The Preprocessor

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The Preprocessor
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The Preprocessor

• Preprocessing occurs before compilation
• Things that occur
• inclusion of other files
• defining constants and macros
• conditional compilation
• conditional execution of preprocessor directives

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Preprocessor (cont)

• All preprocessor directives begin with the
  sign
• Only whitespace characters may appear before this
• They do not end with a semicolon

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include

• Causes a copy of code to be included
• Two forms
• include ltsomething.hgt - something.h is found
  where standard libraries are found.
• include other.h - other.h can be found in
  current directory (of file being compiled) or
  where standard libs are found. Used when
  defining your own header files (.h files)

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define

• Used for defining constants
• Constant variables are IN CAPITAL LETTERS
• Example
• define JEFF Rox the World
• Preprocessor replaces all occurrences of JEFF
  with Rox the World
• define everything you can!!!!

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Macros

• Tiny little functions
• Used commonly in C (C uses inline functions)
• No data type checking in macros
• Example
• define SQUARE ( x ) ( x x )
• myNum SQUARE ( 9 ) // before replacement
• myNum ( 9 9 ) // after replacement

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Problem with Macros

• define SQUARE ( x ) ( x x )
• void main ( )
• int myNum 0
• int someNum 5
• myNum SQUARE ( someNum 7)
• // Whats wrong here?

myNum someNum 7 someNum 7
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Solution

• Use more parentheses
• define SQUARE ( x ) ( ( x ) ( x ) )
• Replacement is now
• myNum (someNum 7) (someNum 7)

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undef

• Used to un-define a constant or macro
• Why would you want to do this?
• Be careful if you undefine a constant or macro
  from another library!

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ifdef, ifndef, endif

• Allows programmer to control how the code is
  compiled
• Used to prevent repeatedly defining and including
  things (when you have multiple files)
• Used with endif

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Example

• include ltiostream.hgt
• ifndef JEFF
• define JEFF Rox the World
• endif

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Another Example

• include ltiostream.hgt
• define DEBUG 1
• // Define the DEBUG constant to be TRUE
• void main (void)
• if (DEBUG)
• cout ltlt DEBUG is WORKING! ltlt endl
• else
• cout ltlt DEBUG is not working ltlt endl

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Note about inline functionsThis isnt the
preprocessor stuff

• Inline functions are similar to macros
• Speeds up code (no function call)
• Text replacement (making executable bigger)
• inline functions can be ignored by compiler - but
  not macros
• inlines have type checking (but not macros)
• Example
• inline int square (int x) return x x

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