Ruby Eigenclass

A presentation for a talk about Ruby eigenglass I gave during Ruby Talks at PGS Software. Ruby Talks are regular workshops of the Ruby team.

Main main source was Ruby Metaprogramming book: https://pragprog.com/book/ppmetr/metaprogramming-ruby

Transcript

1. RUBY EIGENCLASS EVERYTHING YOU WANTED TO KNOW BUT YOU WERE AFRAID TO ASK Created by Maciej Rząsa
2. SINGLETON METHODS Methods defined for a single instance of a class: c l a s s C a t d e f s o u n d " m e o w " e n d e n d C a t . n e w . s o u n d # > " m e o w " b o r e d _ c a t = C a t . n e w d e f b o r e d _ c a t . m o v e " s o l a z y . . . C a n ' t m o v e . . . " e n d b o r e d _ c a t . m o v e # > " S o l a z y . . . C a n ' t m o v e . . . "
3. DIFFERENT SYNTAX b o r e d _ c a t = C a t . n e w c l a s s < < b o r e d _ c a t d e f r u n " A r e y o u k i d d i n g m e ? " e n d e n d Familiar, isn't it?
4. CLASS METHODS Class methods are singleton methods of the class object c l a s s C a t c l a s s < < s e l f d e f n a m e " C a t " e n d e n d d e f s e l f . t a g " [ # { n a m e } ] " e n d d e f s o u n d " m e o w " e n d e n d
5. WHAT IS THE S E L F WE USED? e i g e n _ b o r e d = c l a s s < < b o r e d _ c a t s e l f e n d # > # < C l a s s : # < C a t : 0 x 0 0 0 0 0 0 0 3 e c 9 6 3 8 > > e i g e n _ c a t = c l a s s < < C a t s e l f e n d # > # < C l a s s : C a t > USEFUL SHORTHANDS e i g e n _ b o r e d = = b o r e d _ c a t . s i n g l e t o n _ c l a s s e i g e n _ c a t = = C a t . s i n g l e t o n _ c l a s s
6. EIGENCLASS aka singleton class, metaclass a class of a single object keeps method definitions for the object cannot be instantiated
7. SUPERCLASS OF EIGENCLASS c l a s s A n i m a l d e f s o u n d " a n y n o i s e " e n d e n d c l a s s C a t < A n i m a l d e f s o u n d " m e o w " e n d e n d b o r e d _ c a t = C a t . n e w b o r e d _ c a t . s i n g l e t o n _ c l a s s . s u p e r c l a s s # > C a t C a t . s i n g l e t o n _ c l a s s . s u p e r c l a s s # > < C l a s s : A n i m a l >
8. EIGENCLASS HIERARCHY
9. WHY SHOULD I CARE? understanding of Ruby object model and method dispatch metaprogramming: 'all about self' (Yehuda) private class methods
10. PRIVATE CLASS METHOD c l a s s C a t d e f s e l f . t a g n a m e e n d p r i v a t e d e f s e l f . n a m e " C a t " e n d e n d C a t . n a m e # > " C a t " c l a s s C a t c l a s s < < s e l f d e f t a g n a m e e n d p r i v a t e d e f n a m e " C a t " e n d e n d e n d C a t . n a m e # ! N o M e t h o d E r r o r : p r i v a t e m e t h o d ` n a m e ' c a l l e d f o r C a t : C l a s s
11. PRIVATE CLASS METHOD (2) c l a s s C a t d e f s e l f . t a g n a m e e n d d e f s e l f . n a m e " C a t " e n d p r i v a t e _ c l a s s _ m e t h o d : n a m e e n d C a t . n a m e # ! N o M e t h o d E r r o r : p r i v a t e m e t h o d ` n a m e ' c a l l e d f o r C a t : C l a s s
12. BTW: A BORED CAT source: https://www.flickr.com/photos/wackyvorlon/8761940187
13. SUMMARY Every object (even a class object) has its eigenclass that defines methods for this object.