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| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="scope_exit.alternatives"></a> Alternatives</h2></div></div></div> |
| <a name="alternatives.try_catch"></a><h4> |
| <a name="id761016"></a> |
| try-catch |
| </h4> |
| <p> |
| This is an example of using a badly designed <code class="computeroutput"><span class="identifier">File</span></code> |
| class. An instance of <code class="computeroutput"><span class="identifier">File</span></code> |
| doesn't close a file in a destructor, a programmer is expected to call the |
| <code class="computeroutput"><span class="identifier">close</span></code> member function explicitly. |
| </p> |
| <pre class="programlisting"><span class="identifier">File</span> <span class="identifier">passwd</span><span class="special">;</span> |
| <span class="keyword">try</span> <span class="special">{</span> |
| <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> |
| <span class="comment">// ... |
| </span> <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">();</span> |
| <span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span> |
| <span class="identifier">log</span><span class="special">(</span><span class="string">"could not get user info"</span><span class="special">);</span> |
| <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> |
| <span class="keyword">throw</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Note the following: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| the <code class="computeroutput"><span class="identifier">passwd</span></code> object is defined |
| outside of the <code class="computeroutput"><span class="keyword">try</span></code> block because |
| this object is required inside the <code class="computeroutput"><span class="keyword">catch</span></code> |
| block to close the file, |
| </li> |
| <li class="listitem"> |
| the <code class="computeroutput"><span class="identifier">passwd</span></code> object is not |
| fully constructed until after the <code class="computeroutput"><span class="identifier">open</span></code> |
| member function returns, and |
| </li> |
| <li class="listitem"> |
| 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 |
| to <code class="computeroutput"><span class="identifier">passwd</span><span class="special">.</span><span class="identifier">is_open</span><span class="special">()</span></code>. |
| </li> |
| </ul></div> |
| <p> |
| <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a> doesn't have any of these problems: |
| </p> |
| <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span> |
| <span class="identifier">File</span> <span class="identifier">passwd</span><span class="special">(</span><span class="string">"/etc/passwd"</span><span class="special">);</span> |
| <span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span> <span class="special">(&</span><span class="identifier">passwd</span><span class="special">)</span> <span class="special">)</span> <span class="special">{</span> |
| <span class="identifier">passwd</span><span class="special">.</span><span class="identifier">close</span><span class="special">();</span> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| <span class="comment">// ... |
| </span><span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span> |
| <span class="identifier">log</span><span class="special">(</span><span class="string">"could not get user info"</span><span class="special">);</span> |
| <span class="keyword">throw</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <a name="alternatives.raii"></a><h4> |
| <a name="id761495"></a> |
| RAII |
| </h4> |
| <p> |
| <a href="http://www.research.att.com/~bs/glossary.html#Gresource-acquisition-is-initialization" target="_top">RAII</a> |
| is absolutely perfect for the <code class="computeroutput"><span class="identifier">File</span></code> |
| class introduced above. Use of a properly designed <code class="computeroutput"><span class="identifier">File</span></code> |
| class would look like: |
| </p> |
| <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span> |
| <span class="identifier">File</span> <span class="identifier">passwd</span><span class="special">(</span><span class="string">"/etc/passwd"</span><span class="special">);</span> |
| <span class="comment">// ... |
| </span><span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span> |
| <span class="identifier">log</span><span class="special">(</span><span class="string">"could not get user info"</span><span class="special">);</span> |
| <span class="keyword">throw</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| However, using <a href="http://www.research.att.com/~bs/glossary.html#Gresource-acquisition-is-initialization" target="_top">RAII</a> |
| to build up a <a href="http://www.research.att.com/~bs/glossary.html#Gstrong-guarantee" target="_top">strong |
| 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> |
| types. For example: |
| </p> |
| <pre class="programlisting"><span class="identifier">m_persons</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">person</span><span class="special">);</span> |
| <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">m_persons</span><span class="special">);</span> |
| </pre> |
| <p> |
| The <code class="computeroutput"><span class="identifier">pop_back_if_not_commit</span></code> |
