I don't think this is the same. My C++ is getting rusty, so correct me if I'm wrong, but making a function const is mostly about making the "this" pointer const

I don't think this is the same. My C++ is getting rusty, so correct me if I'm wrong, but making a function const is mostly about making the "this" pointer const. A const function can still mutate global variables or call non-const functions, such as operator<< on cout. A pure D function can do neither of those things.