ABL allows you to define a class as a named block that always begins with the CLASS statement and always ends with the END CLASS statement. The source code for a CLASS statement can appear only in a class definition file (.cls file type), and a class definition file can contain only one such CLASS statement. This statement defines the class type name that any other class or procedure can use to reference the defined class and that any subclass can use to inherit from this class. (For more information on class definition files, see Class definition files and object type names.) This statement can also optionally identify the type name of a super class that the defined class inherits from, as well as one or more interfaces specifying member prototypes (properties, methods, or events) that the class must implement (see the ). The class can define itself as FINAL, which prevents it from being inherited by a subclass. The class can alternatively define itself as ABSTRACT, which requires that it be inherited by a subclass. Classes are always public, which means they are always accessible to other classes and procedures.

A class can contain similar kinds of definitions for data elements and executable code as in procedures. However, the data elements and executable code of a class is organized into strongly-typed class members, including data members, properties, methods, and events, that can be inherited from one class by another. If the class is abstract, it can also define any properties, methods, and events as abstract. Abstract class members must be implemented by a subclass that inherits them, similar to the members of an interface that a class implements.

The main block of a class can contain non-executable statements that define:

  • Any number of data members, including ProDataSets, data-sources, temp-tables, buffers, queries, and simple variables. (See Define data members.)
  • Any number of properties, which are class members similar to simple variables, but with associated behavior. (See Define properties.)
  • Any number of named methods, which are members that define class behavior. (See Define methods.)
  • Any number of class events, which are members that support behavior based on named methods or internal procedures that can be dynamically configured to execute in response to run-time conditions. (See Define class events.)
  • Any number of optional constructors, which are special methods that define initial behavior for a class. (See Define constructors.)
  • An optional destructor, a special method that defines final behavior for a class. (See Define the destructor.)
  • Any number of static definitions for class-scoped handle-based objects, including widgets, streams, and work-tables that provide associated ABL resources to the class, but are not themselves class members. (See Define class-scoped handle-based objects.)
  • Any number of ON statements, which specify ABL triggers for widget and other low-level handle-based object events managed as part of class behavior. (See the ON statement reference entry in ABL Reference.)
  • Any number of user-defined function prototypes (not the function definitions themselves) for user-defined functions called in the class. (See the FUNCTION statement reference entry in ABL Reference.)

The main block of a class cannot contain any executable statements that are outside of a method, constructor, destructor, or ABL trigger definition.

For information on the ABL to use a class type, see Use object types. For more information on defining classes, see Define classes.