1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Examples</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. Boost.LocalFunction 1.0.0"> 8<link rel="up" href="../index.html" title="Chapter 1. Boost.LocalFunction 1.0.0"> 9<link rel="prev" href="advanced_topics.html" title="Advanced Topics"> 10<link rel="next" href="alternatives.html" title="Annex: Alternatives"> 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="advanced_topics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="alternatives.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="boost_localfunction.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a> 28</h2></div></div></div> 29<div class="toc"><dl class="toc"> 30<dt><span class="section"><a href="examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_">GCC 31 Lambdas (without C++11)</a></span></dt> 32<dt><span class="section"><a href="examples.html#boost_localfunction.examples.constant_blocks">Constant 33 Blocks</a></span></dt> 34<dt><span class="section"><a href="examples.html#boost_localfunction.examples.scope_exits">Scope Exits</a></span></dt> 35<dt><span class="section"><a href="examples.html#boost_localfunction.examples.boost_phoenix_functions">Boost.Phoenix 36 Functions</a></span></dt> 37<dt><span class="section"><a href="examples.html#boost_localfunction.examples.closures">Closures</a></span></dt> 38<dt><span class="section"><a href="examples.html#boost_localfunction.examples.gcc_nested_functions">GCC 39 Nested Functions</a></span></dt> 40<dt><span class="section"><a href="examples.html#boost_localfunction.examples.n_papers">N-Papers</a></span></dt> 41</dl></div> 42<p> 43 This section lists some examples that use this library. 44 </p> 45<div class="section"> 46<div class="titlepage"><div><div><h3 class="title"> 47<a name="boost_localfunction.examples.gcc_lambdas__without_c__11_"></a><a class="link" href="examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_" title="GCC Lambdas (without C++11)">GCC 48 Lambdas (without C++11)</a> 49</h3></div></div></div> 50<p> 51 Combing local functions with the non-standard <a href="http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html" target="_top">statement 52 expression</a> extension of the GCC compiler, it is possible to implement 53 lambda functions for GCC compilers even without <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> 54 support. 55 </p> 56<div class="warning"><table border="0" summary="Warning"> 57<tr> 58<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td> 59<th align="left">Warning</th> 60</tr> 61<tr><td align="left" valign="top"><p> 62 This code only works on compilers that support GCC statement expression 63 extension or that support <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 64 lambda functions</a>. 65 </p></td></tr> 66</table></div> 67<p> 68 For example (see also <a href="../../../example/gcc_lambda.cpp" target="_top"><code class="literal">gcc_lambda.cpp</code></a> 69 and <a href="../../../example/gcc_cxx11_lambda.cpp" target="_top"><code class="literal">gcc_cxx11_lambda.cpp</code></a>): 70 </p> 71<div class="informaltable"><table class="table"> 72<colgroup> 73<col> 74<col> 75</colgroup> 76<thead><tr> 77<th> 78 <p> 79 With Local Functions (GCC only) 80 </p> 81 </th> 82<th> 83 <p> 84 C++11 Lambdas 85 </p> 86 </th> 87</tr></thead> 88<tbody><tr> 89<td> 90 <p> 91</p> 92<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 93<span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</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> 94<span class="keyword">int</span><span class="special">*</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">3</span><span class="special">;</span> 95 96<span class="keyword">int</span><span class="special">*</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">end</span><span class="special">,</span> 97 <span class="identifier">GCC_LAMBDA</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">val</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">,</span> <span class="keyword">return</span> <span class="keyword">bool</span><span class="special">)</span> <span class="special">{</span> 98 <span class="keyword">return</span> <span class="identifier">num</span> <span class="special">==</span> <span class="identifier">val</span><span class="special">;</span> 99 <span class="special">}</span> <span class="identifier">GCC_LAMBDA_END</span> 100<span class="special">);</span> 101</pre> 102<p> 103 </p> 104 </td> 105<td> 106 <p> 107</p> 108<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 109<span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</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> 110<span class="keyword">int</span><span class="special">*</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">3</span><span class="special">;</span> 111 112<span class="keyword">int</span><span class="special">*</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">end</span><span class="special">,</span> 113 <span class="special">[</span><span class="identifier">val</span><span class="special">](</span><span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">-></span> <span class="keyword">bool</span> <span class="special">{</span> 114 <span class="keyword">return</span> <span class="identifier">num</span> <span class="special">==</span> <span class="identifier">val</span><span class="special">;</span> 115 <span class="special">}</span> 116<span class="special">);</span> 117</pre> 