Lecture 19 CC Preprocessor

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Lecture 19C/C Preprocessor

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

12/07/01
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Outline

• Components of the build process
• Preprocessor
• Constants
• Conditional compilation
• Header files
• Parameterized macros

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Compiling a Program
Text File Editor
Preprocessor
Build a Program
Compiler
Linker
Executable
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Why a Preprocessor?

• The first C compilers had no constants
• Solution was to create a preprocessor that is run
  on the programs before they are passed to the C
  compiler
• Preprocessor is nothing more than a specialized
  text editor
• Preprocessor replaces defined constants or macros
  with actual code
• Conceptually, preprocessor is not part of the
  C/C compiler
• Preprocessor uses and entirely different systax
• Practically, the preprocessor may be included
  with the main compiler

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define Directive

• General Form
• define Name Substitute-Text
• These two lines perform the same
  functiondefine SIZE 20const int SIZE 20
• The preprocessor changes ALL instances of SIZE to
  the value 20 before sending the file to the
  compiler
• ALL preprocessor commands begin with in column
  1
• No semicolon at the end of a preprocessor line

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define Example
Before Preprocessor
After Preprocessor
define SIZE 20 void main() int
arraySIZE int i for(i0iltSIZEi)
arrayi 10
void main() int array20 int i
for(i0ilt20i) arrayi 10
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define versus const

• const is relatively new, so most older code
  uses define directives
• Using const is better
• Syntax checked immediately
• Uses C syntax instead of preprocessor syntax
• Follows normal C scope rules
• Can be used to define almost any type of C
  constant (including structure classes, etc.)

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Conditional Compilation

• Use the preprocessor to compile only certain
  sections of the code
• Easier than commenting out lines of code
• Debugging Code
• Print messages to help with debug
• When program is finished, turn off messages
• Portable Code
• C/C is not always portable
• The same code may not work on different types of
  machines
• UNIX and Windows

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Debugging Example

• define DEBUG / Turn on debugging /
• undef DEBUG / Turn off debugging /
• for (i0 ilt10 i)
• DataFile gtgt valueifdef DEBUG cout ltlt
  Value ltlt i ltlt is
• cout ltlt value ltlt endlendif

When DEBUG is defined, values will print to
console
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Debugging Example

• define DEBUG / Turn on debugging /
• undef DEBUG / Turn off debugging /
• ifdef DEBUG
• cout ltlt Test Version Debug is on\n
• else
• cout ltlt Production Version\n
• endif

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Portability Example

• define WIN32 / Windows application /
• ifdef WIN32
• Code for Windows OS
• else
• Code for UNIX OS
• endif

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include Directive

• include is actually a preprocessor command
• Examplesinclude ltiostream.hgtinclude
  rectangle.h
• Tells the preprocessor to find the iostream.h
  file and insert it in the current program
• ltgt indicates that the file is a standard header
  file
• indicates that the file is use-defined (local)

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Parameterized Macros

• We can define macros that take parameters
• Exampledefine SQR(x) ( (x) (x) )
• This may simplify the code and make it easier to
  read
• Would it be better to make the macro a function?

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Macro Example
Before Preprocessor
After Preprocessor
define SQR(x)((x)(x)) void main() int
i4 i SQR(i) cout ltlt i ltlt endl
void main() int i4 i ii cout ltlt
i ltlt endl
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Summary

• Components of the build process
• Preprocessor
• Constants
• Conditional compilation
• Header files
• Parameterized macros