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HandlingXML.html
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HandlingXML.html
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<section id="handling-xml">
<h1>Handling XML<a class="headerlink" href="#handling-xml" title="Link to this heading">¶</a></h1>
<p>This page provides an introduction into reading/writing XML effectively
within Mantid. In Mantid, we use Poco to handling XML serialisation. A
useful introductory presentation can be found <a class="reference external" href="http://pocoproject.org/slides/170-XML.pdf">here.</a></p>
<p><strong>This page is a work in progress.</strong></p>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Link to this heading">¶</a></h2>
<section id="parsing">
<h3>Parsing<a class="headerlink" href="#parsing" title="Link to this heading">¶</a></h3>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf"><Poco/DOM/Document.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><Poco/DOM/DOMParser.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><Poco/DOM/Element.h></span>
<span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">DOMParser</span><span class="w"> </span><span class="n">pParser</span><span class="p">;</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">AutoPtr</span><span class="o"><</span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Document</span><span class="o">></span><span class="w"> </span><span class="n">pDoc</span><span class="p">;</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">pDoc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pParser</span><span class="p">.</span><span class="n">parseString</span><span class="p">(</span><span class="n">cuboidStr</span><span class="p">);</span>
<span class="p">}</span><span class="w"> </span><span class="k">catch</span><span class="w"> </span><span class="p">(...)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// Handle the failure as appropriate</span>
<span class="p">}</span>
<span class="c1">// Get pointer to root element</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Element</span><span class="w"> </span><span class="o">*</span><span class="n">pRootElem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pDoc</span><span class="o">-></span><span class="n">documentElement</span><span class="p">();</span>
</pre></div>
</div>
</section>
<section id="iterating-over-an-element-s-children">
<h3>Iterating over an element’s children<a class="headerlink" href="#iterating-over-an-element-s-children" title="Link to this heading">¶</a></h3>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Element</span><span class="w"> </span><span class="o">*</span><span class="n">pRootElem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pDoc</span><span class="o">-></span><span class="n">documentElement</span><span class="p">();</span>
<span class="c1">//Iterate over every child node (non-recursively)</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">Node</span><span class="w"> </span><span class="o">*</span><span class="n">pNode</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pRootElem</span><span class="o">-></span><span class="n">firstChild</span><span class="p">();</span><span class="w"> </span><span class="n">pNode</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">pNode</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pNode</span><span class="o">-></span><span class="n">nextSibling</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">pElem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">dynamic_cast</span><span class="o"><</span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Element</span><span class="o">*></span><span class="p">(</span><span class="n">pNode</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">pElem</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//We can now do something with this element safely</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</section>
<section id="inspecting-an-element">
<h3>Inspecting an element<a class="headerlink" href="#inspecting-an-element" title="Link to this heading">¶</a></h3>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Element</span><span class="w"> </span><span class="o">*</span><span class="n">pElem</span><span class="p">;</span>
<span class="c1">//Reasonably quick operations</span>
<span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">tag</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">tagName</span><span class="p">();</span><span class="w"> </span><span class="c1">//for <foo>bar</foo> the tag is 'foo'</span>
<span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">innerText</span><span class="p">();</span><span class="w"> </span><span class="c1">//for the above example: 'bar'</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Node</span><span class="w"> </span><span class="o">*</span><span class="n">pNext</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">nextSibling</span><span class="p">();</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Node</span><span class="w"> </span><span class="o">*</span><span class="n">pPrev</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">previousSibling</span><span class="p">();</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Node</span><span class="w"> </span><span class="o">*</span><span class="n">pChild</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">firstChild</span><span class="p">();</span><span class="w"> </span><span class="c1">//avoid lastChild, it's expensive</span>
</pre></div>
</div>
</section>
</section>
<section id="avoid-nodelist">
<h2>Avoid NodeList<a class="headerlink" href="#avoid-nodelist" title="Link to this heading">¶</a></h2>
<p>There are numerous functions that return a <code class="docutils literal notranslate"><span class="pre">Poco::XML::NodeList</span></code>. You
should avoid using them and the list they return as best you can.</p>
<p>NodeList is a very inefficient container. Its <code class="docutils literal notranslate"><span class="pre">item</span></code> method has a cost
equivalent to the value of <code class="docutils literal notranslate"><span class="pre">i</span></code> given to it, and its <code class="docutils literal notranslate"><span class="pre">length</span></code> method
a cost of <code class="docutils literal notranslate"><span class="pre">n</span></code>, where <code class="docutils literal notranslate"><span class="pre">n</span></code> is the number of nodes in the list.</p>
<p>This means that running the following is horrendously slow:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// NEVER DO THIS</span>
<span class="n">Poco</span><span class="o">::</span><span class="n">AutoPtr</span><span class="o"><</span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">NodeList</span><span class="o">></span><span class="w"> </span><span class="n">pElems</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElem</span><span class="o">-></span><span class="n">getElementsByTagName</span><span class="p">(</span><span class="s">"foo"</span><span class="p">);</span>
<span class="k">for</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">pElems</span><span class="o">-></span><span class="n">length</span><span class="p">();</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// length costs N, and is called N times (N² cost)</span>
<span class="w"> </span><span class="n">Poco</span><span class="o">::</span><span class="n">XML</span><span class="o">::</span><span class="n">Node</span><span class="o">*</span><span class="w"> </span><span class="n">pNode</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">pElems</span><span class="o">-></span><span class="n">item</span><span class="p">(</span><span class="n">i</span><span class="p">);</span><span class="w"> </span><span class="c1">// item costs at least i and is called N times, with i from 0 -> N-1 (N² + N cost)</span>
<span class="p">}</span>
<span class="c1">// NEVER DO THIS</span>
</pre></div>
</div>
<p>Even if the compiler is smart enough to optimise <code class="docutils literal notranslate"><span class="pre">pElems->length()</span></code> to
a single call, we still have N² + N performance at best. Instead, we
should <strong>always</strong> use the iteration example given earlier, as that runs
in N time, instead of N².</p>
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