118<p> 119 </p> 120 </td> 121</tr></tbody> 122</table></div> 123<p> 124 The GCC lambda function macros are implemented using local functions (see 125 also <a href="../../../example/gcc_lambda.hpp" target="_top"><code class="literal">gcc_lambda.hpp</code></a>): 126 </p> 127<p> 128</p> 129<pre class="programlisting"><span class="preprocessor"># define</span> <span class="identifier">GCC_LAMBDA_</span><span class="special">(</span><span class="identifier">binds</span><span class="special">,</span> <span class="identifier">params</span><span class="special">,</span> <span class="identifier">results</span><span class="special">)</span> <span class="special">\</span> 130 <span class="special">({</span> <span class="comment">/* open statement expression (GCC extension only) */</span> <span class="special">\</span> 131 <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span> <span class="special">\</span> 132 <span class="identifier">BOOST_PP_LIST_ENUM</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_APPEND</span><span class="special">(</span><span class="identifier">binds</span><span class="special">,</span> <span class="special">\</span> 133 <span class="identifier">BOOST_PP_LIST_APPEND</span><span class="special">(</span><span class="identifier">params</span><span class="special">,</span> <span class="special">\</span> 134 <span class="identifier">BOOST_PP_IIF</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_IS_NIL</span><span class="special">(</span><span class="identifier">results</span><span class="special">),</span> <span class="special">\</span> 135 <span class="special">(</span><span class="keyword">return</span> <span class="keyword">void</span><span class="special">,</span> <span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="comment">/* default for lambdas */</span> <span class="special">\</span> 136 <span class="special">,</span> <span class="special">\</span> 137 <span class="identifier">results</span> <span class="special">\</span> 138 <span class="special">)\</span> 139 <span class="special">)</span> <span class="special">\</span> 140 <span class="special">))</span> <span class="special">\</span> 141 <span class="special">)</span> 142</pre> 143<p> 144 </p> 145<p> 146</p> 147<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">GCC_LAMBDA_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span> 148 <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">gcc_lambda_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span> 149 <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">gcc_lambda_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">);</span> <span class="special">\</span> 150 <span class="special">})</span> <span class="comment">/* close statement expression (GCC extension only) */</span> 151</pre> 152<p> 153 </p> 154<p> 155 This is possible because GCC statement expressions allow to use declaration 156 statements within expressions and therefore to declare a local function within 157 an expression. The macros automatically detect if the compiler supports 158 <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 159 lambda functions</a> in which case the implementation uses native lambdas 160 instead of local functions in GCC statement expressions. However, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 161 lambda functions</a> do not support constant binding so it is best to 162 only use <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span> 163 <span class="identifier">variable</span></code> (same as <code class="computeroutput"><span class="special">=</span><span class="identifier">variable</span></code> 164 for <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 165 lambda functions</a>) and <code class="computeroutput"><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">variable</span></code> 166 (same as <code class="computeroutput"><span class="special">&</span><span class="identifier">variable</span></code> 167 for <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 168 lambda functions</a>') because these have the exact same semantic between 169 the local function and the native lambda implementations. Furthermore, local 170 functions allow to bind data members directly while <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 171 lambda functions</a> require to access data members via binding the object 172 <code class="computeroutput"><span class="keyword">this</span></code>. Unfortunately, the short-hand 173 binds <code class="computeroutput"><span class="special">&</span></code> and <code class="computeroutput"><span class="special">=</span></code> of <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 174 lambda functions</a> (which automatically bind all variables in scope 175 either by reference or value) are not supported by these GCC lambda function 176 macros because they are not supported by local functions. Finally, the result 177 type <code class="computeroutput"><span class="keyword">return</span> </code><code class="literal"><span class="emphasis"><em>result-type</em></span></code> 178 is optional and it is assumed <code class="computeroutput"><span class="keyword">void</span></code> 179 when it is not specified (same as with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 180 lambda functions</a>). 