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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_propertytree.tutorial"></a><a class="link" href="tutorial.html" title="Five Minute Tutorial">Five Minute Tutorial</a> |
| </h2></div></div></div> |
| <p> |
| This tutorial uses XML. Note that the library is not specifically bound to |
| XML, and any other supported format (such as INI or JSON) could be used instead. |
| XML was chosen because the author thinks that wide range of people is familiar |
| with it. |
| </p> |
| <p> |
| Suppose we are writing a logging system for some application, and need to read |
| log configuration from a file when the program starts. The file with the log |
| configuration looks like this: |
| </p> |
| <pre class="programlisting"><debug> |
| <filename>debug.log</filename> |
| <modules> |
| <module>Finance</module> |
| <module>Admin</module> |
| <module>HR</module> |
| </modules> |
| <level>2</level> |
| </debug> |
| </pre> |
| <p> |
| It contains the log filename, a list of modules where logging is enabled, and |
| the debug level value. To store the logging configuration in the program we |
| created a debug_settings structure: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">debug_settings</span> |
| <span class="special">{</span> |
| <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> <span class="keyword">int</span> <span class="identifier">m_level</span><span class="special">;</span> <span class="comment">// debug level |
| </span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">set</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> <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> |
| <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> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| All that needs to be done now is to write implementations of load() and save() |
| member functions. Let's first deal with load(). It contains just 7 lines of |
| code, although it does all the necessary things, including error reporting: |
| </p> |
| <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> |
| <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> |
| |
| <span class="comment">// Loads debug_settings structure from the specified XML file |
| </span><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> |
| <span class="special">{</span> |
| <span class="comment">// Create an empty property tree object |
| </span> <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">property_tree</span><span class="special">::</span><code class="computeroutput"><a class="link" href="../boost/property_tree/ptree.html" title="Type definition ptree">ptree</a></code><span class="special">;</span> |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree.html" title="Type definition ptree">ptree</a></code> <span class="identifier">pt</span><span class="special">;</span> |
| |
| <span class="comment">// Load the XML file into the property tree. If reading fails |
| </span> <span class="comment">// (cannot open file, parse error), an exception is thrown. |
| </span> <code class="computeroutput"><a class="link" href="../boost/property_tree/xml_parser/read_xml_id979472.html" title="Function template read_xml">read_xml</a></code><span class="special">(</span><span class="identifier">filename</span><span class="special">,</span> <span class="identifier">pt</span><span class="special">);</span> |
| |
| <span class="comment">// Get the filename and store it in the m_file variable. |
| </span> <span class="comment">// Note that we construct the path to the value by separating |
| </span> <span class="comment">// the individual keys with dots. If dots appear in the keys, |
| </span> <span class="comment">// a path type with a different separator can be used. |
| </span> <span class="comment">// If the debug.filename key is not found, an exception is thrown. |
| </span> <span class="identifier">m_file</span> <span class="special">=</span> <span class="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code><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> |
| |
| <span class="comment">// Get the debug level and store it in the m_level variable. |
| </span> <span class="comment">// This is another version of the get method: if the value is |
| </span> <span class="comment">// not found, the default value (specified by the second |
| </span> <span class="comment">// parameter) is returned instead. The type of the value |
| </span> <span class="comment">// extracted is determined by the type of the second parameter, |
| </span> <span class="comment">// so we can simply write get(...) instead of get<int>(...). |
| </span> <span class="identifier">m_level</span> <span class="special">=</span> <span class="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code><span class="special">(</span><span class="string">"debug.level"</span><span class="special">,</span> <span class="number">0</span><span class="special">);</span> |
| |
| <span class="comment">// Iterate over the debug.modules section and store all found |
| </span> <span class="comment">// modules in the m_modules set. The get_child() function |
| </span> <span class="comment">// returns a reference to the child at the specified path; if |
| </span> <span class="comment">// there is no such child, it throws. Property tree iterators |
| </span> <span class="comment">// are models of BidirectionalIterator. |
| </span> <span class="identifier">BOOST_FOREACH</span><span class="special">(</span><code class="computeroutput"><a class="link" href="../boost/property_tree/ptree.html" title="Type definition ptree">ptree</a></code><span class="special">::</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#boost.property_tree.basic_ptree.value_type">value_type</a></code> <span class="special">&</span><span class="identifier">v</span><span class="special">,</span> |
| <span class="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id973726-bb">get_child</a></code><span class="special">(</span><span class="string">"debug.modules"</span><span class="special">))</span> |
| <span class="identifier">m_modules</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id901510-bb">insert</a></code><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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Now the save() function. It is also 7 lines of code: |
| </p> |
| <pre class="programlisting"><span class="comment">// Saves the debug_settings structure to the specified XML file |
| </span><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> |
| <span class="special">{</span> |
| <span class="comment">// Create an empty property tree object |
| </span> <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">property_tree</span><span class="special">::</span><code class="computeroutput"><a class="link" href="../boost/property_tree/ptree.html" title="Type definition ptree">ptree</a></code><span class="special">;</span> |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree.html" title="Type definition ptree">ptree</a></code> <span class="identifier">pt</span><span class="special">;</span> |
| |
| <span class="comment">// Put log filename in property tree |
| </span> <span class="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code><span class="special">(</span><span class="string">"debug.filename"</span><span class="special">,</span> <span class="identifier">m_file</span><span class="special">);</span> |
| |
| <span class="comment">// Put debug level in property tree |
| </span> <span class="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code><span class="special">(</span><span class="string">"debug.level"</span><span class="special">,</span> <span class="identifier">m_level</span><span class="special">);</span> |
| |
| <span class="comment">// Iterate over the modules in the set and put them in the |
| </span> <span class="comment">// property tree. Note that the put function places the new |
| </span> <span class="comment">// key at the end of the list of keys. This is fine most of |
| </span> <span class="comment">// the time. If you want to place an item at some other place |
| </span> <span class="comment">// (i.e. at the front or somewhere in the middle), this can |
| </span> <span class="comment">// be achieved using a combination of the insert and put_own |
| </span> <span class="comment">// functions. |
| </span> <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> |
| <span class="identifier">pt</span><span class="special">.</span><span class="identifier">__ptree_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> |
| |
| <span class="comment">// Write the property tree to the XML file. |
| </span> <code class="computeroutput"><a class="link" href="../boost/property_tree/xml_parser/write_xml_id898931.html" title="Function template write_xml">write_xml</a></code><span class="special">(</span><span class="identifier">filename</span><span class="special">,</span> <span class="identifier">pt</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The full program <a href="../../../libs/property_tree/examples/debug_settings.cpp" target="_top">debug_settings.cpp</a> |
| is included in the examples directory. |
| </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 © 2008 Marcin Kalicinski<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
| </p> |
| </div></td> |
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