Your First Objective-C Program

Every Objective-C program built with Foundation begins by importing the Foundation umbrella header, #import <Foundation/Foundation.h>, which brings in NSString, NSLog, and the other core Foundation classes, followed by a standard C main function that serves as the program's entry point exactly as it does in plain C. The classic first program prints a greeting using NSLog(@"Hello, World!"); instead of C's printf, because NSLog understands Objective-C's %@ format specifier for objects and automatically writes timestamp and process information alongside the message.

The @autoreleasepool { } block wraps the body of main() to give Objective-C's autorelease mechanism a scope in which temporary objects, like those returned by many Foundation convenience methods, get cleaned up once the block exits, which matters even under ARC because ARC still relies on autorelease pools for certain temporary objects. Inside that block, ordinary Objective-C statements, variable declarations, method calls in bracket syntax, control flow identical to C, make up the actual logic of the program, and the function still ends with return 0; exactly like a C program.

With Foundation imported and Xcode's Command Line Tools installed, a file named hello.m compiles from the terminal with clang -framework Foundation hello.m -o hello, producing an executable binary named hello that can then be run directly with ./hello to print the greeting to the console. If clang reports an error about a missing framework, it usually means the Command Line Tools are missing or the -framework Foundation flag was omitted, since Foundation is not linked by default the way it is inside an Xcode project target.