I don't think it's a little known fact that std::map works that way, ie. that operator [] inserts if the key doesn't exist (or at least, it's surely not little

I don't think it's a little known fact that std::map works that way, ie. that operator [] inserts if the key doesn't exist (or at least, it's surely not little known among people using C++?). Also, I don't think you can have an infinitely expanding std::map in the same way, because all the key/value types have to get rolled into the type signature of the top level map. It's not that hard to write something yourself to achieve it, but it's not going to be the one-liner that it is in Python.