ceph/src/boost/libs/scope_exit/doc/html/scope_exit/alternatives.html

   1 <html>
   2 <head>
   3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   4 <title>Annex: Alternatives</title>
   5 <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
   6 <meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
   7 <link rel="home" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">
   8 <link rel="up" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">
   9 <link rel="prev" href="tutorial.html" title="Tutorial">
  10 <link rel="next" href="no_variadic_macros.html" title="Annex: No Variadic Macros">
  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="tutorial.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="no_variadic_macros.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="scope_exit.alternatives"></a><a class="link" href="alternatives.html" title="Annex: Alternatives">Annex: Alternatives</a>
  28 </h2></div></div></div>
  29 <p>
  30       This section presents some alternative and related work to <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a>.
  31     </p>
  32 <h4>
  33 <a name="scope_exit.alternatives.h0"></a>
  34       <span class="phrase"><a name="scope_exit.alternatives.try_catch"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.try_catch">Try-Catch</a>
  35     </h4>
  36 <p>
  37       This is an example of using a badly designed <code class="computeroutput"><span class="identifier">file</span></code>
  38       class. An instance of <code class="computeroutput"><span class="identifier">file</span></code>
  39       does not close the file in its destructor, a programmer is expected to call
  40       the <code class="computeroutput"><span class="identifier">close</span></code> member function explicitly.
  41       For example (see also <a href="../../../example/try_catch.cpp" target="_top"><code class="literal">try_catch.cpp</code></a>):
  42     </p>
  43 <p>
  44 </p>
  45 <pre class="programlisting"><span class="identifier">file</span> <span class="identifier">passwd</span><span class="special">;</span>
  46 <span class="keyword">try</span> <span class="special">{</span>
  47     <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">open</span><span class="special">(</span><span class="string">"/etc/passwd"</span><span class="special">);</span>
  48     <span class="comment">// ...</span>
  49     <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">();</span>
  50 <span class="special">}</span> <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span>
  51     <span class="identifier">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special">&lt;&lt;</span> <span class="string">"could not get user info"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  52     <span class="keyword">if</span><span class="special">(</span><span class="identifier">passwd</span><span class="special">.</span><span class="identifier">is_open</span><span class="special">())</span> <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">();</span>
  53     <span class="keyword">throw</span><span class="special">;</span>
  54 <span class="special">}</span>
  55 </pre>
  56 <p>
  57     </p>
  58 <p>
  59       Note the following issues with this approach:
  60     </p>
  61 <div class="orderedlist"><ol class="orderedlist" type="1">
  62 <li class="listitem">
  63           The <code class="computeroutput"><span class="identifier">passwd</span></code> object is defined
  64           outside of the <code class="computeroutput"><span class="keyword">try</span></code> block because
  65           this object is required inside the <code class="computeroutput"><span class="keyword">catch</span></code>
  66           block to close the file.
  67         </li>
  68 <li class="listitem">
  69           The <code class="computeroutput"><span class="identifier">passwd</span></code> object is not
  70           fully constructed until after the <code class="computeroutput"><span class="identifier">open</span></code>
  71           member function returns.
  72         </li>
  73 <li class="listitem">
  74           If opening throws, the <code class="computeroutput"><span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">()</span></code> should not be called, hence the call
  75           to <code class="computeroutput"><span class="identifier">passwd</span><span class="special">.</span><span class="identifier">is_open</span><span class="special">()</span></code>.
