cpp.info: Conditional Uses

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Conditional Uses

   There are three general reasons to use a 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.