<p>In <a href="https://github.com/mlpack/mlpack/pull/693#discussion_r66862099">src/mlpack/methods/neighbor_search/ns_model.hpp</a>:</p>
<pre style='color:#555'>> @@ -13,12 +13,24 @@
> #include <mlpack/core/tree/binary_space_tree.hpp>
> #include <mlpack/core/tree/cover_tree.hpp>
> #include <mlpack/core/tree/rectangle_tree.hpp>
> -
> +#include <boost/variant.hpp>
</pre>
<p>You're right, it is header only. I am not sure if it is possible to have a Boost installation where some of the header-only libraries (but not all) are available; I haven't played with b2 (Boost's build system) enough recently to be sure. If it's possible to have a Boost installation with some of the header-only libraries omitted, then we should include <code>variant</code> in there; otherwise, I don't think it's necessary. But I'm not sure whether or not that's possible...</p>
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