Objective-C
What it is
Objective-C is an object-oriented superset of C (Brad Cox, 1980s; NeXT → Apple) that added Smalltalk's object model to C: classes, messages, a dynamic runtime. Any C code is valid Obj-C code, so the language mixes naturally with C/C++ (Objective-C++) and low-level APIs (Core Foundation, POSIX). Until 2014 it was the only language of iOS/macOS; today UIKit, Foundation, Core Data and millions of lines of legacy are written in it.
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