<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69801775">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>&gt; +#include &lt;limits&gt;
&gt; +
&gt; +
&gt; +using namespace std;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace data {
&gt; +
&gt; +/**
&gt; + * Same as increment map policy, but does not change type of features.
&gt; + */
&gt; +class MissingPolicy
&gt; +{
&gt; + public:
&gt; +  // typedef of mapped_type
&gt; +  using mapped_type = double;
</pre>
<p>Also, I'm not sure I get the intention to use an alias here. Why not just use <code>double</code>?</p>

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