Box_intersection_d: Can we make the Callback a const reference ?

[afabri]

Issue Details

In the package for fast box intersections in the API we write Callback callback. Is it acceptable to turn that into const Callback& callback? I would like to do that for performance reasons. I would also document it. This may imply changes on the users code. Even in the PMP package I have to make some data members of callbacks mutable, as the Callback::operator()(Box*,Box*) must be const. Alternatvely, we could switch to Callback& callback.

[lrineau]

Using Callback& callback will break code where the callback is not a l-value.

Using const Callback& will force the callback to be const, and will break user-code, including the examples in Box_intersection_d/examples. If there must be a redirection to a big object, then let it be in the callback object: callbacks should be lightweight objects, like property maps.