Intrusive Types

template<typename I, typename ...Args>
class compressed_invocable_ref

Stores a “compressible” pointer to a std::invocable object.

In the general case, this class wraps a pointer to a std::invocable object. In the case where the invocable is also stateless, it provides a partial template specialization that is empty and points to a static inline instance of the invocable.

The intention is to allow users to store a pointer to an invocable by declaring a compressed_invocable_ref member. By further adding the [[no_unique_address]] attribute, the invocable will consume no additional storage when it points to a stateless invocable.

In many cases, e.g. intrusive lists, an iterator will need to access a member sub-object inside the item it currently points to. This means the iterator must also store a reference to the functor that accesses the intrusive link.

That functor is often stateless – for example, when accessing the item via a reinterpret_cast at a fixed offset (obtained through offsetof) that is baked into the functor type as a non-type template argument. Functors are usually stored in objects having a [[no_unique_address]] attribute, so that they do not require any storage.

However, a reference (or pointer) to the functor must still be passed into the iterator, so the iterator can use it. Reference semantics are needed because in the general case, the functor might be stateful (it depends on the template argument), so we might need the exact functor instance owned by the container. The problem is that even when the functor is empty, the reference is still pointer-sized. This is solved using this helper class, csg::compressed_invocable_ref, which has an explicit template specialization for stateless functors. Since all stateless instances are fungible, the specialization just returns references to a static inline instance of the functor, and the object itself does not store anything.

A compressed_invocable_ref is usually a member variable declared with [[no_unique_address]], so that it requires no storage when it is a reference to a stateless functor.