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26.6.1 Uses of Conditional Directives

There are three usual reasons to use a preprocessing conditional.

• A program may need to use different code depending on the machine or operating system it is to run on. In some cases the code for one operating system may be erroneous on another operating system; for example, it might refer to data types or constants that do not exist on the other system. When this happens, it is not enough to avoid executing the invalid code. Its mere presence will cause the compiler to reject the program. With a preprocessing conditional, the offending code can be effectively excised from the program when it is not valid.
• You may want to be able to compile the same source file into two different programs. One version might make frequent time-consuming consistency checks on its intermediate data, or print the values of those data for debugging, and the other not.
• A conditional whose condition is always false is one way to exclude code from the program but keep it as a sort of comment for future reference.

Simple programs that do not need system-specific logic or complex debugging hooks generally will not need to use preprocessing conditionals.