<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Tutorial</title>
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
<link rel="home" href="../index.html" title="Chapter 1. Boost.ScopeExit">
<link rel="up" href="../index.html" title="Chapter 1. Boost.ScopeExit">
<link rel="prev" href="../index.html" title="Chapter 1. Boost.ScopeExit">
<link rel="next" href="alternatives.html" title="Alternatives">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
<td align="center"><a href="../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../index.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="alternatives.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="scope_exit.tutorial"></a> Tutorial</h2></div></div></div>
<p>
Imagine that you want to make many modifications to data members of the <code class="computeroutput"><span class="identifier">World</span></code> class in the <code class="computeroutput"><span class="identifier">World</span><span class="special">::</span><span class="identifier">addPerson</span></code>
function. You start with adding a new <code class="computeroutput"><span class="identifier">Person</span></code>
object to a vector of persons:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">World</span><span class="special">::</span><span class="identifier">addPerson</span><span class="special">(</span><span class="identifier">Person</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">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">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="comment">// (1) direct action
</span></pre>
<p>
Some operation down the road may throw an exception and all changes to involved
objects should be rolled back. This all-or-nothing semantic is also known as
<a href="http://www.research.att.com/~bs/glossary.html#Gstrong-guarantee" target="_top">strong
guarantee</a>.
</p>
<p>
In particular, last added person must be deleted from <code class="computeroutput"><span class="identifier">m_persons</span></code>
when the function throws. All you need is to define a delayed action (release
of a resource) right after the direct action (resource acquisition):
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">World</span><span class="special">::</span><span class="identifier">addPerson</span><span class="special">(</span><span class="identifier">Person</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aPerson</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">m_persons</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">aPerson</span><span class="special">);</span> <span class="comment">// (1) direct action
</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="identifier">m_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">m_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) turn all rollback actions into no-op
</span><span class="special">}</span>
</pre>
<p>
The block below point <code class="computeroutput"><span class="special">(</span><span class="number">1</span><span class="special">)</span></code> is a <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a>
declaration. Unlike point <code class="computeroutput"><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>, an execution of the <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a>
body will be delayed until the end of the current scope. In this case it will
be executed either after point <code class="computeroutput"><span class="special">(</span><span class="number">4</span><span class="special">)</span></code> or on any
exception.
</p>
<p>
The <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a> declaration starts with <code class="computeroutput"><span class="identifier">BOOST_SCOPE_EXIT</span></code> macro invocation which accepts
<a href="../../../../../libs/preprocessor/index.html" target="_top">Boost.Preprocessor sequence</a>
of captured variables. If a capture starts with the ampersand sign <code class="computeroutput"><span class="special">&</span></code>, a reference to the captured variable
will be available inside the <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit">ScopeExit</a> body;
otherwise, a copy of the variable will be made after the point <code class="computeroutput"><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>
and only the copy will be available inside the body.
</p>
<p>
In the example above, variables <code class="computeroutput"><span class="identifier">commit</span></code>
and <code class="computeroutput"><span class="identifier">m_persons</span></code> are passed by
reference because the final value of the <code class="computeroutput"><span class="identifier">commit</span></code>
variable should be used to determine whether to execute rollback action or
not and the action should modify the <code class="computeroutput"><span class="identifier">m_persons</span></code>
object, not its copy. This is a most common case but passing a variable by
value is sometimes useful as well.
</p>
<p>
Consider a more complex case where <code class="computeroutput"><span class="identifier">World</span><span class="special">::</span><span class="identifier">addPerson</span></code>
can save intermediate states at some points and roll back to the last saved
state. You can use <code class="computeroutput"><span class="identifier">Person</span><span class="special">::</span><span class="identifier">m_evolution</span></code> to store a version of changes
and increment it to cancel all rollback actions associated with those changes.
</p>
<p>
If you pass a current value of <code class="computeroutput"><span class="identifier">m_evolution</span></code>
stored in the <code class="computeroutput"><span class="identifier">checkpoint</span></code> variable
by value, it will remain unchanged until the end of aa scope and you can compare
it with the final value of the <code class="computeroutput"><span class="identifier">m_evolution</span></code>.
If the latter wasn't incremented since you saved it, the rollback action inside
the block should be executed:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">World</span><span class="special">::</span><span class="identifier">addPerson</span><span class="special">(</span><span class="identifier">Person</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aPerson</span><span class="special">)</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">aPerson</span><span class="special">);</span>
<span class="comment">// This block must be no-throw
</span> <span class="identifier">Person</span><span class="special">&</span> <span class="identifier">person</span> <span class="special">=</span> <span class="identifier">m_persons</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span>
<span class="identifier">Person</span><span class="special">::</span><span class="identifier">evolution_t</span> <span class="identifier">checkpoint</span> <span class="special">=</span> <span class="identifier">person</span><span class="special">.</span><span class="identifier">m_evolution</span><span class="special">;</span>
<span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span> <span class="special">(</span><span class="identifier">checkpoint</span><span class="special">)(&</span><span class="identifier">person</span><span class="special">)(&</span><span class="identifier">m_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">checkpoint</span> <span class="special">==</span> <span class="identifier">person</span><span class="special">.</span><span class="identifier">m_evolution</span><span class="special">)</span>
<span class="identifier">m_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">checkpoint</span> <span class="special">=</span> <span class="special">++</span><span class="identifier">person</span><span class="special">.</span><span class="identifier">m_evolution</span><span class="special">;</span>
<span class="comment">// Assign new id to the person
</span> <span class="identifier">World</span><span class="special">::</span><span class="identifier">id_t</span> <span class="keyword">const</span> <span class="identifier">prev_id</span> <span class="special">=</span> <span class="identifier">person</span><span class="special">.</span><span class="identifier">m_id</span><span class="special">;</span>
<span class="identifier">person</span><span class="special">.</span><span class="identifier">m_id</span> <span class="special">=</span> <span class="identifier">m_next_id</span><span class="special">++;</span>
<span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span> <span class="special">(</span><span class="identifier">checkpoint</span><span class="special">)(&</span><span class="identifier">person</span><span class="special">)(&</span><span class="identifier">m_next_id</span><span class="special">)(</span><span class="identifier">prev_id</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">checkpoint</span> <span class="special">==</span> <span class="identifier">person</span><span class="special">.</span><span class="identifier">m_evolution</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">m_next_id</span> <span class="special">=</span> <span class="identifier">person</span><span class="special">.</span><span class="identifier">m_id</span><span class="special">;</span>
<span class="identifier">person</span><span class="special">.</span><span class="identifier">m_id</span> <span class="special">=</span> <span class="identifier">prev_id</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
<span class="comment">// ...
</span>
<span class="identifier">checkpoint</span> <span class="special">=</span> <span class="special">++</span><span class="identifier">person</span><span class="special">.</span><span class="identifier">m_evolution</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Full code listing can be found in <a href="../../../example/world.cpp" target="_top">world.cpp</a>.
</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>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../index.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="alternatives.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
