<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69908461">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>> +#include <limits>
> +
> +
> +using namespace std;
> +
> +namespace mlpack {
> +namespace data {
> +
> +/**
> + * Same as increment map policy, but does not change type of features.
> + */
> +class MissingPolicy
> +{
> + public:
> + // typedef of mapped_type
> + using mapped_type = double;
</pre>
<p>But a problem here is that the actual mapped type is specified by the input type to <code>data::Load()</code>, which takes matrices of any type. So even if <code>mapped_type</code> (which should be named <code>MappedType</code> by the way) is <code>double</code>, if the user passed an <code>arma::Mat<size_t></code> to <code>data::Load()</code>, any NaNs that were mapped will be converted to something else when the <code>double</code> is converted to a <code>size_t</code>.</p>
<p>So I think that you will have to take the type as given by <code>data::Load()</code>, and maybe then you can use a <code>static_assert()</code> to check that the given type has what you need (like for <code>MissingPolicy</code>, you need some kind of NaN).</p>
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