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Formalize wording in variance page, remove any wording that speaks to the audience

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git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@349390 c90b9560-bf6c-de11-be94-00142212c4b1
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Tiffany Taylor 2020-03-12 14:36:14 +00:00
parent 55640d87ac
commit 62be58faa7
1 changed files with 24 additions and 31 deletions
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55
language/oop5/variance.xml
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@ -4,29 +4,23 @@
<title>Covariance and Contravariance</title>
<para>
Prior to PHP 7.4.0, class methods support mainly invariant parameter types and invariant return types.
This means that if a method within a parent class has a parameter type or return
type of <varname>T</varname>, any child method with corresponding parameter type
or return type <emphasis>must also</emphasis> be type <varname>T</varname>.
In PHP 7.2.0, partial contravariance was introduced by removing type restrictions
on parameters in a child method. As of PHP 7.4.0, full covariance and contravariance
support was added.
</para>
<para>
As of PHP 7.4.0, covariance and contravariance is supported. While these concepts
may <emphasis>sound</emphasis> confusing, in practice they're rather simple, and
extremely useful to object-oriented programming.
Covariance allows a child's method to return a more specific type than the return type
of its parent's method. Whereas, contravariance allows a parameter type to be less
specific in a child method, than that of its parent.
</para>
<sect2 xml:id="language.oop5.variance.covariance">
<title>Covariance</title>
<para>
Covariance allows a child's method to return a more specific type than the return
type of its parent's method. This is better illustrated with an example.
</para>
<para>
We'll start with a simple abstract parent class, <varname>Animal</varname>, which is
extended by children classes, <varname>Cat</varname>, and <varname>Dog</varname>.
To illustrate how covariance works, a simple abstract parent class, <varname>Animal</varname>
is created. <varname>Animal</varname> will be extended by children classes,
<varname>Cat</varname>, and <varname>Dog</varname>.
</para>
<informalexample>
@ -66,10 +60,9 @@ class Cat extends Animal
</informalexample>
<para>
Note that there aren't any methods which return values in this example. We will
build upon these classes with a few factories which return a new object of class type
<varname>Animal</varname>, <varname>Cat</varname>, or <varname>Dog</varname>.
Covariance will come into play in the next example.
Note that there aren't any methods which return values in this example. A few factories
will be added which return a new object of class type <varname>Animal</varname>,
<varname>Cat</varname>, or <varname>Dog</varname>.
</para>
<informalexample>
@ -120,11 +113,11 @@ Mavrick barks
<title>Contravariance</title>
<para>
Contravariance, on the other hand, allows a parameter type to be less specific in a child
method, than that of its parent. Continuing with our previous example with the classes
<varname>Animal</varname>, <varname>Cat</varname>, and <varname>Dog</varname>, we're adding
a class called <varname>Food</varname> and <varname>AnimalFood</varname>, and adding a method
<varname>eat(AnimalFood $food)</varname> to our <varname>Animal</varname> abstract class.
Continuing with the previous example with the classes <varname>Animal</varname>,
<varname>Cat</varname>, and <varname>Dog</varname>, a class called
<varname>Food</varname> and <varname>AnimalFood</varname> will be included, and
a method <varname>eat(AnimalFood $food)</varname> is added to the <varname>Animal</varname>
abstract class.
</para>
<informalexample>
@ -147,7 +140,7 @@ abstract class Animal
public function eat(AnimalFood $food)
{
echo $this->name . " nom noms " . get_class($food);
echo $this->name . " eats " . get_class($food);
}
}
]]>
@ -155,9 +148,9 @@ abstract class Animal
</informalexample>
<para>
In order to see the behavior of contravariance, we will override the
<varname>eat</varname> method in the <varname>Dog</varname> class to allow any
<varname>Food</varname> type object. The <varname>Cat</varname> class remains unchanged.
In order to see the behavior of contravariance, the
<varname>eat</varname> method is overridden in the <varname>Dog</varname> class to allow
any <varname>Food</varname> type object. The <varname>Cat</varname> class remains unchanged.
</para>
<informalexample>
@ -176,7 +169,7 @@ class Dog extends Animal
</informalexample>
<para>
Now, we can see how contravariance works.
The next example will show the behavior of contravariance.
</para>
<informalexample>
@ -197,8 +190,8 @@ $doggy->eat($banana);
&example.outputs;
<screen>
<![CDATA[
Ricky nom noms AnimalFood
Mavrick nom noms Food
Ricky eats AnimalFood
Mavrick eats Food
]]>
</screen>