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| <div class="section scope_exit_alternatives"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="scope_exit.alternatives"></a><a class="link" href="alternatives.html" title="Annex: Alternatives">Annex: Alternatives</a> |
| </h2></div></div></div> |
| <p> |
| This section presents some alternative and related work to <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a>. |
| </p> |
| <h4> |
| <a name="scope_exit.alternatives.h0"></a> |
| <span><a name="scope_exit.alternatives.try_catch"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.try_catch">Try-Catch</a> |
| </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> |
| does not close the file in its destructor, a programmer is expected to call |
| the <code class="computeroutput"><span class="identifier">close</span></code> member function explicitly. |
| For example (see also <a href="../../../example/try_catch.cpp" target="_top"><code class="literal">try_catch.cpp</code></a>): |
| </p> |
| <p> |
| </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">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special"><<</span> <span class="string">"could not get user info"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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> |
| </p> |
| <p> |
| Note the following issues with this approach: |
| </p> |
| <div class="orderedlist"><ol class="orderedlist" type="1"> |
| <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. |
| </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> |
| </ol></div> |
| <p> |
| The <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> approach does not have |
| any of these issues. For example (see also <a href="../../../example/try_catch.cpp" target="_top"><code class="literal">try_catch.cpp</code></a>): |
| </p> |
| <p> |
| </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="identifier">passwd</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="special">}</span> <span class="keyword">catch</span><span class="special">(...)</span> <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special"><<</span> <span class="string">"could not get user info"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| <span class="keyword">throw</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <h4> |
| <a name="scope_exit.alternatives.h1"></a> |
| <span><a name="scope_exit.alternatives.raii"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.raii">RAII</a> |
| </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">std</span><span class="special">::</span><span class="identifier">clog</span> <span class="special"><<</span> <span class="string">"could not get user info"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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">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">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">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">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">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">commit_</span><span class="special">)</span> <span class="identifier">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">superman_ptr</span><span class="special">(</span><span class="keyword">new</span> <span class="identifier">superman</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">superman_ptr</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span> |
| <span class="identifier">superman_ptr</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span> <span class="comment">// persons_ successfully took ownership</span> |
| </pre> |
| <p> |
| Or with specialized containers such as <a href="http://www.boost.org/libs/ptr_container" target="_top">Boost.PointerContainer</a> |
| or <a href="http://www.boost.org/libs/multi_index" target="_top">Boost.Multi-Index</a>. |
| </p> |
| <h4> |
| <a name="scope_exit.alternatives.h2"></a> |
| <span><a name="scope_exit.alternatives.scope_guards"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.scope_guards">Scope |
| Guards</a> |
| </h4> |
| <p> |
| Imagine that a new currency rate is introduced before performing a transaction |
| (see also []): |
| </p> |
| <p> |
| </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> |
| <span class="comment">// Transaction...</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| If the transaction does not complete, the currency must be erased from <code class="computeroutput"><span class="identifier">rates</span></code>. This can be done with <a href="http://www.ddj.com/dept/cpp/184403758" target="_top">ScopeGuard</a> |
| and <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> (or |
| <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</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 the following issues with this approach: |
| </p> |
| <div class="orderedlist"><ol class="orderedlist" type="1"> |
| <li class="listitem"> |
| <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> expressions |
| are hard to write correctly (e.g., overloaded functions must be explicitly |
| casted, as demonstrated in the example above). |
| </li> |
| <li class="listitem"> |
| The condition in the <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 reducing readability. |
| </li> |
| <li class="listitem"> |
| Setting a breakpoint inside <code class="computeroutput"><span class="identifier">if_</span><span class="special">[...]</span></code> requires in-depth knowledge of <a href="http://www.boost.org/libs/lambda" target="_top">Boost.Lambda</a> and debugging |
| techniques. |
| </li> |
| </ol></div> |
| <p> |
| This code will look much better with C++11 lambdas: |
| </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">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> |
| |
| <span class="comment">// ...</span> |
| |
| <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| </pre> |
| <p> |
| With <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> we can simply do the |
| following (see also <a href="../../../example/scope_guard.cpp" target="_top"><code class="literal">scope_guard.cpp</code></a>): |
| </p> |
| <p> |
| </p> |
| <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> |
| |
| <span class="comment">// ...</span> |
| |
| <span class="identifier">commit</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <h4> |
| <a name="scope_exit.alternatives.h3"></a> |
| <span><a name="scope_exit.alternatives.the_d_programming_language"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.the_d_programming_language">The |
| D Programming Language</a> |
| </h4> |
| <p> |
| <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> 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 does not 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, these are not possible in C++ because failure or success |
| conditions cannot be determined by calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">uncaught_exception</span></code> |
| (see <a href="http://www.gotw.ca/gotw/047.htm" target="_top">Guru of the Week #47</a> |
| for 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). However, this is not a big problem because these two <a href="http://www.digitalmars.com/d/index.html" target="_top">D</a>'s |
| 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 (similarly to some examples presented in the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a> |
| section). |
| </p> |
| <h4> |
| <a name="scope_exit.alternatives.h4"></a> |
| <span><a name="scope_exit.alternatives.c__11_lambdas"></a></span><a class="link" href="alternatives.html#scope_exit.alternatives.c__11_lambdas">C++11 |
| Lambdas</a> |
| </h4> |
| <p> |
| Using C++11 lambdas, it is relatively easy to implement the <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> |
| construct. For example (see also <a href="../../../example/world_cxx11_lambda.cpp" target="_top"><code class="literal">world_cxx11_lambda.cpp</code></a>): |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">functional</span><span class="special">></span> |
| |
| <span class="keyword">struct</span> <span class="identifier">scope_exit</span> <span class="special">{</span> |
| <span class="identifier">scope_exit</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">f_</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span> <span class="special">{}</span> |
| <span class="special">~</span><span class="identifier">scope_exit</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">f_</span><span class="special">();</span> <span class="special">}</span> |
| <span class="keyword">private</span><span class="special">:</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f_</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <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="identifier">scope_exit</span> <span class="identifier">on_exit1</span><span class="special">([&</span><span class="identifier">commit</span><span class="special">,</span> <span class="keyword">this</span><span class="special">](</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Use C++11 lambda.</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">// `persons_` via captured `this`.</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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| However, this library allows to program the <a class="link" href="../index.html" title="Chapter 1. Boost.ScopeExit 1.1.0">Boost.ScopeExit</a> |
| construct in a way that is portable between C++03 and C++11 compilers. |
| </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 © 2006-2012 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> |
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