Questions about ranges::distance implementation

Question

I'm reading ranges::distance source and don't get the idea that:

• Why is the struct __distance_fn final?
• What is the purpose of void operator&() const = delete (source)

I know that a final class has no derived classes, and the unary & is to get a pointer (or something alike) of an object. But I can't comprehend what they really do here with ranges::distance.

Same case for ranges::advance/prev/next, while other functors don't have such details, e.g. ranges::begin.

• Why doesn't ranges::begin (and other functors) have final or deleted operator &?

Answer 1

ranges::next is a so-called niebloid, unlike customization point objects (CPO), they are not function objects.

To mimic their characteristics and prevent them from being misused as objects, this commit in libstdc++ added final and deleted oeprator& to them so that it is not possible to derive from them and take their addresses.

On the other hand, ranges::begin is specified as CPO in the standard which is a function object that can be copied freely.

There is also a paper P3136: Retiring niebloids that turns niebloids into CPOs to remove their annoying non-object nature.