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Author:halw

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Date:2008-10-22T21:23:04.000000Z


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documentation/current/method/invitation-eiffel-i2e/i2e-genericity.wiki
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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.
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:
<code>ARRAY [G]
LIST [G]
LINKED_LIST [G]</code>
<code>
ARRAY [G]
LIST [G]
LINKED_LIST [G]</code>
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.
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.
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:
<code>il: LIST [INTEGER]
aa: ARRAY [ACCOUNT]
aal: LIST [ARRAY [ACCOUNT]]</code>
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:
<code>
il: LIST [INTEGER]
aa: ARRAY [ACCOUNT]
aal: LIST [ARRAY [ACCOUNT]]</code>
As the last example indicates, an actual generic parameter may itself be generically derived.