  76         </li>
  77 </ol></div>
  78 <p>
  79       The <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> approach does not have
  80       any of these issues. For example (see also <a href="../../../example/try_catch.cpp" target="_top"><code class="literal">try_catch.cpp</code></a>):
  81     </p>
  82 <p>
  83 </p>
  84 <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
  85     <span class="identifier">file</span> <span class="identifier">passwd</span><span class="special">(</span><span class="string">"/etc/passwd"</span><span class="special">);</span>
  86     <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(&amp;</span><span class="identifier">passwd</span><span class="special">)</span> <span class="special">{</span>
  87         <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">();</span>
  88     <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
  89 <span class="special">}</span> <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span>
  90     <span class="identifier">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special">&lt;&lt;</span> <span class="string">"could not get user info"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  91     <span class="keyword">throw</span><span class="special">;</span>
  92 <span class="special">}</span>
  93 </pre>
  94 <p>
  95     </p>
  96 <h4>
  97 <a name="scope_exit.alternatives.h1"></a>
  98       <span class="phrase"><a name="scope_exit.alternatives.raii"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.raii">RAII</a>
  99     </h4>
 100 <p>
 101       <a href="http://www.research.att.com/~bs/glossary.html#Gresource-acquisition-is-initialization" target="_top">RAII</a>
 102       is absolutely perfect for the <code class="computeroutput"><span class="identifier">file</span></code>
 103       class introduced above. Use of a properly designed <code class="computeroutput"><span class="identifier">file</span></code>
 104       class would look like:
 105     </p>
 106 <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
 107     <span class="identifier">file</span> <span class="identifier">passwd</span><span class="special">(</span><span class="string">"/etc/passwd"</span><span class="special">);</span>
 108     <span class="comment">// ...</span>
 109 <span class="special">}</span> <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span>
 110     <span class="identifier">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special">&lt;&lt;</span> <span class="string">"could not get user info"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 111     <span class="keyword">throw</span><span class="special">;</span>
 112 <span class="special">}</span>
 113 </pre>
 114 <p>
 115       However, using <a href="http://www.research.att.com/~bs/glossary.html#Gresource-acquisition-is-initialization" target="_top">RAII</a>
 116       to build up a <a href="http://www.research.att.com/~bs/glossary.html#Gstrong-guarantee" target="_top">strong
 117       guarantee</a> could introduce a lot of non-reusable <a href="http://www.research.att.com/~bs/glossary.html#Gresource-acquisition-is-initialization" target="_top">RAII</a>
 118       types. For example:
 119     </p>
 120 <pre class="programlisting"><span class="identifier">persons_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a_person</span><span class="special">);</span>
 121 <span class="identifier">pop_back_if_not_commit</span> <span class="identifier">pop_back_if_not_commit_guard</span><span class="special">(</span><span class="identifier">commit</span><span class="special">,</span> <span class="identifier">persons_</span><span class="special">);</span>
 122 </pre>
 123 <p>
 124       The <code class="computeroutput"><span class="identifier">pop_back_if_not_commit</span></code>
 125       class is either defined out of the scope or as a local class:
 126     </p>
 127 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">pop_back_if_not_commit</span> <span class="special">{</span>
 128     <span class="keyword">bool</span> <span class="identifier">commit_</span><span class="special">;</span>
 129     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">person</span><span class="special">&gt;&amp;</span> <span class="identifier">vec_</span><span class="special">;</span>
 130     <span class="comment">// ...</span>
 131     <span class="special">~</span><span class="identifier">pop_back_if_not_commit</span><span class="special">()</span> <span class="special">{</span>
 132         <span class="keyword">if</span><span class="special">(!</span><span class="identifier">commit_</span><span class="special">)</span> <span class="identifier">vec_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span>
 133     <span class="special">}</span>
 134 <span class="special">};</span>
 135 </pre>
 136 <p>
 137       In some cases <a href="http://www.research.att.com/~bs/glossary.html#Gstrong-guarantee" target="_top">strong
 138       guarantee</a> can be accomplished with standard utilities:
 139     </p>
 140 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">auto_ptr</span><span class="special">&lt;</span><span class="identifier">Person</span><span class="special">&gt;</span> <span class="identifier">superman_ptr</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">superman</span><span class="special">());</span>
 141 <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">superman_ptr</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span>
 142 <span class="identifier">superman_ptr</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span> <span class="comment">// persons_ successfully took ownership</span>
 143 </pre>
 144 <p>
 145       Or with specialized containers such as <a href="http://www.boost.org/libs/ptr_container" target="_top">Boost.PointerContainer</a>
 146       or <a href="http://www.boost.org/libs/multi_index" target="_top">Boost.Multi-Index</a>.
