PHP: 对象继承

对象继承

继承已为大家所熟知的一个程序设计特性，PHP 的对象模型也使用了继承。继承将会影响到类与类，对象与对象之间的关系。

比如，当扩展一个类，子类就会继承父类所有 public 和 protected 的方法，属性和常量。除非子类覆盖了父类的方法，被继承的方法都会保留其原有功能。

继承有助于功能的设计和抽象，在实现类似的对象、增加新功能时，无须重复编写这些公用的功能。

子类无法访问父类的私有方法。因此，子类无需考虑正常的继承规则而重新实现私有方法。然而，在 PHP 8.0.0 之前， final 和 static 的限制会应用于 private 方法。从 PHP 8.0.0 开始，仅 private final 的构造器是唯一受限的 private 方法；想要“禁用”构造器，我们通常用静态工厂方法作为代替。

方法，属性和常量的可见性可以放宽，例如 protected 方法可以标记为 public，但不能增加限制，例如标记 public 属性为 private。有个例外是构造方法，可以限制其可见性，例如 public 构造方法可以在子类中标记为 private。

注意:
除非使用了自动加载，否则一个类必须在使用之前被定义。如果一个类扩展了另一个，则父类必须在子类之前被声明。此规则适用于类继承其它类与接口。

注意:
不允许使用只读属性覆盖可读写属性，反之亦然。

<?php class A { public int $prop; } class B extends A { // Illegal: read-write -> readonly public readonly int $prop; } ?>

示例 #1 继承示例

<?php

class Foo
{
    public function printItem($string)
    {
        echo 'Foo: ' . $string . PHP_EOL;
    }
    
    public function printPHP()
    {
        echo 'PHP is great.' . PHP_EOL;
    }
}

class Bar extends Foo
{
    public function printItem($string)
    {
        echo 'Bar: ' . $string . PHP_EOL;
    }
}

$foo = new Foo();
$bar = new Bar();
$foo->printItem('baz'); // 输出: 'Foo: baz'
$foo->printPHP();       // 输出: 'PHP is great' 
$bar->printItem('baz'); // 输出: 'Bar: baz'
$bar->printPHP();       // 输出: 'PHP is great'

?>

返回类型与内部类兼容

PHP 8.1 之前，大多数内部类或方法没有声明其返回类型，并且在继承它们时允许返回任何类型。

自 PHP 8.1.0 起，大多数内部方法开始“暂时”声明其返回类型，在这种情况下，方法的返回类型应该与继承的父级方法兼容；否则，将发出弃用通知。注意，没有指定返回声明也会视为签名不匹配，从而导致弃用通知。

如果由于 PHP 跨版本兼容性问题而无法为重写方法声明返回类型，则可以添加 ReturnTypeWillChange 注解来消除弃用通知。

示例 #2 The overriding method does not declare any return type

<?php
class MyDateTime extends DateTime
{
    public function modify(string $modifier) { return false; }
}

// "Deprecated: Return type of MyDateTime::modify(string $modifier) should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0
?>

示例 #3 The overriding method declares a wrong return type

<?php
class MyDateTime extends DateTime
{
    public function modify(string $modifier): ?DateTime { return null; }
}

// "Deprecated: Return type of MyDateTime::modify(string $modifier): ?DateTime should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0
?>

示例 #4 The overriding method declares a wrong return type without a deprecation notice

<?php
class MyDateTime extends DateTime
{
    /**
     * @return DateTime|false
     */
    #[\ReturnTypeWillChange]
    public function modify(string $modifier) { return false; }
}

// No notice is triggered
?>

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＋添加备注

用户贡献的备注 6 notes

down

214

jackdracona at msn dot com ¶

15 years ago

Here is some clarification about PHP inheritance – there is a lot of bad information on the net.  PHP does support Multi-level inheritance.  (I tested it using version 5.2.9).  It does not support multiple inheritance.
 
