podio silently skips over non-existant products

[tmadlener]

When getting a product from the EventStore, e.g. via store.get<edm4hep::TrackCollection>("tracks"), podio currently does not even issue any warning that the product is not available. Instead a default constructed object of the desired type is returned. The most trivial way this can happen is a simple typo. However, it might actually be the case that a user requires a product that is simply not present in the EventStore.

I think podio should at least print a warning statement that the desired product is not found. Arguably, it should be possible for the user to check whether the object is present in the EventStore or not. (I know this is in principle possible with bool EventStore::get(const std::string&, const T*&), but I think this should also be possible using the more ergonomic EventStore::get(const std::string&)). I just realized that this is actually easily possible by asking the returned collection whether it is valid via collection.isValid(). The question then becomes do we want to force the user to check whether the collection is valid or not?

This would be an excellent use case for optional, but that would require at least c++17, unless we want to pull in boost.

[andresailer]

We agreed that we can use c++17 for key4hep at least:
https://codimd.web.cern.ch/lkFym47kSdKGWqmPLCpf-w#Build-system--Target-compiler--dependencies