(cpp.info.gz) Conditional Uses

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(cpp.info.gz) Conditionals

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 4.1 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.
 

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(cpp.info.gz) Conditionals

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