Boost Serialization minimal example with polymorphism, abstract base class, container, smart pointer, separate header and source files, custom save and load functions

Layout:

main.cc -- main()
util.cc -- saving to and loading from tmp file
base.cc -- abstract base class, split into header and source
derived_one.cc -- derived from Base
holder.cc -- holds a smart pointer, shared_ptr, to Base
filesystem_path_archive.hh -- custom serialization for std::filesystem::path

The sample is documented, see the files directly. It:

shows how to explicitly instantiate serialize() functions in an abstract base class implementation
shows that to serialize a container, e.g. std::shared_ptr or std::string, you need to include that container from boost serialization headers
shows how to serialize a normal class, e.g. Holder, including a derived class, e.g. DerivedOne, that is split into header and source
shows how to write custom serialize and deserialize functions for classes that cannot be edited, e.g. std::filesystem::path
shows that archivers, e.g. text_iarchive, text_oarchive, must be included with normal classes' source files, besides places where they are explicitly required by the user

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