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Date:2008-09-19T07:54:43.000000Z git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@25 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
28 lines
1.9 KiB
Plaintext
28 lines
1.9 KiB
Plaintext
[[Property:title|9 Genericity]]
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[[Property:link_title|I2E: Genericity]]
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[[Property:weight|-6]]
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[[Property:uuid|091c0b65-73de-b454-b3f2-d8752983780e]]
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Building software components (classes) as implementations of abstract data types yields systems with a solid architecture but does not in itself ensure reusability and extendibility. Two key techniques address the problem: generosity (unconstrained or constrained) and inheritance. Let us look first at the unconstrained form.
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To make a class generic is to give it '''formal generic parameters''' representing as unknown types, as in these examples from EiffelBase, an open-source library covering basic data structures and algorithms:
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<code>ARRAY [G]
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LIST [G]
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LINKED_LIST [G]</code>
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These classes describe data structures -- arrays, lists without commitment to a specific representation, lists in linked representation -- containing objects of a certain type. The formal generic parameter <code> G </code> denotes this type.
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A class such as these doesn't quite yet describe a type, but a type template, since <code> G </code> itself denotes an unknown type. To derive a directly usable list or array type, you must provide a type corresponding to <code> G </code>, called an '''actual generic parameter'''; this may be either an expanded type, including basic types such as <code> INTEGER </code>, or a reference type. Here are some possible generic derivations:
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<code>il: LIST [INTEGER]
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aa: ARRAY [ACCOUNT]
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aal: LIST [ARRAY [ACCOUNT]]</code>
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As the last example indicates, an actual generic parameter may itself be generically derived.
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It would not be possible, without genericity, to have static type checking in a realistic object-oriented language.
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A variant of this mechanism, constrained genericity, will enable a class to place specific requirements on possible actual generic parameters. Constrained genericity will be described after inheritance.
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