Run Time Environment. Implementing Object-Oriented Languages

Transcription

1 Run Time Environment Implementing Objet-Oriented Languages Copright 2017, Pedro C. Diniz, all rights reserved. Students enrolled in the Compilers lass at the Universit of Southern California have epliit permission to make opies of these materials for their personal use. 1

2 What Have We Learned? Reapitulation AR is a Run-Time struture to hold State regarding the eeution of a proedure AR an be alloated in Stati, Stak or even Heap Links to allow Call-Return and Aess to Non-loal Variables Smbol- plas important role Not Yet Done with Proedures Saving Contet before all and restoring after the all Need to understand how to generate ode for bod 2

3 Implementing Objet-Oriented Lang. Mapping message or names to methods Stati mapping, known at ompile-time (Java, C++) Fied offsets & indiret alls Dnami mapping, unknown until run-time (Smalltalk) Look up name in lass of methods Want uniform plaement of standard servies (NEW, PRINT, ) This is reall a Data-Strutures Problem Build a Table of Funtion Pointers Use a Standard Invoation Sequene 3

4 Implementing Objet-Oriented Lang. With stati, ompile-time mapped lasses Class A fee() fie() foe() fum() z i j k i j k i j k objets of lass A Class B band() bend() bind() bond() Message dispath beomes an indiret all through a funtion m n a b a b objets of lass B 4

5 Implementing Objet-Oriented Lang. Method Code: void A:fee(){ ->i = 0; Class A fee() fie() foe() i j k t0 = & + 4; *t0 = 0; fum() z i j i j k k objets of lass A Method Aesses Objet Data as Offsets from the Referene 5

6 Single Inheritane & Dnami Dispath Use prefiing of s Class Point Class Point { int, ; publi void draw(); publi void d2o(); draw d2o Point:draw Point:d20 Class ColorPoint draw ColorPoint:draw Class ColorPoint etends Point { Color ; publi void draw(); publi void rev(); d2o rev ColorPoint:rev 6

7 The Inheritane Hierarh To simplif objet reation, we allow a lass to inherit methods from an anestor, or superlass. The desendant lass is alled the sublass of its anestor. The Conept: a fee() fie() One Method is an etension of from One b fee() foe() fie() foe() fum() d e Two z Three Priniple: B sublass A Þ d Î B an be used wherever a Î A is epeted B has all the methods defined in its super lass B ma override a method definition from A Sublass provides all the interfaes of superlass 7

8 The Inheritane Hierarh Two distint Implementation Philosophies Stati Class Struture Can map name to ode at ompile time Leads to 1-level jump vetor Cop superlass methods Fied offsets & indiret alls Less fleible & epressive Dnami Class Struture Cannot map name to ode at ompile time Multiple jump vetor (1/lass) Must searh for method Run-time lookups ahing Muh more epensive to run Impat on name spae Method an see instane variables of, lass, & superlasses Man different levels where a value an reside In essene, OOL differs from ALL in the shape of its name spae AND in the mehanism used to bind names to implementations 8

9 Multiple Inheritane The Idea: Allow more fleible sharing of methods & attributes Rela the inlusion requirement If B is a sublass of A, it need not implement all of A s methods Need a linguisti mehanism for speifing partial inheritane Problems when C inherits from both A & B C s method an etend A or B, but not both Laout of an objet reord for C beomes trik Other lasses, sa D, an inherit from C & B Adjustments to offsets beome omple Sa, both A & B might provide fum() whih is seen in C? C++ produes a snta error when fum() is used 9

10 Multiple Inheritane Eample Use Prefiing of Storage Class Point { int, ; Class CThing { Color ; Class CPoint etends Point, CThing { Issue: does asting work properl? 10

11 Multiple Inheritane Eample Use Prefiing of Storage Class Point Class Point { int, ; void draw(); void d2o(); draw d2o Class CThing Point: draw Point: d2o Class CThing { Color ; void rev(); rev Class CPoint CThing: rev draw CPoint:draw Class Cpoint etends Point, CThing { void draw() d2o rev +=12 11

12 Casting with Multiple Inheritane Usage as Point: No etra ation (prefiing does everthing) Usage as CThing: Inrement b 12 Usage as CPoint: La out data for CThing at + 12 When alling rev Call in points to a trampoline funtion that adds 12 to, then alls rev Ensures that rev, whih assumes that points to a CThing data area, gets the right data 12

13 Multiple Inheritane (Eample) Assume that C inherits fee() from A, fie() from B, & defines both foe() and fum() A vars B vars C vars fee() fie() foe() fum() + offset This implementation Uses trampoline funtions Optimizes well with inlining Adds overhead where needed (Zero offsets go awa) Folds inheritane into data struture, rather than linkage fie() ode ode ode ode Trampoline funtion Assumes stati lass struture For dnami, wh not rebuild on a hange in struture? 13

14 Implementing Objet-Oriented Lang. So, what an an Eeuting Method see? The Objet s own Attributes & Private Variables The Attributes & Private Variables of Classes that define it Ma be man suh lasses, in man ombinations Class variables are visible to methods that inherit the lass Objet defined in the Global Name Spae (or sope) Objets ma ontain other objets Objets that ontain their definition A lass as an instane variable of another lass,... An Eeuting Method might referene an of these Making this work requires ompile-time elaboration for stati ase and run-time elaboration for dnami ase Making it run quikl takes are, planning, and triker 14

15 Summar Support for Objet-Oriented Languages: Mostl fous on message dispathing Finding the orret funtion in the Class Hierarh Adjusting Objet Laout and Class Referenes An Invoation Stak is still needed... 15