181 </p> 182</div> 183<div class="section"> 184<div class="titlepage"><div><div><h3 class="title"> 185<a name="boost_localfunction.examples.constant_blocks"></a><a class="link" href="examples.html#boost_localfunction.examples.constant_blocks" title="Constant Blocks">Constant 186 Blocks</a> 187</h3></div></div></div> 188<p> 189 It is possible to use local functions to check assertions between variables 190 that are made constant within the asserted expressions. This is advantageous 191 because assertions are not supposed to change the state of the program and 192 ideally the compiler will not compile assertions that modify variables. 193 </p> 194<p> 195 For example, consider the following assertion where by mistake we programmed 196 <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> 197 instead of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code>: 198 </p> 199<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 200<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Mistakenly `=` instead of `==`.</span> 201</pre> 202<p> 203 Ideally this code will not compile instead this example not only compiles 204 but the assertion even passes the run-time check and no error is generated 205 at all. The <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2004/n1613.pdf" target="_top">[N1613]</a> 206 paper introduces the concept of a <span class="emphasis"><em>const-block</em></span> which 207 could be used to wrap the assertion above and catch the programming error 208 at compile-time. Similarly, the following code will generate a compile-time 209 error when <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> 210 is mistakenly used instead of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> because both <code class="computeroutput"><span class="identifier">x</span></code> 211 and <code class="computeroutput"><span class="identifier">y</span></code> are made constants 212 (using local functions) within the block of code performing the assertion 213 (see also <a href="../../../example/const_block_error.cpp" target="_top"><code class="literal">const_block_error.cpp</code></a>): 214 </p> 215<div class="informaltable"><table class="table"> 216<colgroup> 217<col> 218<col> 219</colgroup> 220<thead><tr> 221<th> 222 <p> 223 With Local Functions 224 </p> 225 </th> 226<th> 227 <p> 228 N1613 Const-Blocks 229 </p> 230 </th> 231</tr></thead> 232<tbody><tr> 233<td> 234 <p> 235</p> 236<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 237<span class="identifier">CONST_BLOCK</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Constant block.</span> 238 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Compiler error.</span> 239<span class="special">}</span> <span class="identifier">CONST_BLOCK_END</span> 240</pre> 241<p> 242 </p> 243 </td> 244<td> 245 <p> 246</p> 247<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 248<span class="keyword">const</span> <span class="special">{</span> <span class="comment">// Constant block.</span> 249 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Compiler error.</span> 250<span class="special">}</span> 251</pre> 252<p> 253 </p> 254 </td> 255</tr></tbody> 256</table></div> 257<p> 258 The constant block macros are implemented using local functions (see also 259 <a href="../../../example/const_block.hpp" target="_top"><code class="literal">const_block.hpp</code></a>): 260 </p> 261<p> 262</p> 263<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">CONST_BLOCK_</span><span class="special">(</span><span class="identifier">variables</span><span class="special">)</span> <span class="special">\</span> 264 <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span> <span class="special">\</span> 265 <span class="identifier">BOOST_PP_IIF</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_IS_NIL</span><span class="special">(</span><span class="identifier">variables</span><span class="special">),</span> <span class="special">\</span> 266 <span class="keyword">void</span> <span class="identifier">BOOST_PP_TUPLE_EAT</span><span class="special">(</span><span class="number">3</span><span class="special">)</span> <span class="special">\</span> 267 <span class="special">,</span> <span class="special">\</span> 268 <span class="identifier">BOOST_PP_LIST_FOR_EACH_I</span> <span class="special">\</span> 269 <span class="special">)(</span><span class="identifier">CONST_BLOCK_BIND_</span><span class="special">,</span> <span class="special">~,</span> <span class="identifier">variables</span><span class="special">)</span> <span class="special">\</span> 270 <span class="special">)</span> 271</pre> 272<p> 273 </p> 274<p> 275</p> 276<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">CONST_BLOCK_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span> 277 <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">const_block_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span> 278 <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">const_block_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">)();</span> <span class="comment">/* call local function immediately */</span> 279</pre> 280<p> 281 </p> 282<p> 283 The constant block macros are implemented using a local function which binds 284 by constant reference <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span></code> 285 all the specified variables (so the variables are constant within the code 286 block but they do not need to be <code class="computeroutput"><span class="identifier">CopyConstructible</span></code> 287 and no extra copy is performed). The local function executes the <code class="computeroutput"><span class="identifier">assert</span></code> instruction in its body and it is 288 called immediately after it is defined. More in general, constant blocks 289 can be used to evaluate any instruction (not just assertions) within a block 290 were all specified variables are constant. 