Editing Constructor (object-oriented programming) (section)

=== Eiffel ===
In [[Eiffel (programming language)|Eiffel]], the routines which initialize new objects are called ''creation procedures''. Creation procedures have the following traits:

* Creation procedures have no explicit return type (by definition of ''procedure'').{{Efn|Eiffel ''routines'' are either ''procedures'' or ''functions''. Procedures never have a return type. Functions always have a return type.}}
* Creation procedures are named.
* Creation procedures are designated by name as creation procedures in the text of the class.
* Creation procedures can be explicitly invoked to re-initialize existing objects.
* Every effective (i.e., concrete or non-abstract) class must designate at least one creation procedure.
* Creation procedures must leave the newly initialized object in a state that satisfies the class invariant.{{Efn|Because the inherited class invariant must be satisfied, there is no mandatory call to the parents' constructors.}}

Although object creation involves some subtleties,<ref name="eiffel standard">{{Cite web|url=http://www.ecma-international.org/publications/standards/Ecma-367.htm|title=Eiffel ISO/ECMA specification document}}</ref> the creation of an attribute with a typical declaration <code lang="eiffel">x: T</code> as expressed in a creation instruction <code lang="eiffel">create x.make</code> consists of the following sequence of steps:

* Create a new direct instance of type <code lang="eiffel">T</code>.{{Efn|The Eiffel standard requires fields to be initialized on first access, so it is not necessary to perform default field initialization during object creation.}}
* Execute the creation procedure <code lang="eiffel">make</code> to the newly created instance.
* Attach the newly initialized object to the entity <code lang="eiffel">x</code>.

In the first snippet below, class <code lang="eiffel">POINT</code> is defined. The procedure <code lang="eiffel">make</code> is coded after the keyword <code lang="eiffel">feature</code>.

The keyword <code lang="eiffel">create</code> introduces a list of procedures which can be used to initialize instances. In this case the list includes <code lang="eiffel">default_create</code>, a procedure with an empty implementation inherited from class <code lang="eiffel">ANY</code>, and the <code lang="eiffel">make</code> procedure coded within the class.

<syntaxhighlight lang="eiffel">
class
    POINT
create
    default_create, make

feature

    make (a_x_value: REAL; a_y_value: REAL)
        do
            x := a_x_value
            y := a_y_value
        end

    x: REAL
            -- X coordinate

    y: REAL
            -- Y coordinate
        ...
</syntaxhighlight>

In the second snippet, a class which is a client to <code lang="eiffel">POINT</code> has a declarations <code lang="eiffel">my_point_1</code> and <code lang="eiffel">my_point_2</code> of type <code lang="eiffel">POINT</code>.

In procedural code, <code lang="eiffel">my_point_1</code> is created as the origin (0.0, 0.0). Because no creation procedure is specified, the procedure <code lang="eiffel">default_create</code> inherited from class <code lang="eiffel">ANY</code> is used. This line could have been coded <code lang="eiffel">create my_point_1.default_create</code> .
Only procedures named as creation procedures can be used in an instruction with the <code lang="eiffel">create</code> keyword.
Next is a creation instruction for <code lang="eiffel">my_point_2</code>, providing initial values for the <code lang="eiffel">my_point_2</code>'s coordinates.
The third instruction makes an ordinary instance call to the <code lang="eiffel">make</code> procedure to reinitialize the instance attached to <code lang="eiffel">my_point_2</code> with different values.

<syntaxhighlight lang="eiffel">
    my_point_1: POINT
    my_point_2: POINT
        ...

            create my_point_1
            create my_point_2.make (3.0, 4.0)
            my_point_2.make (5.0, 8.0)
        ...
</syntaxhighlight>