<p>In <a href="https://github.com/mlpack/mlpack/pull/683#discussion_r66455634">src/mlpack/methods/edge_boxes/feature_extraction_impl.hpp</a>:</p>
<pre style='color:#555'>&gt; + *
&gt; + * Implementation of feature extraction methods.
&gt; + */
&gt; +#ifndef MLPACK_METHODS_EDGE_BOXES_STRUCTURED_TREE_IMPL_HPP
&gt; +#define MLPACK_METHODS_EDGE_BOXES_STRUCTURED_TREE_IMPL_HPP
&gt; +
&gt; +
&gt; +#include &quot;feature_extraction.hpp&quot;
&gt; +#include &lt;map&gt;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace structured_tree {
&gt; +
&gt; +template&lt;typename MatType, typename CubeType&gt;
&gt; +StructuredForests&lt;MatType, CubeType&gt;::
&gt; +StructuredForests(const std::map&lt;std::string, int&gt;&amp; inMap)
</pre>
<p>Since you need to copy the inMap no matter what, maybe you could pass by value, this is more effecient(<a href="http://stackoverflow.com/questions/21605579/how-true-is-want-speed-pass-by-value">Want performance</a>? pass by value)</p>

<p><code>StructuredForests(std::map&lt;std::string, int&gt; inMap) : options(std::move(inMap)) {}</code></p>

<p>I use initialization list to initialize the data member.The class/struct of c++ do the initialization task at initialization list, not constructor. If you assign the inMap to options in constructor, it is "assign" but not "initialize".</p>

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