What is this?

null pointer is required?

const?

Why const & here?

It is a so called forward declaration. We don't actually need to know the ins and outs of how this class looks like, just that it exists somewhere.

it's good practice to make sure the value is properly initialized. otherwise, it will contain random uninitialized memory garbage

yes, it's a getter and does not mutate the observable state of this object. thus it should be const

that is following the Qt coding guidelines, which says that you take nearly all (except small) values by const&. Not doing so would incur a copy operation, which may be expensive.

Why this instead of

#include <KActionCollection>

??

Understood, thanks.

See: http://stackoverflow.com/a/9906429