Conditional Compilation

The Objective-C preprocessor offers a feature known as conditional compilation. Conditional compilation is often used to create one program that can be compiled to run on different computer systems. It is also often used to switch on or off various statements in the program, such as debugging statements that print the values of variables or trace the flow of program execution.

The #ifdef, #endif, #else, and #ifndef Statements

Unfortunately, a program sometimes must rely on system-dependent parameters that need to be specified differently on different processors (for example, Power PC versus Intel) or on a particular version of the operating system (for example, Tiger versus Leopard).