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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_propertytree.accessing"></a><a class="link" href="accessing.html" title="How to Access Data in a Property Tree">How to Access Data in a Property |
| Tree</a> |
| </h2></div></div></div> |
| <p> |
| Property tree resembles (almost is) a standard container with value type of |
| <code class="computeroutput"><span class="identifier">pair</span><span class="special"><</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">ptree</span><span class="special">></span></code>. |
| It has the usual member functions, such as <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id901510-bb">insert</a></code>, |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id901637-bb">push_back</a></code>, |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id959996-bb">find</a></code>, |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id901572-bb">erase</a></code>, |
| etc. These can of course be used to populate and access the tree. For example |
| the following code adds key <code class="computeroutput"><span class="string">"pi"</span></code> |
| with data (almost) equal to mathematical <span class="emphasis"><em>pi</em></span> value: |
| </p> |
| <pre class="programlisting"><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="identifier">pt</span><span class="special">.</span><code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id901637-bb">push_back</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="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="string">"pi"</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><span class="string">"3.14159"</span><span class="special">)));</span> |
| </pre> |
| <p> |
| To find the value of <code class="computeroutput"><span class="identifier">pi</span></code> we |
| might do the following: |
| </p> |
| <pre class="programlisting"><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">const_iterator</code> <span class="identifier">it</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#id959996-bb">find</a></code><span class="special">(</span><span class="string">"pi"</span><span class="special">);</span> |
| <span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="identifier">it</span><span class="special">-></span><span class="identifier">second</span><span class="special">.</span><span class="identifier">_ptree_data__</span><span class="special">());</span> |
| </pre> |
| <p> |
| This looks quite cumbersome, and would be even more so if <code class="computeroutput"><span class="identifier">pi</span></code> |
| value was not stored so near the top of the tree, and we cared just a little |
| bit more about errors. Fortunately, there is another, correct way of doing |
| it: |
| </p> |
| <pre class="programlisting"><span class="identifier">ptree</span> <span class="identifier">pt</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#id960399-bb">put</a></code><span class="special">(</span><span class="string">"pi"</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span> <span class="comment">// put double |
| </span><span class="keyword">double</span> <span class="identifier">pi</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="keyword">double</span><span class="special">>(</span><span class="string">"pi"</span><span class="special">);</span> <span class="comment">// get double |
| </span></pre> |
| <p> |
| It doesn't get simpler than that. Basically, there are 2 families of member |
| functions, <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code> |
| and <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code>, |
| which allow intuitive access to data stored in the tree (direct children or |
| not). |
| </p> |
| <a name="boost_propertytree.accessing.three_ways_of_getting_data"></a><h4> |
| <a name="id2112019"></a> |
| <a class="link" href="accessing.html#boost_propertytree.accessing.three_ways_of_getting_data">Three |
| Ways of Getting Data</a> |
| </h4> |
| <p> |
| There are three versions of get: get, get (default-value version), and get_optional, |
| which differ by failure handling strategy. All versions take path specifier, |
| which determines in which key to search for a value. It can be a single key, |
| or a path to key, where path elements are separated with a special character |
| (a '.' if not specified differently). For example debug.logging.errorlevel |
| might be a valid path with dot as a separator. |
| </p> |
| <div class="orderedlist"><ol class="orderedlist" type="1"> |
| <li class="listitem"> |
| The throwing version (<code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code>): |
| |
| <pre class="programlisting"><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">/* ... */</span> |
| <span class="keyword">float</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#id900429-bb">get</a></code><span class="special"><</span><span class="keyword">float</span><span class="special">>(</span><span class="string">"a.path.to.float.value"</span><span class="special">);</span> |
| </pre> |
| This call locates the proper node in the tree and tries to translate its |
| data string to a float value. If that fails, exception is thrown. If path |
| does not exist, it will be <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree_bad_path.html" title="Class ptree_bad_path">ptree_bad_path</a></code> |
| exception. If value could not be translated, it will be <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree_bad_data.html" title="Class ptree_bad_data">ptree_bad_data</a></code>. |
| Both of them derive from <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree_error.html" title="Class ptree_error">ptree_error</a></code> |
| to make common handling possible. |
| </li> |
| <li class="listitem"> |
| The default-value version (<code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code>): |
| |
| <pre class="programlisting"><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">/* ... */</span> |
| <span class="keyword">float</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#id900429-bb">get</a></code><span class="special">(</span><span class="string">"a.path.to.float.value"</span><span class="special">,</span> <span class="special">-</span><span class="number">1.f</span><span class="special">);</span> |
| </pre> |
| It will do the same as above, but if it fails, it will return the default |
| value specified by second parameter (here -1.f) instead of throwing. This |
| is very useful in common situations where one wants to allow omitting of |
| some keys. Note that type specification needed in throwing version is normally |
| not necessary here, because type is determined by the default value parameter. |
| </li> |
| <li class="listitem"> |
| The optional version (<code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900639-bb">get_optional</a></code>): |
| |
