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<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>
<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 -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>)
</p>
</div></td>
</tr></table>
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