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| <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> |
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