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25<div class="section">
26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="scope_exit.no_variadic_macros"></a><a class="link" href="no_variadic_macros.html" title="Annex: No Variadic Macros">Annex: No Variadic Macros</a>
28</h2></div></div></div>
29<p>
30 This section presents an alternative syntax for compilers without variadic
31 macro support.
32 </p>
33<h4>
34<a name="scope_exit.no_variadic_macros.h0"></a>
35 <span class="phrase"><a name="scope_exit.no_variadic_macros.sequence_syntax"></a></span><a class="link" href="no_variadic_macros.html#scope_exit.no_variadic_macros.sequence_syntax">Sequence
36 Syntax</a>
37 </h4>
38<p>
39 Most modern compilers support variadic macros (notably, these include GCC,
40 MSVC, and all C++11 compilers). <a href="#ftn.scope_exit.no_variadic_macros.f0" class="footnote" name="scope_exit.no_variadic_macros.f0"><sup class="footnote">[10]</sup></a> However, in the rare case that programmers need to use this library
41 on a complier without variaidc macros, this library also allows to specify
42 the capture list using a <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a>
43 sequence where tokens are separated by round parenthesis <code class="computeroutput"><span class="special">()</span></code>:
44 </p>
45<pre class="programlisting"><span class="special">(</span><span class="identifier">capture1</span><span class="special">)</span> <span class="special">(</span><span class="identifier">capture2</span><span class="special">)</span> <span class="special">...</span> <span class="comment">// All compilers.</span>
46</pre>
47<p>
48 Instead of the comma-separated list that we have seen so far which requires
49 variadic macros:
50 </p>
51<pre class="programlisting"><span class="identifier">capture1</span><span class="special">,</span> <span class="identifier">capture2</span><span class="special">,</span> <span class="special">...</span> <span class="comment">// Only compilers with variadic macros.</span>
52</pre>
53<p>
54 For example, the following syntax is accepted on all compilers with and without
55 variadic macros (see also <a href="../../../test/world_seq.cpp" target="_top"><code class="literal">world_seq.cpp</code></a>
56 and <a href="../../../test/world.cpp" target="_top"><code class="literal">world.cpp</code></a>):
57 </p>
58<div class="informaltable"><table class="table">
59<colgroup>
60<col>
61<col>
62</colgroup>
63<thead><tr>
64<th>
65 <p>
66 Boost.Preprocessor Sequence (All Compilers)
67 </p>
68 </th>
69<th>
70 <p>
71 Comma-Separated List (Variadic Macros Only)
72 </p>
73 </th>
74</tr></thead>
75<tbody><tr>
76<td>
77 <p>
78</p>
79<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><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>
80 <span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span>
81
82 <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> <span class="comment">// (1) direct action</span>
83 <span class="comment">// Following block is executed when the enclosing scope exits.</span>
84 <span class="identifier">BOOST_SCOPE_EXIT</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">persons_</span><span class="special">)</span> <span class="special">)</span> <span class="special">{</span>
85 <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">// (2) rollback action</span>
86 <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
87
88 <span class="comment">// ... // (3) other operations</span>
89
90 <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> <span class="comment">// (4) disable rollback actions</span>
91<span class="special">}</span>
92</pre>
93<p>
94 </p>
95 </td>
96<td>
97 <p>
98</p>
99<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><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>
100 <span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span>
101
102 <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> <span class="comment">// (1) direct action</span>
103 <span class="comment">// Following block is executed when the enclosing scope exits.</span>
104 <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(&amp;</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">persons_</span><span class="special">)</span> <span class="special">{</span>
105 <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">// (2) rollback action</span>
106 <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
107
108 <span class="comment">// ... // (3) other operations</span>
109
110 <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> <span class="comment">// (4) disable rollback actions</span>
111<span class="special">}</span>
112</pre>
113<p>
114 </p>
115 </td>
116</tr></tbody>
117</table></div>
118<p>
119 Note how the same macros accept both syntaxes on compilers with variadic macros
120 and only the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a>
121 sequence syntax on compilers without variadic macros. Older versions of this
122 library used to only support the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a>
123 sequence syntax so this syntax is supported also for backward compatibility.
124 However, in the current version of this library and on compilers with variadic
125 macros, the comma-separated syntax is preferred because it is more readable.
