cpp: Conditional Uses

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1 4.1 Conditional Uses
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1 There are three general reasons to use a conditional.
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1    * A program may need to use different code depending on the machine
1      or operating system it is to run on.  In some cases the code for
1      one operating system may be erroneous on another operating system;
1      for example, it might refer to data types or constants that do not
1      exist on the other system.  When this happens, it is not enough to
1      avoid executing the invalid code.  Its mere presence will cause the
1      compiler to reject the program.  With a preprocessing conditional,
1      the offending code can be effectively excised from the program when
1      it is not valid.
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1    * You may want to be able to compile the same source file into two
1      different programs.  One version might make frequent time-consuming
1      consistency checks on its intermediate data, or print the values of
1      those data for debugging, and the other not.
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1    * A conditional whose condition is always false is one way to exclude
1      code from the program but keep it as a sort of comment for future
1      reference.
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1    Simple programs that do not need system-specific logic or complex
1 debugging hooks generally will not need to use preprocessing
1 conditionals.
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