CPP Preprocessor commands

Preprocessor and the Compiler

• Every time you run your compiler, your preprocessor runs first.
• The preprocessor looks for preprocessor instructions, each of which begins with the symbol #.
• The #define Preprocessor Directive.
  Syntax:

  #define macroName replacementString

  This causes the compiler to go through the file, replacing every occurrence of macroName with replacementString.
  Example:

  #define BIG 512
  int myArray[BIG]; // will replace BIG with 512 

• Using #define for Tests/Debug:

  #define DEBUG

  Later in your listing . . . .

  #if defined DEBUG
  cout << "Debug defined";
  #endif 

• Using #define for Macro function:

  #define TWICE(x) ( (x) * 2 )

  and then in your code you write . . . .

  TWICE(4)    // and the value ( (4) * 2 ) is substituted

• To avoid inclusion of header file more then once you can use inclusion guards:
  Example:

  #ifndef PERSON_HPP    // if this header is included (defined)
  #define PERSON_HPP    // define it and include all up to the #endif
  ...                   // the whole header file (*.hpp) goes here
  #endif

• The stringizing operator (#) puts quotes around any characters following the operator, up to the next whitespace.

  #define WRITESTRING(x) cout << #x

  and then in your code you write . . . .

  WRITESTRING(This is my string);
  // turns it into: cout << "This is my string";