To call mmap, you are calling a function that is not in the collection of translation units that makes the program. Libraries are beyond the Standard. Include files not listed in the Standard, likewise. So you rely on Posix, there.

For objects got by casting void* to a known type, you rely on the compiler being unable to prove that the objects didn't exist already, somewhere in the program. Pray the compiler doesn't get smart enough to notice no constructor for that type is linked, meaning you that you couldn't have made that object.