<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r66954641">src/mlpack/core/data/impute_strategies/custom_strategy.hpp</a>:</p>
<pre style='color:#555'>&gt; +#include &lt;mlpack/core.hpp&gt;
&gt; +
&gt; +
&gt; +using namespace std;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace data {
&gt; +
&gt; +class CustomStrategy
&gt; +{
&gt; +  // empty class
&gt; +};
&gt; +
&gt; +} // namespace data
&gt; +} // namespace mlpack
&gt; +
</pre>
<p>it is used for calling a overload of <a href="https://github.com/keonkim/mlpack/blob/5a517c25ef55de1f4814dc3605190d17f868ff82/src/mlpack/core/data/imputer.hpp#L86">Imputer::impute()</a>, which lets users to define a number that takes place of the missing value.</p>

<p>It basically does nothing but to fill the place of the template parameter.<br>
It is used like <a href="https://github.com/keonkim/mlpack/blob/5a517c25ef55de1f4814dc3605190d17f868ff82/src/mlpack/methods/preprocess/preprocess_imputer_main.cpp#L101">this</a>. but I was not clear how to make this clean.</p>

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