| <pre class="programlisting"><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">/* ... */</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">float</span><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#id900639-bb">get_optional</a></code><span class="special"><</span><span class="keyword">float</span><span class="special">>(</span><span class="string">"a.path.to.float.value"</span><span class="special">);</span> |
| </pre> |
| This version uses boost::optional class to handle extraction failure. On |
| successful extraction, it will return boost::optional initialized with |
| extracted value. Otherwise, it will return uninitialized boost::optional. |
| </li> |
| </ol></div> |
| <p> |
| To retrieve value from this tree (not some subkey), use <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id988473-bb">get_value</a></code>, |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id988473-bb">get_value</a></code> |
| (default-value version), and <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id931247-bb">get_value_optional</a></code>. |
| They have identical semantics to <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code> |
| functions, except they don't take the <code class="computeroutput"><a class="link" href="../boost/property_tree/path.html" title="Type definition path">path</a></code> |
| parameter. Don't call <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code> |
| with and empty <code class="computeroutput"><a class="link" href="../boost/property_tree/path.html" title="Type definition path">path</a></code> |
| to do this as it will try to extract contents of subkey with empty name. |
| </p> |
| <p> |
| To use separator character other than default '<code class="literal">.</code>', each |
| of the <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code> |
| versions has another form, which takes an additional parameter in front of |
| path. This parameter of type <code class="computeroutput"><span class="keyword">char</span><span class="special">/</span><span class="keyword">wchar_t</span></code> specifies |
| the separating character. This is a lifesaving device for those who may have |
| dots in their keys: |
| </p> |
| <pre class="programlisting"><span class="identifier">pt</span><span class="special">.</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">float</span><span class="special">>(</span><span class="char">'/'</span><span class="special">,</span> <span class="string">"p.a.t.h/t.o/v.a.l.u.e"</span><span class="special">);</span> |
| <span class="identifier">pt</span><span class="special">.</span><span class="identifier">get</span><span class="special">(</span><span class="char">'/'</span><span class="special">,</span> <span class="string">"p.a.t.h/t.o/v.a.l.u.e"</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">NULL</span><span class="special">);</span> |
| <span class="identifier">pt</span><span class="special">.</span><span class="identifier">get_optional</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="char">'/'</span><span class="special">,</span> <span class="string">"p.a.t.h/t.o/v.a.l.u.e"</span><span class="special">);</span> |
| </pre> |
| <a name="boost_propertytree.accessing.one_way_of_putting_data"></a><h4> |
| <a name="id2112741"></a> |
| <a class="link" href="accessing.html#boost_propertytree.accessing.one_way_of_putting_data">One Way |
| of Putting Data</a> |
| </h4> |
| <p> |
| To complement <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code>, |
| there is a <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| function. Contrary to <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code>, |
| it has only one variant. The reason is, there is no need to handle put failures |
| so often (normally, <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| will only fail if conversion of the supplied value to <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#boost.property_tree.basic_ptree.data_type">data_type</a></code> |
| fails or the system runs out of memory. In the former case, <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| will throw <code class="computeroutput"><a class="link" href="../boost/property_tree/ptree_bad_data.html" title="Class ptree_bad_data">ptree_bad_data</a></code>). |
| Sample usage of <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| might appear as: |
| </p> |
| <pre class="programlisting"><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="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">"a.path.to.float.value"</span><span class="special">,</span> <span class="number">3.14f</span><span class="special">);</span> |
| </pre> |
| <p> |
| Calling <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| will insert a new value at specified path, so that a call to <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900429-bb">get</a></code> |
| specifying the same path will retrieve it. Further, <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| will insert any missing path elements during path traversal. For example, calling |
| <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a><span class="special">(</span><span class="string">"key1.key2.key3"</span><span class="special">,</span> <span class="number">3.14f</span><span class="special">)</span></code> |
| on an empty tree will insert three new children: <code class="computeroutput"><span class="identifier">key1</span></code>, |
| <code class="computeroutput"><span class="identifier">key1</span><span class="special">.</span><span class="identifier">key2</span></code> and <code class="computeroutput"><span class="identifier">key1</span><span class="special">.</span><span class="identifier">key2</span><span class="special">.</span><span class="identifier">key3</span></code>. The last one will receive a string |
| <code class="computeroutput"><span class="string">"3.14"</span></code> as data, while |
| the two former ones will have empty data strings. <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| always inserts new keys at the back of the existing sequences. |
| </p> |
| <p> |
| Similar to <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id988473-bb">get_value</a></code>, |
| there is also a <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id900356-bb">put_value</a></code> |
| function. It does the same for this property tree what <code class="computeroutput"><a class="link" href="../boost/property_tree/basic_ptree.html#id960399-bb">put</a></code> |
| does for its children. Thus, it does not require <code class="computeroutput"><a class="link" href="../boost/property_tree/path.html" title="Type definition path">path</a></code>: |
| </p> |
| <pre class="programlisting"><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="identifier">pt</span><span class="special">.</span><span class="identifier">__ptree_put_own__</span><span class="special">(</span><span class="number">3.14f</span><span class="special">);</span> |
| </pre> |
| </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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