<p>Hi there, sorry for the slow response. This is a tricky issue...</p>
<p>mlpack uses template metaprogramming in <code>src/mlpack/core/data/serialization_shim.hpp</code> so that you can write a method with <code>Serialize()</code> instead of <code>serialize()</code> which meshes with the style guide (member functions should start with a capital letter). The details of how this is done is that the <code>data::CreateNVP()</code> function essentially wraps the given type <code>T</code> in a <code>FirstShim<T></code> object if <code>T::Serialize()</code> exists, and this <code>FirstShim<T></code> type then does the necessary wrangling to make the whole thing work.</p>
<p>So this means that for any class that has <code>Serialize()</code>, for it to be properly serialized, then <code>data::CreateNVP()</code> must be called to serialize it, and not <code>BOOST_SERIALIZATION_NVP()</code>. But the serialization code for <code>std::map<></code> uses <code>BOOST_SERIALIZATION_NVP()</code>, so a map that holds any type with <code>Serialize()</code> cannot be serialized using the <code>std::map<>::serialize()</code> implementation.</p>
<p>This means that there are two choices: manually implement <code>std::map<>::serialize()</code> (i.e. don't call <code>ar & data::CreateNVP(root, "root")</code> but instead do the implementation by hand), or rename your <code>Serialize()</code> function to <code>serialize()</code> and it should all work.</p>
<p>Sorry for the long explanation---the topic is complex. :) I hope this helps, and if I can clarify anything, let me know.</p>
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