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Added non-conforming inheritance section

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Author:halw
Date:2010-01-14T17:09:43.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@394 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
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halw
2010-01-14 17:09:43 +00:00
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documentation/current/method/eiffel-tutorial-et/et-inheritance.wiki
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@@ -879,6 +879,31 @@ Likewise the compiler in EiffelStudio will produce warnings in cases in which ca
===Non-conforming inheritance===
So far, our experience with inheritance is that of "conforming" inheritance ... the most commonly used type of inheritance. Conforming inheritance is what allows a direct instance (in the '''catcall''' example above) of <code>COW</code> to be attached at runtime to an entity of type <code>ANIMAL</code>. This can be a powerful modeling capability, but it is this same polymorphism facilitated by conforming inheritance that puts us in the danger of using polymorphic '''catcalls'''.
In cases in which polymorphic attachment is not anticipated, the possibility of catcalls can be avoided by using '''non-conforming inheritance'''. Non-conforming inheritance is just a more restrictive form of inheritance. It allows features to be inherited from parent to heir, but it disallows polymorphic attachment of a direct instance of an heir to an entity based on a non-conforming parent.
In order to use non-conforming inheritance for a particular parent, we use the marker <code>{NONE}</code> in the appropriate inheritance part of the class:
<code>
class
MY_HEIR_CLASS
inherit
MY_CONFORMING_PARENT
inherit
{NONE} MY_NON_CONFORMING_PARENT
...
</code>
Here there are two inherit clauses, one to specify conforming parents, and one to specify non-conforming parents. The clause specifying the conforming inheritance must precede the one specifying the non-conforming inheritance.
So, in this case, at runtime it is valid for a direct instance of <code>MY_HEIR_CLASS</code> to be attached to an entity of type <code>MY_CONFORMING_PARENT</code>, but not to an entity of type <code>MY_NON_CONFORMING_PARENT</code>. Accordingly, the compiler would reject any code in which an instance of <code>MY_HEIR_CLASS</code> could become attached to an entity of type <code>MY_NON_CONFORMING_PARENT</code>. Because the polymorphic attachment cannot be made, the possibility of a catcall is avoided.