| class is either defined out of the scope or as a local class: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">pop_back_if_not_commit</span> <span class="special">{</span> |
| <span class="keyword">bool</span> <span class="identifier">m_commit</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">m_vec</span><span class="special">;</span> |
| <span class="comment">// ... |
| </span> <span class="special">~</span><span class="identifier">pop_back_if_not_commit</span><span class="special">()</span> <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(!</span><span class="identifier">m_commit</span><span class="special">)</span> |
| <span class="identifier">m_vec</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| In some cases <a href="http://www.research.att.com/~bs/glossary.html#Gstrong-guarantee" target="_top">strong |
| guarantee</a> can be accomplished with standard utilities: |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">auto_ptr</span><span class="special"><</span><span class="identifier">Person</span><span class="special">></span> <span class="identifier">spSuperMan</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">Superman</span><span class="special">);</span> |
| <span class="identifier">m_persons</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">spSuperMan</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span> |
| <span class="identifier">spSuperMan</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span> <span class="comment">// m_persons successfully took ownership. |
| </span></pre> |
| <p> |
| or with specialized containers such as <a href="../../../../../libs/ptr_container/doc/ptr_container.html" target="_top">Boost |
| Pointer Container Library</a> or <a href="../../../../../libs/multi_index/doc/index.html" target="_top">Boost |
| Multi-Index Containers Library</a>. |
| </p> |
| <a name="alternatives._ulink_url__http___www_ddj_com_dept_cpp_184403758__scopeguard__ulink_"></a><h4> |
| <a name="id761994"></a> |
| <a href="http://www.ddj.com/dept/cpp/184403758" target="_top">ScopeGuard</a> |
| </h4> |
| <p> |
| Imagine that you add a new currency rate: |
| </p> |
| <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> |
| <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> |
| <span class="keyword">double</span> <span class="identifier">rate</span> <span class="special">=</span> <span class="number">1.3326</span><span class="special">;</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</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">></span> <span class="identifier">rates</span><span class="special">;</span> |
| <span class="keyword">bool</span> <span class="identifier">currency_rate_inserted</span> <span class="special">=</span> |
| <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> |
| </pre> |
| <p> |
| and then continue a transaction. If it cannot be completed, you erase the currency |
| from <code class="computeroutput"><span class="identifier">rates</span></code>. This is how you |
| can do this with <a href="http://www.ddj.com/dept/cpp/184403758" target="_top">ScopeGuard</a> |
| and <a href="../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a>: |
| </p> |
| <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> |
| |
| <span class="identifier">ON_BLOCK_EXIT</span><span class="special">(</span> |
| <span class="identifier">if_</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">_1</span><span class="special">)</span> <span class="special">[</span> |
| <span class="identifier">bind</span><span class="special">(</span> |
| <span class="keyword">static_cast</span><span class="special"><</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</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">>::</span><span class="identifier">size_type</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</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">>::*)(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&)</span> |
| <span class="special">>(&</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</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">>::</span><span class="identifier">erase</span><span class="special">)</span> |
| <span class="special">,</span> <span class="special">&</span><span class="identifier">rates</span> |
| <span class="special">,</span> <span class="identifier">currency</span> |
| <span class="special">)</span> |
| <span class="special">]</span> |
| <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> |
| <span class="special">);</span> |
| |
| <span class="comment">// ... |
| </span> |
| <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| </pre> |
| <p> |
| Note that |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Boost.lambda expressions are hard to write correctly, for example, overloaded |
| function must be explicitly casted, as demonstrated in this example, |
| </li> |
| <li class="listitem"> |
| condition in <code class="computeroutput"><span class="identifier">if_</span></code> expression |
| refers to <code class="computeroutput"><span class="identifier">commit</span></code> variable |
| indirectly through the <code class="computeroutput"><span class="identifier">_1</span></code> |
| placeholder, |
| </li> |
| <li class="listitem"> |
| setting a breakpoint inside <code class="computeroutput"><span class="identifier">if_</span><span class="special">[</span> <span class="special">...</span> <span class="special">]</span></code> |
| requires in-depth knowledge of <a href="../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a> |