291 </p> 292<p> 293 Unfortunately, constant blocks cannot be implemented with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 294 lambda functions</a> because these do not support constant binding. Variables 295 bound by value using <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 296 lambda functions</a> (<code class="computeroutput"><span class="identifier">variable</span></code>, 297 <code class="computeroutput"><span class="special">=</span><span class="identifier">variable</span></code>, 298 and <code class="computeroutput"><span class="special">=</span></code>) are constant but they 299 are required to be <code class="computeroutput"><span class="identifier">CopyConstructible</span></code> 300 and they introduce potentially expensive copy operations. <a href="#ftn.boost_localfunction.examples.constant_blocks.f0" class="footnote" name="boost_localfunction.examples.constant_blocks.f0"><sup class="footnote">[29]</sup></a> Of course it is always possible to introduce extra constant variables 301 and bind these variables to the <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 302 lambda functions</a> but the constant block code will then have to manage 303 the declaration and initialization of these extra variables plus it will 304 have to use the extra variable names instead of the original variable names: 305 </p> 306<p> 307</p> 308<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> 309<span class="keyword">const</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&</span> <span class="identifier">const_x</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">;</span> <span class="comment">// Constant so cannot be modified</span> 310<span class="keyword">const</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">y</span><span class="special">)&</span> <span class="identifier">const_y</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// and reference so no copy.</span> 311<span class="special">[&</span><span class="identifier">const_x</span><span class="special">,</span> <span class="special">&</span><span class="identifier">const_y</span><span class="special">]()</span> <span class="special">{</span> <span class="comment">// Lambda functions (C++11 only).</span> 312 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">const_x</span> <span class="special">=</span> <span class="identifier">const_y</span><span class="special">);</span> <span class="comment">// Unfortunately, `const_` names.</span> 313<span class="special">}();</span> 314</pre> 315<p> 316 </p> 317<p> 318 In many cases the use of an extra constant variable <code class="computeroutput"><span class="identifier">const_x</span></code> 319 can be acceptable but in other cases it might be preferable to maintain the 320 same variable name <code class="computeroutput"><span class="identifier">x</span></code> within 321 the function body. 322 </p> 323</div> 324<div class="section"> 325<div class="titlepage"><div><div><h3 class="title"> 326<a name="boost_localfunction.examples.scope_exits"></a><a class="link" href="examples.html#boost_localfunction.examples.scope_exits" title="Scope Exits">Scope Exits</a> 327</h3></div></div></div> 328<p> 329 Scope exits allow to execute arbitrary code at the exit of the enclosing 330 scope and they are provided by the <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a> 331 library. 332 </p> 333<p> 334 For curiosity, here we show how to re-implement scope exits using local functions. 335 One small advantage of scope exits that use local functions is that they 336 support constant binding. <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a> 337 does not directly support constant binding (however, it is always possible 338 to introduce an extra <code class="computeroutput"><span class="keyword">const</span></code> 339 local variable, assign it to the value to bind, and then bind the <code class="computeroutput"><span class="keyword">const</span></code> variable so to effectively have constant 340 binding with <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a> 341 as well). 342 </p> 343<div class="note"><table border="0" summary="Note"> 344<tr> 345<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> 346<th align="left">Note</th> 347</tr> 348<tr><td align="left" valign="top"><p> 349 In general, the authors recommend to use <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a> 350 instead of the code listed by this example whenever possible (because 351 <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a> 352 is a library deliberately designed to support the scope exit construct). 