73<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span> 74</pre> 75<p> 76 <code class="computeroutput"><span class="identifier">_a</span></code> assumes the type of 77 <code class="computeroutput"><span class="identifier">arg1</span></code>: a reference to an 78 argument, while <code class="computeroutput"><span class="identifier">_b</span></code> has 79 type <code class="computeroutput"><span class="keyword">int</span></code>. 80 </p> 81<p> 82 Consider this: 83 </p> 84<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> 85 86<span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">)</span> 87<span class="special">[</span> 88 <span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span> 89<span class="special">]</span> 90<span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 91 92<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> 93</pre> 94<p> 95 the output of above is : 0 0 96 </p> 97<p> 98 While with this: 99 </p> 100<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> 101 102<span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">))</span> 103<span class="special">[</span> 104 <span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span> 105<span class="special">]</span> 106<span class="special">(</span><span class="identifier">i</span><span class="special">);</span> 107 108<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> 109</pre> 110<p> 111 the output is : 0 1 112 </p> 113<p> 114 Reference preservation is necessary because we need to have L-value access 115 to outer lambda-scopes (especially the arguments). <code class="computeroutput"><span class="identifier">arg</span></code>s 116 and <code class="computeroutput"><span class="identifier">ref</span></code>s are L-values. 117 <code class="computeroutput"><span class="identifier">val</span></code>s are R-values. 118 </p> 119<p> 120 <span class="bold"><strong>Visibility</strong></span> <a name="phoenix.modules.scope.let.visibility"></a> 121 </p> 122<p> 123 The scope and lifetimes of the local variables is limited within the let-body. 124 <code class="computeroutput"><span class="identifier">let</span></code> blocks can be nested. 125 A local variable may hide an outer local variable. For example: 126 </p> 127<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_y</span> <span class="special">=</span> <span class="identifier">_2</span><span class="special">)</span> 128<span class="special">[</span> 129 <span class="comment">// _x here is an int: 1</span> 130 131 <span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="identifier">_3</span><span class="special">)</span> <span class="comment">// hides the outer _x</span> 132 <span class="special">[</span> 133 <span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_x</span> <span class="special"><<</span> <span class="identifier">_y</span> <span class="comment">// prints "Hello, World"</span> 134 <span class="special">]</span> 135<span class="special">](</span><span class="number">1</span><span class="special">,</span><span class="string">" World"</span><span class="special">,</span><span class="string">"Hello,"</span><span class="special">);</span> 136</pre> 137<p> 138 The actual values of the parameters _1, _2 and _3 are supplied from the 139 bracketed list at the end of the <code class="computeroutput"><span class="identifier">let</span></code>. 140 </p> 141<p> 142 There is currently a limitation that the inner <code class="computeroutput"><span class="identifier">let</span></code> 143 cannot be supplied with a constant e.g. <code class="computeroutput"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="number">1</span><span class="special">)</span></code>. 144 </p> 145<p> 146 The RHS (right hand side lambda-expression) of each local-declaration cannot 147 refer to any LHS local-id. At this point, the local-ids are not in scope 148 yet; they will only be in scope in the let-body. The code below is in error: 149 </p> 150<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span> 151 <span class="identifier">_a</span> <span class="special">=</span> <span class="number">1</span> 152 <span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="identifier">_a</span> <span class="comment">// Error: _a is not in scope yet</span> 153<span class="special">)</span> 154<span class="special">[</span> 155 <span class="comment">// _a and _b's scope starts here</span> 156 <span class="comment">/*. body .*/</span> 157<span class="special">]</span> 158</pre> 159<p> 160 However, if an outer let scope is available, this will be searched. Since 161 the scope of the RHS of a local-declaration is the outer scope enclosing 162 the let, the RHS of a local-declaration can refer to a local variable of 163 an outer scope: 164 </p> 165<pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="number">1</span><span class="special">)</span> 166<span class="special">[</span> 167 <span class="identifier">let</span><span class="special">(</span> 168 <span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">_1</span> 169 <span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="identifier">_a</span> <span class="comment">// Ok. _a refers to the outer _a</span> 170 <span class="special">)</span> 171 <span class="special">[</span> 172 <span class="comment">/*. body .*/</span> 173 <span class="special">]</span> 174<span class="special">](</span><span class="number">1</span><span class="special">)</span> 175</pre> 176</div> 177<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 178<td align="left"></td> 179<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 180 Heller, John Fletcher<p> 181 Distributed under the Boost Software License, Version 1.0. (See accompanying 182 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>) 183 </p> 184</div></td> 185</tr></table> 186<hr> 187<div class="spirit-nav"> 188<a accesskey="p" href="local_variables.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../scope.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="lambda.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 189</div> 190</body> 191</html> 192
