Hello. `mongocxx::pool::entry` is designed to mimic `unique_ptr`. However it is created by a factory and therefor cannot be default constructed with a `nullptr` like how you would normally do with a `unique_ptr`.

This creates an awkward situation where you have to wrap the `entry` inside another unique_ptr to be able to separate declaration and definition (i.e. when using it as a class member). This means that you would need to have code that looks like this
`(**entry).list_databases():`

or
`(*entry)->list_databases():`

Adding an extra level of indirection as well as being hard to read.

I think the entry either should not mimic a `unique_ptr`, or if it does, then like a `unique_ptr` you should be able to default construct an invalid entry from a `nullptr` and move construct later on from what the pool factory gives you.