353 </p></td></tr> 354</table></div> 355<p> 356 The following example binds <code class="computeroutput"><span class="identifier">p</span></code> 357 by constant reference so this variable cannot be modified within the scope 358 exit body but it is not copied and it will present the value it has at the 359 exit of the enclosing scope and not at the scope exit declaration (see also 360 <a href="../../../example/scope_exit.cpp" target="_top"><code class="literal">scope_exit.cpp</code></a>): 361 </p> 362<div class="informaltable"><table class="table"> 363<colgroup> 364<col> 365<col> 366</colgroup> 367<thead><tr> 368<th> 369 <p> 370 With Local Functions 371 </p> 372 </th> 373<th> 374 <p> 375 Boost.ScopeExit 376 </p> 377 </th> 378</tr></thead> 379<tbody><tr> 380<td> 381 <p> 382</p> 383<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">person</span><span class="special">&</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span> 384<span class="identifier">person</span><span class="special">::</span><span class="identifier">evolution_t</span> <span class="identifier">checkpoint</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">;</span> 385 386<span class="identifier">SCOPE_EXIT</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">checkpoint</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">{</span> 387 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">checkpoint</span> <span class="special">==</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-></span><span class="identifier">persons_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span> 388<span class="special">}</span> <span class="identifier">SCOPE_EXIT_END</span> 389</pre> 390<p> 391 </p> 392 </td> 393<td> 394 <p> 395</p> 396<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">person</span><span class="special">&</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span> 397<span class="identifier">person</span><span class="special">::</span><span class="identifier">evolution_t</span> <span class="identifier">checkpoint</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">;</span> 398 399<span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span><span class="identifier">checkpoint</span><span class="special">,</span> <span class="special">&</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Or extra variable `const_p`.</span> 400 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">checkpoint</span> <span class="special">==</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-></span><span class="identifier">persons_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span> 401<span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> 402</pre> 403<p> 404 </p> 405 </td> 406</tr></tbody> 407</table></div> 408<p> 409 The scope exit macros are implemented by passing a local function when constructing 410 an object of the following class (see also <a href="../../../example/scope_exit.hpp" target="_top"><code class="literal">scope_exit.hpp</code></a>): 411 </p> 412<p> 413</p> 414<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">scope_exit</span> <span class="special">{</span> 415 <span class="identifier">scope_exit</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f</span><span class="special">):</span> <span class="identifier">f_</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span> <span class="special">{}</span> 416 <span class="special">~</span><span class="identifier">scope_exit</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">f_</span><span class="special">();</span> <span class="special">}</span> 417<span class="keyword">private</span><span class="special">:</span> 418 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f_</span><span class="special">;</span> 419<span class="special">};</span> 420</pre> 421<p> 422 </p> 423<p> 424</p> 425<pre class="programlisting"><span class="preprocessor"># define</span> <span class="identifier">SCOPE_EXIT</span><span class="special">(...)</span> <span class="special">\</span> 426 <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">__VA_ARGS__</span><span class="special">)</span> 427</pre> 428<p> 429 </p> 430<p> 431</p> 432<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">SCOPE_EXIT_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span> 433 <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_func_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span> 434 <span class="identifier">scope_exit</span> <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">)(</span> <span class="special">\</span> 435 <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_func_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">));</span> 436</pre> 437<p> 438 </p> 439<p> 440 A local variable within the enclosing scope is used to hold the object so 441 the destructor will be invoked at the exit of the enclosing scope and it 442 will in turn call the local function executing the scope exit instructions. 443 The scope exit local function has no parameter and <code class="computeroutput"><span class="keyword">void</span></code> 444 result type but it supports binding and constant binding. 445 </p> 446</div> 447<div class="section"> 448<div class="titlepage"><div><div><h3 class="title"> 449<a name="boost_localfunction.examples.boost_phoenix_functions"></a><a class="link" href="examples.html#boost_localfunction.examples.boost_phoenix_functions" title="Boost.Phoenix Functions">Boost.Phoenix 450 Functions</a> 451</h3></div></div></div> 452<p> 453 Local functions can be used to create <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a> 454 functions. For example (see also <a href="../../../example/phoenix_factorial_local.cpp" target="_top"><code class="literal">phoenix_factorial_local.cpp</code></a> 455 and <a href="../../../example/phoenix_factorial.cpp" target="_top"><code class="literal">phoenix_factorial.cpp</code></a>): 456 </p> 457<div class="informaltable"><table class="table"> 458<colgroup> 459<col> 460<col> 461</colgroup> 462<thead><tr> 463<th> 464 <p> 465 Local Functions 466 </p> 467 </th> 468<th> 469 <p> 470 Global Functor 471 </p> 472 </th> 473</tr></thead> 474<tbody><tr> 475<td> 476 <p> 477</p> 478<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> 479 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">arg_names</span><span class="special">::</span><span class="identifier">arg1</span><span class="special">;</span> 480 481 <span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Unfortunately, monomorphic.