| and debugging techniques. |
| </li> |
| </ul></div> |
| <p> |
| This code will look much better with native lambda expressions proposed for |
| C++0x: |
| </p> |
| <pre class="programlisting"><span class="identifier">ON_BLOCK_EXIT</span><span class="special">(</span> |
| <span class="special">[</span><span class="identifier">currency_rate_inserted</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">rates</span><span class="special">,</span> <span class="special">&</span><span class="identifier">currency</span><span class="special">]()</span> <span class="special">-></span> <span class="keyword">void</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&&</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> |
| <span class="special">}</span> |
| <span class="special">);</span> |
| </pre> |
| <p> |
| With <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a> you can simply do |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span> <span class="special">(</span><span class="identifier">currency_rate_inserted</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">currency</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">currency_rate_inserted</span> <span class="special">&&</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> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| |
| <span class="comment">// ... |
| </span> |
| <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| </pre> |
| <a name="alternatives.c__0x"></a><h4> |
| <a name="id762994"></a> |
| C++0x |
| </h4> |
| <p> |
| In future releases <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a> will take advantages |
| of C++0x features. |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| Passing capture list as <a href="../../../../../libs/preprocessor/index.html" target="_top">Boost.Preprocessor |
| sequence</a> will be replaced with a traditional macro invocation style: |
| </li></ul></div> |
| <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">&</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&</span><span class="identifier">rates</span><span class="special">,</span> <span class="special">&</span><span class="identifier">currency</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&&</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> |
| <span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span> |
| </pre> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| <code class="computeroutput"><span class="identifier">BOOST_SCOPE_EXIT_END</span></code> will |
| be replaced with a semicolon: |
| </li></ul></div> |
| <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">&</span><span class="identifier">commit</span><span class="special">,</span> <span class="special">&</span><span class="identifier">rates</span><span class="special">,</span> <span class="special">&</span><span class="identifier">currency</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&&</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> |
| <span class="special">};</span> |
| </pre> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| Users will be able to capture local variables implicitly with lambda capture |
| defaults <code class="computeroutput"><span class="special">&</span></code> and <code class="computeroutput"><span class="special">=</span></code>: |
| </li></ul></div> |
| <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">{</span> |
| <span class="keyword">if</span><span class="special">(</span><span class="identifier">currency_rate_inserted</span> <span class="special">&&</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> |
| <span class="special">};</span> |
| </pre> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| It will be possible to capture <code class="computeroutput"><span class="keyword">this</span></code> |
| pointer. |
| </li></ul></div> |
| <a name="alternatives.the_d_programming_language"></a><h4> |
| <a name="id763445"></a> |
| The D Programming Language |
| </h4> |
| <p> |
| <span class="underline">ScopeExit</span> is similar to <a href="http://www.digitalmars.com/d/2.0/statement.html#ScopeGuardStatement" target="_top">scope(exit)</a> |
| feature built into the <a href="http://www.digitalmars.com/d/index.html" target="_top">D</a> |
| programming language. |
| </p> |
| <p> |
| A curious reader may notice that the library doesn't 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> |
| language. Unfortunately, it's not possible in C++ because failure or success |
| condition cannot be determined by calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">uncaught_exception</span></code>. |
| It's not a big problem, though. These two constructs can be expressed in terms |
| of <a href="http://www.digitalmars.com/d/2.0/statement.html#ScopeGuardStatement" target="_top">scope(exit)</a> |
| and a <code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">commit</span></code> |
| variable as explained in <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>. |
| Refer to <a href="http://www.gotw.ca/gotw/047.htm" target="_top">Guru of the Week #47</a> |
| for more 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 use at all. |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2006 -2009 Alexander Nasonov<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <ulink url="http://www.boost.org/LICENSE_1_0.txt"> |
| http://www.boost.org/LICENSE_1_0.txt </ulink>) |
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