126 </p>
127<p>
128 Finally, an empty capture list is always specified using <code class="computeroutput"><span class="keyword">void</span></code>
129 on compilers with and without variaidc macros (see also <a href="../../../test/world_void.cpp" target="_top"><code class="literal">world_void.cpp</code></a>):
130 </p>
131<p>
132</p>
133<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">world_t</span> <span class="special">{</span>
134 <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;</span> <span class="identifier">persons</span><span class="special">;</span>
135 <span class="keyword">bool</span> <span class="identifier">commit</span><span class="special">;</span>
136<span class="special">}</span> <span class="identifier">world</span><span class="special">;</span> <span class="comment">// Global variable.</span>
137
138<span class="keyword">void</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>
139 <span class="identifier">world</span><span class="special">.</span><span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span>
140 <span class="identifier">world</span><span class="special">.</span><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>
141
142 <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// No captures.</span>
143 <span class="keyword">if</span><span class="special">(!</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">commit</span><span class="special">)</span> <span class="identifier">world</span><span class="special">.</span><span class="identifier">persons</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span>
144 <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
145
146 <span class="comment">// ...</span>
147
148 <span class="identifier">world</span><span class="special">.</span><span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
149<span class="special">}</span>
150</pre>
151<p>
152 </p>
153<h4>
154<a name="scope_exit.no_variadic_macros.h1"></a>
155 <span class="phrase"><a name="scope_exit.no_variadic_macros.examples"></a></span><a class="link" href="no_variadic_macros.html#scope_exit.no_variadic_macros.examples">Examples</a>
156 </h4>
157<p>
158 For reference, the following is a list of most of the examples presented in
159 this documentation reprogrammed using the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a>
160 sequence syntax instead of comma-separated lists (in alphabetic order):
161 </p>
162<div class="informaltable"><table class="table">
163<colgroup><col></colgroup>
164<thead><tr><th>
165 <p>
166 Files
167 </p>
168 </th></tr></thead>
169<tbody>
170<tr><td>
171 <p>
172 <a href="../../../test/same_line_seq.cpp" target="_top"><code class="literal">same_line_seq.cpp</code></a>
173 </p>
174 </td></tr>
175<tr><td>
176 <p>
177 <a href="../../../example/scope_guard_seq.cpp" target="_top"><code class="literal">scope_guard_seq.cpp</code></a>
178 </p>
179 </td></tr>
180<tr><td>
181 <p>
182 <a href="../../../example/try_catch_seq.cpp" target="_top"><code class="literal">try_catch_seq.cpp</code></a>
183 </p>
184 </td></tr>
185<tr><td>
186 <p>
187 <a href="../../../test/world_checkpoint_all_seq.cpp" target="_top"><code class="literal">world_checkpoint_all_seq.cpp</code></a>
188 </p>
189 </td></tr>
190<tr><td>
191 <p>
192 <a href="../../../test/world_checkpoint_seq.cpp" target="_top"><code class="literal">world_checkpoint_seq.cpp</code></a>
193 </p>
194 </td></tr>
195<tr><td>
196 <p>
197 <a href="../../../test/world_this_seq.cpp" target="_top"><code class="literal">world_this_seq.cpp</code></a>
198 </p>
199 </td></tr>
200<tr><td>
201 <p>
202 <a href="../../../test/world_tpl_seq.cpp" target="_top"><code class="literal">world_tpl_seq.cpp</code></a>
203 </p>
204 </td></tr>
205</tbody>
206</table></div>
207<div class="footnotes">
208<br><hr style="width:100; text-align:left;margin-left: 0">
209<div id="ftn.scope_exit.no_variadic_macros.f0" class="footnote"><p><a href="#scope_exit.no_variadic_macros.f0" class="para"><sup class="para">[10] </sup></a>
210 A C++ compiler does not support variadic macros if the <a href="http://www.boost.org/libs/config" target="_top">Boost.Config</a>
211 macro <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_VARIADIC_MACROS</span></code>
212 is defined for that compiler.
213 </p></div>
214</div>
215</div>
216<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
217<td align="left"></td>
218<td align="right"><div class="copyright-footer">Copyright &#169; 2006-2012 Alexander Nasonov, Lorenzo Caminiti<p>
219 Distributed under the Boost Software License, Version 1.0 (see accompanying
220 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>)
221 </p>
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