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<div class="title">Common pitfalls </div>  </div>
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<div class="textblock"><h1><a class="anchor" id="TopicPitfalls_template_keyword"></a>
Compilation error with template methods</h1>
<p>See this <a class="el" href="TopicTemplateKeyword.html">page </a>.</p>
<h1><a class="anchor" id="TopicPitfalls_auto_keyword"></a>
C++11 and the auto keyword</h1>
<p>In short: do not use the auto keywords with <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a>'s expressions, unless you are 100% sure about what you are doing. In particular, do not use the auto keyword as a replacement for a Matrix&lt;&gt; type. Here is an example:</p>
<div class="fragment"><div class="line"><a class="code" href="group__matrixtypedefs.html#ga99b41a69f0bf64eadb63a97f357ab412">MatrixXd</a> A, B;</div><div class="line"><span class="keyword">auto</span> C = A*B;</div><div class="line"><span class="keywordflow">for</span>(...) { ... w = C * v;  ...}</div></div><!-- fragment --><p>In this example, the type of C is not a MatrixXd but an abstract expression representing a matrix product and storing references to A and B. Therefore, the product of A*B will be carried out multiple times, once per iteration of the for loop. Moreover, if the coefficients of A or B change during the iteration, then C will evaluate to different values.</p>
<p>Here is another example leading to a segfault: </p><div class="fragment"><div class="line"><span class="keyword">auto</span> C = ((A+B).eval()).transpose();</div><div class="line"><span class="comment">// do something with C</span></div></div><!-- fragment --><p> The problem is that eval() returns a temporary object (in this case a MatrixXd) which is then referenced by the Transpose&lt;&gt; expression. However, this temporary is deleted right after the first line, and there the C expression reference a dead object. The same issue might occur when sub expressions are automatically evaluated by <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> as in the following example: </p><div class="fragment"><div class="line"><a class="code" href="group__matrixtypedefs.html#ga8554c6170729f01c7572574837ecf618">VectorXd</a> u, v;</div><div class="line"><span class="keyword">auto</span> C = u + (A*v).normalized();</div><div class="line"><span class="comment">// do something with C</span></div></div><!-- fragment --><p> where the normalized() method has to evaluate the expensive product A*v to avoid evaluating it twice. On the other hand, the following example is perfectly fine: </p><div class="fragment"><div class="line"><span class="keyword">auto</span> C = (u + (A*v).normalized()).eval();</div></div><!-- fragment --><p> In this case, C will be a regular VectorXd object. </p>
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