This means that you cannot have one class extend 2 other classes (see the extends keyword).  However, you can have one class extend another, which extends another, and so on. 
 
Example:
 
<?php
class A {
        // more code here
}
 
class B extends A {
        // more code here
}
 
class C extends B {
        // more code here
}
 
 
$someObj = new A();  // no problems
$someOtherObj = new B(); // no problems
$lastObj = new C(); // still no problems
 
?>

down

102

Mohammad Istanbouly ¶

8 years ago

I think the best way for beginners to understand inheritance is through a real example so here is a simple example I can gave to you 

<?php

class Person
{
    public $name;
    protected $age;
    private $phone;

    public function talk(){
        //Do stuff here
    }

    protected function walk(){
        //Do stuff here
    }

    private function swim(){
        //Do stuff here
    }
}

class Tom extends Person
{
    /*Since Tom class extends Person class this means 
        that class Tom is a child class and class person is 
        the parent class and child class will inherit all public 
        and protected members(properties and methods) from
        the parent class*/

     /*So class Tom will have these properties and methods*/

     //public $name;
     //protected $age;
     //public function talk(){}
     //protected function walk(){}

     //but it will not inherit the private members 
     //this is all what Object inheritance means
}

down

akashwebdev at gmail dot com ¶

10 years ago

The Idea that multiple inheritence is not supported is correct but with tratits this can be reviewed.

for e.g.
 
<?php
trait  custom
{
     public function hello()
     {
          echo "hello";
     }
}

trait custom2
{
       public function hello()
       {
            echo "hello2";
       }
}

class inheritsCustom
{
        use custom, custom2
        {
              custom2::hello insteadof custom;
        }
}

$obj = new inheritsCustom();
$obj->hello();
?>

down

jarrod at squarecrow dot com ¶

16 years ago

You can force a class to be strictly an inheritable class by using the "abstract" keyword. When you define a class with abstract, any attempt to instantiate a separate instance of it will result in a fatal error. This is useful for situations like a base class where it would be inherited by multiple child classes yet you want to restrict the ability to instantiate it by itself.

Example........

<?php

abstract class Cheese
{
      //can ONLY be inherited by another class
}

class Cheddar extends Cheese
{
}

$dinner = new Cheese; //fatal error
$lunch = new Cheddar; //works!

?>

down

strata_ranger at hotmail dot com ¶

15 years ago

I was recently extending a PEAR class when I encountered a situation where I wanted to call a constructor two levels up the class hierarchy, ignoring the immediate parent.  In such a case, you need to explicitly reference the class name using the :: operator.

Fortunately, just like using the 'parent' keyword PHP correctly recognizes that you are calling the function from a protected context inside the object's class hierarchy.

E.g:

<?php
class foo
{
  public function something()
  {
    echo __CLASS__; // foo
    var_dump($this);
  }
}

class foo_bar extends foo
{
  public function something()
  {
    echo __CLASS__; // foo_bar
    var_dump($this);
  }
}

class foo_bar_baz extends foo_bar
{
  public function something()
  {
    echo __CLASS__; // foo_bar_baz
    var_dump($this);
  }

  public function call()
  {
    echo self::something(); // self
    echo parent::something(); // parent
    echo foo::something(); // grandparent
  }
}

error_reporting(-1);

$obj = new foo_bar_baz();
$obj->call();

// Output similar to:
// foo_bar_baz
// object(foo_bar_baz)[1]
// foo_bar
// object(foo_bar_baz)[1]
// foo
// object(foo_bar_baz)[1]

?>

down

ignacio at inek dot com dot ar ¶

9 months ago

In case you have a public readonly property in a class you need to extend, adding other properties, this can be a way to do it:

<?php

class A {
    public function __construct(
        public readonly int $prop
    ) {}
}

class B extends A {
    public function __construct(
        int $prop,
        public readonly int $prop2
    ) {
        parent::__construct($prop);
    }
}
?>