26.6 Conditionals

A conditional is a preprocessing directive that controls whether or not to include a chunk of code in the final token stream that is compiled. Preprocessing conditionals can test arithmetic expressions, or whether a name is defined as a macro, or both together using the special defined operator.

A preprocessing conditional in C resembles in some ways an if statement in C, but it is important to understand the difference between them. The condition in an if statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on. The condition in a preprocessing conditional directive is tested when your program is compiled. Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation.

Sometimes this distinction makes no practical difference. GCC and other modern compilers often do test if statements when a program is compiled, if their conditions are known not to vary at run time, and eliminate code that can never be executed. If you can count on your compiler to do this, you may find that your program is more readable if you use if statements with constant conditions (perhaps determined by macros). Of course, you can only use this to exclude code, not type definitions or other preprocessing directives, and you can only do it if the file remains syntactically valid when that code is not used.