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";
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