They are separate compilers. Obj-C is compiled by clang, Swift by the Swift compiler. You enable bridging in Xcode and then get access to your Obj-C code in Swi

They are separate compilers. Obj-C is compiled by clang, Swift by the Swift compiler. You enable bridging in Xcode and then get access to your Obj-C code in Swift through the use of a bridging header, and Swift in Obj-C through the use of the *-Swift.h generated header. You can write classes in Swift that are visible to Obj-C and vice versa. You can't expose everything from Swift to Obj-C since Obj-C is missing most of Swift capabilities, but it's easy to call back and forth.