Editing Method overriding (section)

===Eiffel===

In [[Eiffel (programming language)|Eiffel]], '''feature redefinition''' is analogous to method overriding in C++ and Java. Redefinition is one of three forms of feature adaptation classified as '''redeclaration'''. Redeclaration also covers '''effecting''', in which an implementation is provided for a feature which was deferred (abstract) in the parent class, and '''undefinition''', in which a feature that was effective (concrete) in the parent becomes deferred again in the heir class. When a feature is redefined, the feature name is kept by the heir class, but properties of the feature such as its signature, contract (respecting restrictions for [[precondition]]s and [[postcondition]]s), and/or implementation will be different in the heir. If the original feature in the parent class, called the heir feature's '''precursor''', is effective, then the redefined feature in the heir will be effective. If the precursor is deferred, the feature in the heir will be deferred.<ref name="meyer">Meyer 2009, page 572-575</ref>

The intent to redefine a feature, as {{Eiffel|message}} in the example below, must be explicitly declared in the {{Eiffel|inherit}} clause of the heir class.

<syntaxhighlight lang="eiffel">
class
    THOUGHT
feature
    message
            -- Display thought message
        do
            print ("I feel like I am diagonally parked in a parallel universe.%N")
        end
end

class
    ADVICE
inherit
    THOUGHT
        redefine
            message
        end
feature
    message
            -- Precursor
        do
            print ("Warning: Dates in calendar are closer than they appear.%N")
        end
end
</syntaxhighlight>

In class {{Eiffel|ADVICE}} the feature {{Eiffel|message}} is given an implementation that differs from that of its precursor in class {{Eiffel|THOUGHT}}.

Consider a class which uses instances for both {{Eiffel|THOUGHT}} and {{Eiffel|ADVICE}}:

<syntaxhighlight lang="eiffel">
class
    APPLICATION
create
    make
feature 
    make
            -- Run application.
        do
            (create {THOUGHT}).message;
            (create {ADVICE}).message
        end
end
</syntaxhighlight>

When instantiated, class {{Eiffel|APPLICATION}} produces the following output:

<syntaxhighlight lang="output">
I feel like I am diagonally parked in a parallel universe.
Warning: Dates in calendar are closer than they appear.
</syntaxhighlight>

Within a redefined feature, access to the feature's precursor can be gained by using the language keyword {{Eiffel|Precursor}}. Assume the implementation of {{Eiffel|{ADVICE}.message}} is altered as follows:

<syntaxhighlight lang="eiffel">
    message
            -- Precursor
        do
            print ("Warning: Dates in calendar are closer than they appear.%N")
            Precursor
        end
</syntaxhighlight>

Invocation of the feature now includes the execution of {{Eiffel|{THOUGHT}.message}}, and produces the following output:

<syntaxhighlight lang="output">
Warning: Dates in calendar are closer than they appear.
I feel like I am diagonally parked in a parallel universe.
</syntaxhighlight>