As I explained, the difference is that in one case you have one vtable per object in the vector, in the other case you have one vtable for the whole vector. With one vtable per object you can have one vector containing objects of two different types that implement the same trait with a different vtable. With one vtable for the whole vector you cannot. The same difference exists in many languages, e.g. C# List&lt;IFoo&gt; vs List&lt;T&gt; where T:IFoo.