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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="property_tree.tutorial"></a><a class="link" href="tutorial.html" title="Five Minute Tutorial">Five Minute Tutorial</a>
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</h2></div></div></div>
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<p>
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This tutorial uses XML. Note that the library is not specifically bound to
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XML, and any other supported format (such as INI or JSON) could be used instead.
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XML was chosen because the author thinks that a wide range of people is familiar
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with it.
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</p>
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<p>
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Suppose we are writing a logging system for some application, and need to read
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log configuration from a file when the program starts. The file with the log
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configuration looks like this:
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</p>
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<pre class="programlisting"><debug>
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<filename>debug.log</filename>
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<modules>
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<module>Finance</module>
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<module>Admin</module>
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<module>HR</module>
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</modules>
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<level>2</level>
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</debug>
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</pre>
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<p>
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It contains the log filename, a list of modules where logging is enabled, and
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the debug level value.
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</p>
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<p>
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First we need some includes:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">property_tree</span><span class="special">/</span><span class="identifier">ptree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">property_tree</span><span class="special">/</span><span class="identifier">xml_parser</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">foreach</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">set</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">exception</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="keyword">namespace</span> <span class="identifier">pt</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">property_tree</span><span class="special">;</span>
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</pre>
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<p>
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To store the logging configuration in the program we create a debug_settings
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structure:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">debug_settings</span>
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<span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">m_file</span><span class="special">;</span> <span class="comment">// log filename</span>
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<span class="keyword">int</span> <span class="identifier">m_level</span><span class="special">;</span> <span class="comment">// debug level</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">set</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="identifier">m_modules</span><span class="special">;</span> <span class="comment">// modules where logging is enabled</span>
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<span class="keyword">void</span> <span class="identifier">load</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&</span><span class="identifier">filename</span><span class="special">);</span>
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<span class="keyword">void</span> <span class="identifier">save</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&</span><span class="identifier">filename</span><span class="special">);</span>
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<span class="special">};</span>
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</pre>
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<p>
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All that needs to be done now is to write implementations of load() and save()
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member functions. Let's first deal with load(). It contains just 7 lines of
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code, although it does all the necessary things, including error reporting:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">debug_settings</span><span class="special">::</span><span class="identifier">load</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&</span><span class="identifier">filename</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="comment">// Create empty property tree object</span>
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<span class="identifier">pt</span><span class="special">::</span><span class="identifier">ptree</span> <span class="identifier">tree</span><span class="special">;</span>
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<span class="comment">// Parse the XML into the property tree.</span>
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<span class="identifier">pt</span><span class="special">::</span><span class="identifier">read_xml</span><span class="special">(</span><span class="identifier">filename</span><span class="special">,</span> <span class="identifier">tree</span><span class="special">);</span>
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<span class="comment">// Use the throwing version of get to find the debug filename.</span>
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<span class="comment">// If the path cannot be resolved, an exception is thrown.</span>
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<span class="identifier">m_file</span> <span class="special">=</span> <span class="identifier">tree</span><span class="special">.</span><span class="identifier">get</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="string">"debug.filename"</span><span class="special">);</span>
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<span class="comment">// Use the default-value version of get to find the debug level.</span>
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<span class="comment">// Note that the default value is used to deduce the target type.</span>
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<span class="identifier">m_level</span> <span class="special">=</span> <span class="identifier">tree</span><span class="special">.</span><span class="identifier">get</span><span class="special">(</span><span class="string">"debug.level"</span><span class="special">,</span> <span class="number">0</span><span class="special">);</span>
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<span class="comment">// Use get_child to find the node containing the modules, and iterate over</span>
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<span class="comment">// its children. If the path cannot be resolved, get_child throws.</span>
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<span class="comment">// A C++11 for-range loop would also work.</span>
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<span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">pt</span><span class="special">::</span><span class="identifier">ptree</span><span class="special">::</span><span class="identifier">value_type</span> <span class="special">&</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">tree</span><span class="special">.</span><span class="identifier">get_child</span><span class="special">(</span><span class="string">"debug.modules"</span><span class="special">))</span> <span class="special">{</span>
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<span class="comment">// The data function is used to access the data stored in a node.</span>
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<span class="identifier">m_modules</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">second</span><span class="special">.</span><span class="identifier">data</span><span class="special">());</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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<p>
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Now the save() function. It is also 7 lines of code:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">debug_settings</span><span class="special">::</span><span class="identifier">save</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&</span><span class="identifier">filename</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="comment">// Create an empty property tree object.</span>
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<span class="identifier">pt</span><span class="special">::</span><span class="identifier">ptree</span> <span class="identifier">tree</span><span class="special">;</span>
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<span class="comment">// Put the simple values into the tree. The integer is automatically</span>
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<span class="comment">// converted to a string. Note that the "debug" node is automatically</span>
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<span class="comment">// created if it doesn't exist.</span>
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<span class="identifier">tree</span><span class="special">.</span><span class="identifier">put</span><span class="special">(</span><span class="string">"debug.filename"</span><span class="special">,</span> <span class="identifier">m_file</span><span class="special">);</span>
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<span class="identifier">tree</span><span class="special">.</span><span class="identifier">put</span><span class="special">(</span><span class="string">"debug.level"</span><span class="special">,</span> <span class="identifier">m_level</span><span class="special">);</span>
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<span class="comment">// Add all the modules. Unlike put, which overwrites existing nodes, add</span>
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<span class="comment">// adds a new node at the lowest level, so the "modules" node will have</span>
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<span class="comment">// multiple "module" children.</span>
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<span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&</span><span class="identifier">name</span><span class="special">,</span> <span class="identifier">m_modules</span><span class="special">)</span>
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<span class="identifier">tree</span><span class="special">.</span><span class="identifier">add</span><span class="special">(</span><span class="string">"debug.modules.module"</span><span class="special">,</span> <span class="identifier">name</span><span class="special">);</span>
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<span class="comment">// Write property tree to XML file</span>
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<span class="identifier">pt</span><span class="special">::</span><span class="identifier">write_xml</span><span class="special">(</span><span class="identifier">filename</span><span class="special">,</span> <span class="identifier">tree</span><span class="special">);</span>
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<span class="special">}</span>
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</pre>
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<p>
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The full program <a href="../../../libs/property_tree/examples/debug_settings.cpp" target="_top">debug_settings.cpp</a>
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is included in the examples directory.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2008-2010 Marcin Kalicinski<br>Copyright © 2010-2013 Sebastian
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Redl<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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</tr></table>
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