The subclass fails to inherit its parent class constructor

Nov 10, 2020
Knowledge

Title

URL Name

The-subclass-fails-to-inherit-its-parent-class-constructor

Article Number

000196325

Environment

Product:OpenEdge Version:11.x, 12.x OS: All supported platforms

Question/Problem Description

The subclass fails to inherit its parent class constructor

The application is trying to inherit a constructor from a 'Parent' super class, to a 'Child' sub class. But the inheritance isn't working properly.

The parent class has overloaded constructors and the compiler raises a syntax error, mismatched number of parameters. The code is as follows:

CLASS Parent:
    CONSTRUCTOR Parent ():
        DISP "Parent constructor with no parameter" THIS-OBJECT.
    END CONSTRUCTOR.

    CONSTRUCTOR Parent (i AS INT):
        DISP "Parent constructor with integer parameter" THIS-OBJECT.
    END CONSTRUCTOR.
END CLASS.

CLASS Child INHERITS Parent:
END CLASS.

// Test.p
DEF VAR p  AS Parent.
DEF VAR c1 AS Child.
DEF VAR c2 AS Child.

DISP "TEST: Child with inherited constructor" WITH FRAME a.
c1 = NEW Child().
DISP "TEST: Child with inherited constructor with parameter (Compile ERROR)" WITH FRAME b.
c2 = NEW Child(1).  // Raises error 2680

Steps to Reproduce

Clarifying Information

Error Message

Mismatched number of parameters supplied to 'Child'. Expecting 0 but 1 were specified. (2680)

Defect Number

Enhancement Number

Cause

This is expected behaviour. A subclass can inherit the overloaded methods of a super class, but it cannot inherit the overloaded constructors of a super class. In the OpenEdge Development: Develop Object-oriented ABL Applications manual it states:

"A class definition can overload its own methods and constructors. As a subclass, it can also inherit the overloaded methods (except the constructors) of a super class. It can also overload methods inherited from a super class, and it can override any existing overloadings of the methods that it inherits."

Resolution

Overload the constructor in the sub class:

CLASS Child INHERITS Parent:
    CONSTRUCTOR Child ():
      DISP "Child constructor" THIS-OBJECT.
    END CONSTRUCTOR.

    CONSTRUCTOR Child (i AS INT):
      DISP "Child constructor with integer parameter" THIS-OBJECT.
    END CONSTRUCTOR.
END CLASS.

Workaround

Notes

References to other documentation:
OpenEdge Development: Develop Object-oriented ABL Applications, Chapter 3: Design objects: inheritance, polymorphism, and delegation

Keyword Phrase

Last Modified Date

11/10/2020 3:17 PM

The subclass fails to inherit its parent class constructor

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