 147     </p>
 148 <h4>
 149 <a name="scope_exit.alternatives.h2"></a>
 150       <span class="phrase"><a name="scope_exit.alternatives.scope_guards"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.scope_guards">Scope
 151       Guards</a>
 152     </h4>
 153 <p>
 154       Imagine that a new currency rate is introduced before performing a transaction
 155       (see also []):
 156     </p>
 157 <p>
 158 </p>
 159 <pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span>
 160 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">currency</span><span class="special">(</span><span class="string">"EUR"</span><span class="special">);</span>
 161 <span class="keyword">double</span> <span class="identifier">rate</span> <span class="special">=</span> <span class="number">1.3326</span><span class="special">;</span>
 162 <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">rates</span><span class="special">;</span>
 163 <span class="keyword">bool</span> <span class="identifier">currency_rate_inserted</span> <span class="special">=</span>
 164         <span class="identifier">rates</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">currency</span><span class="special">,</span> <span class="identifier">rate</span><span class="special">)).</span><span class="identifier">second</span><span class="special">;</span>
 165 <span class="comment">// Transaction...</span>
 166 </pre>
 167 <p>
 168     </p>
 169 <p>
 170       If the transaction does not complete, the currency must be erased from <code class="computeroutput"><span class="identifier">rates</span></code>. This can be done with <a href="http://www.ddj.com/dept/cpp/184403758" target="_top">ScopeGuard</a>
 171       and <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> (or
 172       <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>):
 173     </p>
 174 <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">;</span>
 175
 176 <span class="identifier">ON_BLOCK_EXIT</span><span class="special">(</span>
 177     <span class="identifier">if_</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&amp;&amp;</span> <span class="special">!</span><span class="identifier">_1</span><span class="special">)</span> <span class="special">[</span>
 178         <span class="identifier">bind</span><span class="special">(</span>
 179             <span class="keyword">static_cast</span><span class="special">&lt;</span>
 180                 <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">size_type</span>
 181                 <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;::*)(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;)</span>
 182             <span class="special">&gt;(&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">erase</span><span class="special">)</span>
 183           <span class="special">,</span> <span class="special">&amp;</span><span class="identifier">rates</span>
 184           <span class="special">,</span> <span class="identifier">currency</span>
 185         <span class="special">)</span>
 186     <span class="special">]</span>
 187   <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cref</span><span class="special">(</span><span class="identifier">commit</span><span class="special">)</span>
 188   <span class="special">);</span>
 189
 190 <span class="comment">// ...</span>
 191
 192 <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
 193 </pre>
 194 <p>
 195       Note the following issues with this approach:
 196     </p>
 197 <div class="orderedlist"><ol class="orderedlist" type="1">
 198 <li class="listitem">
 199           <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> expressions
 200           are hard to write correctly (e.g., overloaded functions must be explicitly
 201           casted, as demonstrated in the example above).
 202         </li>
 203 <li class="listitem">
 204           The condition in the <code class="computeroutput"><span class="identifier">if_</span></code>
 205           expression refers to <code class="computeroutput"><span class="identifier">commit</span></code>
 206           variable indirectly through the <code class="computeroutput"><span class="identifier">_1</span></code>
 207           placeholder reducing readability.
 208         </li>
 209 <li class="listitem">
 210           Setting a breakpoint inside <code class="computeroutput"><span class="identifier">if_</span><span class="special">[...]</span></code> requires in-depth knowledge of <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> and debugging
 211           techniques.
 212         </li>
 213 </ol></div>
 214 <p>
 215       This code will look much better with C++11 lambdas:
 216     </p>
 217 <pre class="programlisting"><span class="identifier">ON_BLOCK_EXIT</span><span class="special">(</span>
 218     <span class="special">[</span><span class="identifier">currency_rate_inserted</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">rates</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">currency</span><span class="special">]()</span> <span class="special">{</span>
 219         <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&amp;&amp;</span> <span class="special">!</span><span class="identifier">commit</span><span class="special">)</span> <span class="identifier">rates</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">currency</span><span class="special">);</span>
 220     <span class="special">}</span>
 221 <span class="special">);</span>
 222
 223 <span class="comment">// ...</span>
 224
 225 <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
 226 </pre>
 227 <p>
 228       With <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> we can simply do the
 229       following (see also <a href="../../../example/scope_guard.cpp" target="_top"><code class="literal">scope_guard.cpp</code></a>):
 230     </p>
 231 <p>
 232 </p>
 233 <pre class="programlisting"><span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">rates</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">currency</span><span class="special">)</span> <span class="special">{</span>
 234     <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&amp;&amp;</span> <span class="special">!</span><span class="identifier">commit</span><span class="special">)</span> <span class="identifier">rates</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">currency</span><span class="special">);</span>
 235 <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
 236
 237 <span class="comment">// ...</span>
 238
 239 <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
 240 </pre>
 241 <p>
 242     </p>
 243 <h4>
 244 <a name="scope_exit.alternatives.h3"></a>
 245       <span class="phrase"><a name="scope_exit.alternatives.the_d_programming_language"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.the_d_programming_language">The
 246       D Programming Language</a>
 247     </h4>
 248 <p>
 249       <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> is similar to <a href="http://www.digitalmars.com/d/2.0/statement.html#ScopeGuardStatement" target="_top">scope(exit)</a>
 250       feature built into the <a href="http://www.digitalmars.com/d/index.html" target="_top">D</a>
 251       programming language.
