Options.h Singleton Pattern

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Options.h(Singleton Pattern)

• By
• Kirit Patel
• James Vanderhyde

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Why use Singleton Patterns?

• Ensures a class has only one instance.
• Providing a global point of access to it.
• Sometimes we want to make sure that there is only
  one instance of a particular class/system. (i.e.
  file system, window manager,)
• Singleton was a nice person.

3
Options.h

• //This class is a Singleton. It contains exactly
• // one instance that is statically referenced.
• // It is useful for holding global information
• // without the dangers of global variables.
• class Options
• public // interfaces available to other
  classes
• //Returns a pointer to the singleton object
  of this class
• static Options instance (void)
• //Parses and stores options from main
• void parse_args (int argc, char argv)

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Options.h (Contd.)

• //Four options that may be on or off
• //These options are stored in octal order
• // so that we can use them as bitmasks!
• enum Option
• FOLD 01, //compare strings case insensitively
• NUMERIC 02, //compare string as if numbers
• REVERSE 04, //sort in reverse order (?)
• NORMAL 010 //compare strings case
  senstively
• //Pivot-selection strategy for quicksort
• enum Pivot_Strategy
• MEDIAN, //median of first, middle, and last
• RANDOM, //randomly choose a pivot
• FIRST //use first element

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Options.h (contd.)

• //Returns whether Option o is set in this
  singleton
• bool enabled (Option o)
• //Returns value of a global variable for
• // how far we have progressed through the
  input
• int field_offset (void) // Offset from BOL.
• //Returns the pivot strategy in this
  singleton
• Pivot_Strategy pivot_strat (void)
• //Function pointer for comparing two strings
• int (compare) (const char l, const char
  r)

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Options.h (Contd.)

• protected //interfaces available to this class
  and classes derived from it
• //The default constructor for this class
• //I don't know how it ensures a singleton.
• Options (void) // Ensure Singleton.
• //The place the maskable options are stored
• u_long options_ //Maintains options bitmask
  . . .
• //How far we have progresses through the
  input
• int field_offset_
• //The single instance of the class
• //Since it's static, it is referenced through
  the class
• static Options instance_ // Singleton.

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Options.H (Contd.)

• //This operator is used to compare two Line_Ptrs
  objects.
• int Line_Ptrsoperatorlt (const Line_Ptrs rhs)
• //Get a handy reference to the global
  information.
• Options options Optionsinstance ()
• //If we are under normal operation,
• if (options-gtenabled (OptionsNORMAL))
• //we compare with strcmp.
• return strcmp (this-gtbof_, rhs.bof_) lt 0
• //Otherwise, if we are under case-insensitive
  operation,
• else if (options-gtenabled (OptionsFOLD))
• //we compare with strcasecmp.
• return strcasecmp (this-gtbof_, rhs.bof_) lt 0

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Options.H (Final)

• //Otherwise, we must be under number comparing
  operation,
• else
• // assert (options-gtenabled (OptionsNUMERIC))
• //so we use numcp.
• return numcmp (this-gtbof_, rhs.bof_) lt 0