cpp.info: Conditional Uses

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Why Conditionals are Used

   Generally there are three kinds of reason 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 library routines that do not exist
     on the other system.  When this happens, it is not enough to avoid
     executing the invalid code: merely having it in the program makes
     it impossible to link the program and run it.  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.  Sometimes the difference between the programs
     is that one makes frequent time-consuming consistency checks on its
     intermediate data, or prints the values of those data for
     debugging, while the other does not.
   * A conditional whose condition is always false is a good way to
     exclude code from the program but keep it as a sort of comment for
     future reference.
   Most simple programs that are intended to run on only one machine
will not need to use preprocessing conditionals.