</span> 482 <span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="identifier">factorial_impl</span><span class="special">(</span><span class="identifier">n</span> <span class="special">-</span> <span class="number">1</span><span class="special">);</span> 483 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="identifier">factorial_impl</span><span class="special">)</span> 484 485 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)></span> <span class="special">></span> 486 <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">factorial_impl</span><span class="special">);</span> <span class="comment">// Phoenix function from local function.</span> 487 488 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span> 489 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Call.</span> 490 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Lazy call.</span> 491 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span> 492<span class="special">}</span> 493</pre> 494<p> 495 </p> 496 </td> 497<td> 498 <p> 499</p> 500<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">struct</span> <span class="identifier">factorial_impl</span> <span class="special">{</span> <span class="comment">// Phoenix function from global functor.</span> 501 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 502 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 503 504 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 505 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">)></span> <span class="special">:</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span><span class="special">&)></span> <span class="special">{};</span> 506 507 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 508 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">&)></span> <span class="special">{</span> <span class="keyword">typedef</span> <span class="identifier">Arg</span> <span class="identifier">type</span><span class="special">;</span> <span class="special">};</span> 509 510 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> <span class="comment">// Polymorphic.</span> 511 <span class="identifier">Arg</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> 512 <span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)(</span><span class="identifier">n</span> <span class="special">-</span> <span class="number">1</span><span class="special">);</span> 513 <span class="special">}</span> 514<span class="special">};</span> 515 516<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> 517 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">arg_names</span><span class="special">::</span><span class="identifier">arg1</span><span class="special">;</span> 518 519 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">factorial_impl</span><span class="special">></span> <span class="identifier">factorial</span><span class="special">;</span> 520 521 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span> 522 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Call.</span> 523 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Lazy call.</span> 524 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span> 525<span class="special">}</span> 526</pre> 527<p> 528 </p> 529 </td> 530</tr></tbody> 531</table></div> 532<p> 533 This is presented here mainly as a curiosity because <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a> 534 functions created from local functions have the important limitation that 535 they cannot be polymorphic. <a href="#ftn.boost_localfunction.examples.boost_phoenix_functions.f0" class="footnote" name="boost_localfunction.examples.boost_phoenix_functions.f0"><sup class="footnote">[30]</sup></a> Therefore, in many cases creating the <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a> 536 function from global functors (possibly with the help of <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a> 537 adaptor macros) might be a more useful. 538 </p> 539</div> 540<div class="section"> 541<div class="titlepage"><div><div><h3 class="title"> 542<a name="boost_localfunction.examples.closures"></a><a class="link" href="examples.html#boost_localfunction.examples.closures" title="Closures">Closures</a> 543</h3></div></div></div> 544<p> 545 The following are examples of <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)" target="_top">closures</a> 546 that illustrate how to return local functions to the calling scope (note 547 how extra care is taken in order to ensure that all bound variables remain 548 valid at the calling scope): 549 </p> 550<div class="informaltable"><table class="table"> 551<colgroup><col></colgroup> 552<thead><tr><th> 553 <p> 554 Files 555 </p> 556 </th></tr></thead> 557<tbody> 558<tr><td> 559 <p> 560 <a href="../../../test/return_inc.cpp" target="_top"><code class="literal">return_inc.cpp</code></a> 561 </p> 562 </td></tr> 563<tr><td> 564 <p> 565 <a href="../../../test/return_this.cpp" target="_top"><code class="literal">return_this.cpp</code></a> 566 </p> 567 </td></tr> 568<tr><td> 569 <p> 570 <a