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#### 4.4.4 Class and Keyword Inheritance

Class inheritance affects which methods are called in IDL. An object of a class which inherits methods from one or more superclasses can override that method by defining its own method of the same name, extend the method by calling the method(s) of its superclass(es) in its version, or inherit the method directly by making no modifications. IDLWAVE examines class definitions during completion and routine information display, and records all inheritance information it finds. This information is displayed if appropriate with the calling sequence for methods (see Routine Info), as long as variable `idlwave-support-inheritance` is non-`nil`.

In many class methods, keyword inheritance (`_EXTRA` and `_REF_EXTRA`) is used hand-in-hand with class inheritance and method overriding. E.g., in a `SetProperty` method, this technique allows a single call `obj->SetProperty` to set properties up the entire class inheritance chain. This is often referred to as chaining, and is characterized by chained method calls like `self->MySuperClass::SetProperty,_EXTRA=e` .

IDLWAVE can accommodate this special synergy between class and keyword inheritance: if `_EXTRA` or `_REF_EXTRA` is detected among a method's keyword parameters, all keywords of superclass versions of the method being considered can be included in completion. There is of course no guarantee that this type of keyword chaining actually occurs, but for some methods it's a very convenient assumption. The variable `idlwave-keyword-class-inheritance` can be used to configure which methods have keyword inheritance treated in this simple, class-driven way. By default, only `Init` and `(Get|Set)Property` are. The completion buffer will label keywords based on their originating class.

— User Option: idlwave-support-inheritance (`t`)

> Non-`nil` means consider inheritance during completion, online help etc.

— User Option: idlwave-keyword-class-inheritance

> A list of regular expressions to match methods for which simple class-driven keyword inheritance will be used for Completion.