 252     </p>
 253 <p>
 254       A curious reader may notice that the library does not implement <code class="computeroutput"><span class="identifier">scope</span><span class="special">(</span><span class="identifier">success</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">scope</span><span class="special">(</span><span class="identifier">failure</span><span class="special">)</span></code> of the <a href="http://www.digitalmars.com/d/index.html" target="_top">D</a>
 255       language. Unfortunately, these are not possible in C++ because failure or success
 256       conditions cannot be determined by calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">uncaught_exception</span></code>
 257       (see <a href="http://www.gotw.ca/gotw/047.htm" target="_top">Guru of the Week #47</a>
 258       for details about <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">uncaught_exception</span></code> and if it has any good
 259       use at all). However, this is not a big problem because these two <a href="http://www.digitalmars.com/d/index.html" target="_top">D</a>'s
 260       constructs can be expressed in terms of <a href="http://www.digitalmars.com/d/2.0/statement.html#ScopeGuardStatement" target="_top">scope(exit)</a>
 261       and a <code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">commit</span></code>
 262       variable (similarly to some examples presented in the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
 263       section).
 264     </p>
 265 <h4>
 266 <a name="scope_exit.alternatives.h4"></a>
 267       <span class="phrase"><a name="scope_exit.alternatives.c__11_lambdas"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.c__11_lambdas">C++11
 268       Lambdas</a>
 269     </h4>
 270 <p>
 271       Using C++11 lambdas, it is relatively easy to implement the <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a>
 272       construct. For example (see also <a href="../../../example/world_cxx11_lambda.cpp" target="_top"><code class="literal">world_cxx11_lambda.cpp</code></a>):
 273     </p>
 274 <p>
 275 </p>
 276 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">functional</span><span class="special">&gt;</span>
 277
 278 <span class="keyword">struct</span> <span class="identifier">scope_exit</span> <span class="special">{</span>
 279     <span class="identifier">scope_exit</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)&gt;</span> <span class="identifier">f</span><span class="special">)</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>
 280     <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>
 281 <span class="keyword">private</span><span class="special">:</span>
 282     <span class="identifier">std</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)&gt;</span> <span class="identifier">f_</span><span class="special">;</span>
 283 <span class="special">};</span>
 284
 285 <span class="keyword">void</span> <span class="identifier">world</span><span class="special">::</span><span class="identifier">add_person</span><span class="special">(</span><span class="identifier">person</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a_person</span><span class="special">)</span> <span class="special">{</span>
 286     <span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span>
 287
 288     <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a_person</span><span class="special">);</span>
 289     <span class="identifier">scope_exit</span> <span class="identifier">on_exit1</span><span class="special">([&amp;</span><span class="identifier">commit</span><span class="special">,</span> <span class="keyword">this</span><span class="special">](</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Use C++11 lambda.</span>
 290         <span class="keyword">if</span><span class="special">(!</span><span class="identifier">commit</span><span class="special">)</span> <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span> <span class="comment">// `persons_` via captured `this`.</span>
 291     <span class="special">});</span>
 292
 293     <span class="comment">// ...</span>
 294
 295     <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
 296 <span class="special">}</span>
 297 </pre>
 298 <p>
 299     </p>
 300 <p>
 301       However, this library allows to program the <a class="link" href="../index.html" title="Chapter&#160;1.&#160;Boost.ScopeExit 1.1.0">Boost.ScopeExit</a>
 302       construct in a way that is portable between C++03 and C++11 compilers.
 303     </p>
 304 </div>
 305 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 306 <td align="left"></td>
 307 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2012 Alexander Nasonov, Lorenzo Caminiti<p>
 308         Distributed under the Boost Software License, Version 1.0 (see accompanying
 309         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>)
 310       </p>
 311 </div></td>
 312 </tr></table>
 313 <hr>
 314 <div class="spirit-nav">
 315 <a accesskey="p" href="tutorial.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="no_variadic_macros.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
 316 </div>
 317 </body>
 318 </html>