href="../../../test/return_setget.cpp" target="_top"><code class="literal">return_setget.cpp</code></a> 571 </p> 572 </td></tr> 573<tr><td> 574 <p> 575 <a href="../../../test/return_derivative.cpp" target="_top"><code class="literal">return_derivative.cpp</code></a> 576 </p> 577 </td></tr> 578</tbody> 579</table></div> 580</div> 581<div class="section"> 582<div class="titlepage"><div><div><h3 class="title"> 583<a name="boost_localfunction.examples.gcc_nested_functions"></a><a class="link" href="examples.html#boost_localfunction.examples.gcc_nested_functions" title="GCC Nested Functions">GCC 584 Nested Functions</a> 585</h3></div></div></div> 586<p> 587 The GCC C compiler supports local functions as a non-standard extension under 588 the name of <a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html" target="_top">nested 589 functions</a>. Note that nested functions are exclusively a C extension 590 of the GCC compiler (they are not supported for C++ not even by the GCC compiler, 591 and they are not part of any C or C++ standard, nor they are supported by 592 other compilers like MSVC). 593 </p> 594<p> 595 The following examples are taken form the GCC nested function documentation 596 and programmed using local functions: 597 </p> 598<div class="informaltable"><table class="table"> 599<colgroup><col></colgroup> 600<thead><tr><th> 601 <p> 602 Files 603 </p> 604 </th></tr></thead> 605<tbody> 606<tr><td> 607 <p> 608 <a href="../../../example/gcc_square.cpp" target="_top"><code class="literal">gcc_square.cpp</code></a> 609 </p> 610 </td></tr> 611<tr><td> 612 <p> 613 <a href="../../../example/gcc_access.cpp" target="_top"><code class="literal">gcc_access.cpp</code></a> 614 </p> 615 </td></tr> 616<tr><td> 617 <p> 618 <a href="../../../example/gcc_store.cpp" target="_top"><code class="literal">gcc_store.cpp</code></a> 619 </p> 620 </td></tr> 621</tbody> 622</table></div> 623</div> 624<div class="section"> 625<div class="titlepage"><div><div><h3 class="title"> 626<a name="boost_localfunction.examples.n_papers"></a><a class="link" href="examples.html#boost_localfunction.examples.n_papers" title="N-Papers">N-Papers</a> 627</h3></div></div></div> 628<p> 629 The following examples are taken from different C++ "N-papers" 630 and programmed using local functions: 631 </p> 632<div class="informaltable"><table class="table"> 633<colgroup> 634<col> 635<col> 636</colgroup> 637<thead><tr> 638<th> 639 <p> 640 Files 641 </p> 642 </th> 643<th> 644 <p> 645 Notes 646 </p> 647 </th> 648</tr></thead> 649<tbody> 650<tr> 651<td> 652 <p> 653 <a href="../../../example/n2550_find_if.cpp" target="_top"><code class="literal">n2550_find_if.cpp</code></a> 654 </p> 655 </td> 656<td> 657 <p> 658 This example is adapted from <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2550.pdf" target="_top">[N2550]</a> 659 (<a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 660 lambda functions</a>): It passes a local function to the STL 661 algorithm <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span></code>. 662 </p> 663 </td> 664</tr> 665<tr> 666<td> 667 <p> 668 <a href="../../../example/n2529_this.cpp" target="_top"><code class="literal">n2529_this.cpp</code></a> 669 </p> 670 </td> 671<td> 672 <p> 673 This example is adapted from <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2529.pdf" target="_top">[N2529]</a> 674 (<a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 675 lambda functions</a>): It binds the object in scope <code class="computeroutput"><span class="keyword">this</span></code> to a local function. 676 </p> 677 </td> 678</tr> 679</tbody> 680</table></div> 681</div> 682<div class="footnotes"> 683<br><hr style="width:100; text-align:left;margin-left: 0"> 684<div id="ftn.boost_localfunction.examples.constant_blocks.f0" class="footnote"><p><a href="#boost_localfunction.examples.constant_blocks.f0" class="para"><sup class="para">[29] </sup></a> 685 Ideally, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11 686 lambda functions</a> would allow to bind variables also using <code class="computeroutput"><span class="keyword">const</span><span class="special">&</span> <span class="identifier">variable</span></code> (constant reference) and <code class="computeroutput"><span class="keyword">const</span><span class="special">&</span></code> 687 (all variables by constant reference). 688 </p></div> 689<div id="ftn.boost_localfunction.examples.boost_phoenix_functions.f0" class="footnote"><p><a href="#boost_localfunction.examples.boost_phoenix_functions.f0" class="para"><sup class="para">[30] </sup></a> 690 <span class="bold"><strong>Rationale.</strong></span> Local functions can only be 691 monomorphic because they are implemented using local classes and local 692 classes cannot be templates in C++ (not even in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>). 693 </p></div> 694</div> 695</div> 696<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 697<td align="left"></td> 698<td align="right"><div class="copyright-footer">Copyright © 2009-2012 Lorenzo 699 Caminiti<p> 700 Distributed under the Boost Software License, Version 1.0 (see accompanying 701 file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 702 </p> 703</div></td> 704</tr></table> 705<hr> 706<div class="spirit-nav"> 707<a accesskey="p" href="advanced_topics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="alternatives.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 708</div> 709</body> 710</html> 711
