.CBL File Format – Object-oriented COBOL

COBOL source code

COBOL 2002 added classes, interfaces, methods and user-defined functions, drawing on the object COBOL already shipped by Micro Focus, IBM and Fujitsu. The source file shape changes: instead of four divisions around one program, a class file is an IDENTIFICATION DIVISION with CLASS-ID, then a FACTORY (shared) part and an OBJECT (per instance) part, each holding methods.

Class definition

       CLASS-ID. Account INHERITS Base.
       ENVIRONMENT DIVISION.
       CONFIGURATION SECTION.
       REPOSITORY.
           CLASS Base AS "java.lang.Object"
           CLASS Account AS "Account".
       FACTORY.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  open-count   PIC 9(5) COMP-5 VALUE 0.
       PROCEDURE DIVISION.
       METHOD-ID. NewAccount.
       DATA DIVISION.
       LINKAGE SECTION.
       01  result       OBJECT REFERENCE Account.
       PROCEDURE DIVISION RETURNING result.
           ADD 1 TO open-count
           INVOKE SELF "NEW" RETURNING result
       END METHOD NewAccount.
       END FACTORY.

       OBJECT.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  balance      PIC S9(9)V99 COMP-3 VALUE 0.
       PROCEDURE DIVISION.
       METHOD-ID. Deposit.
       DATA DIVISION.
       LINKAGE SECTION.
       01  amount       PIC S9(9)V99 COMP-3.
       PROCEDURE DIVISION USING amount.
           ADD amount TO balance
       END METHOD Deposit.
       END OBJECT.
       END CLASS Account.
ElementMeaning
CLASS-ID. name [AS "external"] [INHERITS parent…] [FINAL] [ABSTRACT] [USING type-params]Starts a class. The 2002 standard has single and multiple inheritance, FINAL (cannot be extended) and parameterised classes through USING. Compilers differ in which they support.
IMPLEMENTS interface…Names the interfaces the class provides.
FACTORY. … END FACTORY.Class-level data and methods (like static).
OBJECT. … END OBJECT.Instance data and methods.
METHOD-ID. name [AS "external"] [OVERRIDE] [FINAL] [PROPERTY | STATIC]A method; ends with END METHOD name.. Properties generate GET and SET accessors.
INTERFACE-ID. name [INHERITS …]A set of method signatures with no bodies; END INTERFACE.
REPOSITORY.Names the classes, interfaces and functions used, and optionally their external names (CLASS Foo AS "com.example.Foo").
OBJECT REFERENCE [class | interface]The usage of a variable holding an object.
SELF, SUPER, NULL, EXCEPTION-OBJECTThe receiving object; the parent class’s version of a method; no object; the object of the exception being handled.

Invoking

           INVOKE Account "NewAccount" RETURNING acct
           INVOKE acct "Deposit" USING 100.00
           INVOKE acct "GetBalance" RETURNING bal
           MOVE acct::"GetBalance" TO bal      *> inline invocation (IBM)
           SET acct TO NULL
  • INVOKE obj "method" USING … RETURNING … calls a method by name (a literal, or an item that holds the name). INVOKE class "NEW" creates an instance.
  • Arguments are passed by reference, by content or by value as for CALL; RETURNING names the result.
  • ON EXCEPTION / NOT ON EXCEPTION and END-INVOKE exist as for CALL.
  • The conditions obj IS INSTANCE OF class and obj IS NULL test an object.

User-defined functions and program types

  • FUNCTION-ID. name [AS "external"]. starts a function: a program whose PROCEDURE DIVISION USING a b RETURNING r has a result and which is called with FUNCTION name(a, b) from an expression. It ends with END FUNCTION name.
  • PROGRAM-ID. name [AS "external"] [IS] [COMMON | INITIAL | RECURSIVE | PROTOTYPE]. The program attributes: COMMON (a nested program callable by siblings), INITIAL (state reset at each call), RECURSIVE (may call itself), PROTOTYPE (a signature only).
  • ENTRY "name" USING … gives a program a second name with its own parameters (IBM, Micro Focus, GnuCOBOL).

Exceptions

The 2002 standard defines exception conditions (EC-SIZE, EC-BOUND, EC-I-O, EC-ARGUMENT…), checked after >>TURN … CHECKING ON and handled with USE AFTER EXCEPTION CONDITION declaratives, RAISE EXCEPTION and RESUME (see Statements). Object exceptions travel as objects of a class. Managed COBOL has TRY/CATCH/FINALLY statements, below.

COBOL for .NET and the JVM

Micro Focus Visual COBOL (and its predecessors Net Express and Server Express) compiles COBOL to .NET assemblies and JVM classes; Fujitsu NetCOBOL for .NET did the same for the CLR. The language adds, beyond the standard:

  • CLASS-ID. Name AS "Namespace.Name"; $set ilusing"System" or ILUSING/JVMLANG directives to import namespaces.
  • 01 s TYPE String (or a framework class name) as the declaration of a typed variable; DECLARE s AS TYPE String, and inline declaration in the procedure division.
  • TRY … CATCH ex AS TYPE Exception … FINALLY … END-TRY and RAISE / THROW.
  • Method call with parenthesis: invoke s::ToUpper(), declare n = s::Length, properties by name.
  • Generics, delegates, lambdas, attributes, PERFORM over FOR EACH / PERFORM VARYING … OF collection forms, operator overloading, using statements.
       CLASS-ID. Hello.
       METHOD-ID. Main STATIC.
       PROCEDURE DIVISION.
           DECLARE name AS STRING = "World"
           TRY
               DISPLAY "Hello, " & name::ToUpper()
           CATCH ex AS TYPE Exception
               DISPLAY ex::Message
           END-TRY
       END METHOD Main.
       END CLASS Hello.

This syntax is documented by the compilers’ vendors (Micro Focus Visual COBOL language reference); I could not read it, so the list is from memory of that product and may be incomplete. The viewer colours TRY, CATCH, FINALLY, THROW, DECLARE and END-TRY as keywords; other managed-only words (AS forms of generics, lambdas) get no special colour.