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| <div class="section scope_exit_no_variadic_macros"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <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> |
| </h2></div></div></div> |
| <p> |
| This section presents an alternative syntax for compilers without variadic |
| macro support. |
| </p> |
| <h4> |
| <a name="scope_exit.no_variadic_macros.h0"></a> |
| <span><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 |
| Syntax</a> |
| </h4> |
| <p> |
| Most modern compilers support variadic macros (notably, these include GCC, |
| MSVC, and all C++11 compilers). <sup>[<a name="scope_exit.no_variadic_macros.f0" href="#ftn.scope_exit.no_variadic_macros.f0" class="footnote">10</a>]</sup> However, in the rare case that programmers need to use this library |
| on a complier without variaidc macros, this library also allows to specify |
| the capture list using a <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a> |
| sequence where tokens are separated by round parenthesis <code class="computeroutput"><span class="special">()</span></code>: |
| </p> |
| <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> |
| </pre> |
| <p> |
| Instead of the comma-separated list that we have seen so far which requires |
| variadic macros: |
| </p> |
| <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> |
| </pre> |
| <p> |
| For example, the following syntax is accepted on all compilers with and without |
| variadic macros (see also <a href="../../../test/world_seq.cpp" target="_top"><code class="literal">world_seq.cpp</code></a> |
| and <a href="../../../test/world.cpp" target="_top"><code class="literal">world.cpp</code></a>): |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Boost.Preprocessor Sequence (All Compilers) |
| </p> |
| </th> |
| <th> |
| <p> |
| Comma-Separated List (Variadic Macros Only) |
| </p> |
| </th> |
| </tr></thead> |
| <tbody><tr> |
| <td> |
| <p> |
| </p> |
| <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">&</span> <span class="identifier">a_person</span><span class="special">)</span> <span class="special">{</span> |
| <span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</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> <span class="comment">// (1) direct action</span> |
| <span class="comment">// Following block is executed when the enclosing scope exits.</span> |
| <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span> <span class="special">(&</span><span class="identifier">commit</span><span class="special">)</span> <span class="special">(&</span><span class="identifier">persons_</span><span class="special">)</span> <span class="special">)</span> <span class="special">{</span> |
| <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> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| |
| <span class="comment">// ... // (3) other operations</span> |
| |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| </td> |
| <td> |
| <p> |
| </p> |
| <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">&</span> <span class="identifier">a_person</span><span class="special">)</span> <span class="special">{</span> |
| <span class="keyword">bool</span> <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">false</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> <span class="comment">// (1) direct action</span> |
| <span class="comment">// Following block is executed when the enclosing scope exits.</span> |
| <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(&</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&</span><span class="identifier">persons_</span><span class="special">)</span> <span class="special">{</span> |
| <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> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| |
| <span class="comment">// ... // (3) other operations</span> |
| |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <p> |
| Note how the same macros accept both syntaxes on compilers with variadic macros |
| and only the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a> |
| sequence syntax on compilers without variadic macros. Older versions of this |
| library used to only support the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a> |
| sequence syntax so this syntax is supported also for backward compatibility. |
| However, in the current version of this library and on compilers with variadic |
| macros, the comma-separated syntax is preferred because it is more readable. |
| </p> |
| <p> |
| Finally, an empty capture list is always specified using <code class="computeroutput"><span class="keyword">void</span></code> |
| 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>): |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">world_t</span> <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">person</span><span class="special">></span> <span class="identifier">persons</span><span class="special">;</span> |
| <span class="keyword">bool</span> <span class="identifier">commit</span><span class="special">;</span> |
| <span class="special">}</span> <span class="identifier">world</span><span class="special">;</span> <span class="comment">// Global variable.</span> |
| |
| <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">&</span> <span class="identifier">a_person</span><span class="special">)</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="keyword">false</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">push_back</span><span class="special">(</span><span class="identifier">a_person</span><span class="special">);</span> |
| |
| <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> |
| <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> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| |
| <span class="comment">// ...</span> |
| |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <h4> |
| <a name="scope_exit.no_variadic_macros.h1"></a> |
| <span><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> |
| </h4> |
| <p> |
| For reference, the following is a list of most of the examples presented in |
| this documentation reprogrammed using the <a href="http://www.boost.org/libs/preprocessor" target="_top">Boost.Preprocessor</a> |
| sequence syntax instead of comma-separated lists (in alphabetic order): |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup><col></colgroup> |
| <thead><tr><th> |
| <p> |
| Files |
| </p> |
| </th></tr></thead> |
| <tbody> |
| <tr><td> |
| <p> |
| <a href="../../../test/same_line_seq.cpp" target="_top"><code class="literal">same_line_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../example/scope_guard_seq.cpp" target="_top"><code class="literal">scope_guard_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../example/try_catch_seq.cpp" target="_top"><code class="literal">try_catch_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../test/world_checkpoint_all_seq.cpp" target="_top"><code class="literal">world_checkpoint_all_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../test/world_checkpoint_seq.cpp" target="_top"><code class="literal">world_checkpoint_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../test/world_this_seq.cpp" target="_top"><code class="literal">world_this_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| <tr><td> |
| <p> |
| <a href="../../../test/world_tpl_seq.cpp" target="_top"><code class="literal">world_tpl_seq.cpp</code></a> |
| </p> |
| </td></tr> |
| </tbody> |
| </table></div> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a id="ftn.scope_exit.no_variadic_macros.f0" href="#scope_exit.no_variadic_macros.f0" class="para">10</a>] </sup> |
| A C++ compiler does not support variadic macros if the <a href="http://www.boost.org/libs/config" target="_top">Boost.Config</a> |
| macro <code class="computeroutput"><span class="identifier">BOOST_NO_VARIADIC_MACROS</span></code> |
| is defined for that compiler. |
| </p></div> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2006-2012 Alexander Nasonov, Lorenzo Caminiti<p> |
| Distributed under the Boost Software License, Version 1.0 (see accompanying |
| 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>) |
| </p> |
| </div></td> |
| </tr></table> |
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