That involves a few diagrams, but essentially, we have two layers:

- the libcu++ layer, which has some of our extensions and implementations specific to our platform. - the libc++ layer, which is a modified upstream libc++.

A header in the libcu++ layer defines the libc++ internal macros in a certain way, and then includes the applicable libc++ header.

This is the current architecture, but we're moving away towards a more integrated approach where almost everything is in the libc++ layer.