[mlpack-git] [mlpack/mlpack] Serialize a class with "map<int, Node*>" (#811)

[Ryan Curtin]

Hi there, sorry for the slow response.  This is a tricky issue...

mlpack uses template metaprogramming in `src/mlpack/core/data/serialization_shim.hpp` so that you can write a method with `Serialize()` instead of `serialize()` which meshes with the style guide (member functions should start with a capital letter).  The details of how this is done is that the `data::CreateNVP()` function essentially wraps the given type `T` in a `FirstShim<T>` object if `T::Serialize()` exists, and this `FirstShim<T>` type then does the necessary wrangling to make the whole thing work.

So this means that for any class that has `Serialize()`, for it to be properly serialized, then `data::CreateNVP()` must be called to serialize it, and not `BOOST_SERIALIZATION_NVP()`.  But the serialization code for `std::map<>` uses `BOOST_SERIALIZATION_NVP()`, so a map that holds any type with `Serialize()` cannot be serialized using the `std::map<>::serialize()` implementation.

This means that there are two choices: manually implement `std::map<>::serialize()` (i.e. don't call `ar & data::CreateNVP(root, "root")` but instead do the implementation by hand), or rename your `Serialize()` function to `serialize()` and it should all work.

Sorry for the long explanation---the topic is complex. :)  I hope this helps, and if I can clarify anything, let me know.

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