02 An Introduction To Clascal Jul84

An Introduction to Clascal Susan Keohan nacintoshUser Education Apple Co.puter, Inc. Preface This is a conceptual introduction to ClascaL not a reference manual. Example programs and syntax diagrams follow the text. The purpose of this document is to introduce experienced Pascal programmers to the concepts necessary to make the transition from a traditional procedure-oriented environment to Clascal's . object-oriented environment. This document assumes you are familiar with Lisa Pascal and QuickDraw. All programs and program fragments are boldface in this document. for example, thisShape.Erase is a program fragment. Table of Contents Introduction ...... . Comparing Pascal and Clascal Pascal. Clascal Class Types. Objects Methods 1 1 ••2 ••4 . , ,. 8 ·.9 12 SELF. 16 Class Hierarchy and Inheritance Assignment Checking and Typecasting . . Creating and Freeing Objects StPERSELF and Extensibility ..... . Clascal vs. Pascal .....•.. Clascal and the Lisa Applications ToolKit. .hen to Use the Clascal Extensions Advanced Topics .... The Example Programs 17 20 21 22 25 26 26 27 30 .. J./~ AI7 InlrDtb:fion to CI6SC61 "1y198*, An Introduction to Clascal Introduction ellsoll is I set of extensions to Lisl Pasoal that adds objects and classes of objects to the language. The sesaantic extensions of elascal were inspired by the language s.Blltalk 76. The syntactic extensions were influenced by the language SillJII 67.. It is not necessary to know s.alltalk or Simula before learning elascal. elascal differs little fro. Pascal syntactically. The ..jor difference between the languages is in progr_ing technique. bn you write a Pascal proor., your procedure and fooction specifications are separate fro. your data structure specifications. When a Pasoal proor. is rCA data is passed to a procedure or function, and the procedure or function acts on the data. In elascal the data structures of an object and its procedures and functions'ar~ specified together 1n a class declaration. Ihen the program is rWl, objects are created. These objects use the functions and procedures that the program.er specified when defining the data structure. All the operations that an object eM perf0111 are defined by the object's class. As a result, progr.. modularity is i~roYed. In Pascal, to add new variations to an old data type, you ..st either define new types inc~atible rith the old type, or. edit existing code to add new cases to a variant record type and to procedures that act on that type. In ClasoaL you can define subclasses of existing classes without editing existing code Met without introducing incDq)atible types. As a result, progr_ extensibility is illproved. CCJIIparing Pasc.l and Clascal Clascal is a superset of Lisa Pascal. Because the readers of this doc\IIent are Pascal progr...ers, CI.scal oonoepts are CGIIPIred to Pascal concepts throughout. this doouIent. As a Pascal progrlllller, you should have little difficulty adjusting to ClesoaL because III)St of w…