Make clearer that this is expected behaviour ;-)
Add dtor.
Use consistent syntax.
Add RAII example.
Add list initialiser to vec.
Add clean task to vec make file.
Pedantically we really ought to static cast this.

It’ll be of type `initializer_list<double>::size_type` (I expect!)
which is not the same as `int`.
Use far more succinct (+ probably more performant) copy function.

Now matching the example from the book.
Add explanation for use of `static_cast<>`.