How to implement a hashmap of vectors of any type?

[vitalyd]

@ptodorov

What I don’t fully understand is why do I need a known size in that inner vector? From the book I gathered that a vec’s data is stored on the heap and the vec struct on the stack keeps a pointer to that data. And since it’s already on the heap why do I need an additional pointer (Box) to point to my actual data?

The additional pointer is to create a trait object, which provides the necessary type erasure to store heterogeneous types in an otherwise homogeneous container.

Your case sounds a bit like this recent thread, and you may be able to use the same approach of storing an entire Vec behind a fat ptr rather than each element in the vec.