Editing Generic programming (section)

====Genericity in Eiffel====
Generic classes have been a part of [[Eiffel (programming language)|Eiffel]] since the original method and language design. The foundation publications of Eiffel,<ref>''Object-Oriented Software Construction,'' Prentice Hall, 1988, and ''Object-Oriented Software Construction, second edition,'' Prentice Hall, 1997.</ref><ref>''Eiffel: The Language,'' Prentice Hall, 1991.</ref> use the term ''genericity'' to describe creating and using generic classes.

=====Basic, unconstrained genericity=====
Generic classes are declared with their class name and a list of one or more ''formal generic parameters''. In the following code, class <code lang=Eiffel>LIST</code> has one formal generic parameter <code lang=Eiffel>G</code>

<syntaxhighlight lang="eiffel">
class
    LIST [G]
            ...
feature   -- Access
    item: G
            -- The item currently pointed to by cursor
            ...
feature   -- Element change
    put (new_item: G)
            -- Add `new_item' at the end of the list
            ...
</syntaxhighlight>

The formal generic parameters are placeholders for arbitrary class names that will be supplied when a declaration of the generic class is made, as shown in the two ''generic derivations'' below, where <code lang=Eiffel>ACCOUNT</code> and <code lang=Eiffel>DEPOSIT</code> are other class names. <code lang=Eiffel>ACCOUNT</code> and <code lang=Eiffel>DEPOSIT</code> are considered ''actual generic parameters'' as they provide real class names to substitute for <code lang=Eiffel>G</code> in actual use.

<syntaxhighlight lang="eiffel">
    list_of_accounts: LIST [ACCOUNT]
            -- Account list

    list_of_deposits: LIST [DEPOSIT]
            -- Deposit list
</syntaxhighlight>

Within the Eiffel type system, although class <code lang=Eiffel>LIST [G]</code> is considered a class, it is not considered a type. However, a generic derivation of <code lang=Eiffel>LIST [G]</code> such as <code lang=Eiffel>LIST [ACCOUNT]</code> is considered a type.

=====Constrained genericity=====

For the list class shown above, an actual generic parameter substituting for <code lang=Eiffel>G</code> can be any other available class. To constrain the set of classes from which valid actual generic parameters can be chosen, a ''generic constraint'' can be specified. In the declaration of class <code lang=Eiffel>SORTED_LIST</code> below, the generic constraint dictates that any valid actual generic parameter will be a class that inherits from class <code lang=Eiffel>COMPARABLE</code>. The generic constraint ensures that elements of a <code lang=Eiffel>SORTED_LIST</code> can in fact be sorted.

<syntaxhighlight lang="eiffel">
class
    SORTED_LIST [G -> COMPARABLE]
</syntaxhighlight>