Detailed explanation of problems encountered in inheritance of PHP singleton mode

Detailed explanation of PHPSingle case modeInheritanceProblems encountered

<?php
// 单例模式之继承

class Singleton
{
	protected static $ins = null;

	private final function construct() { }

	protected final function clone() { }

	// public static function getIns() {
		// if(self::$ins === null){
			// self::$ins = new self();
		// }
		// return self::$ins;
	// }
	
	public static function getIns() {
		if(static::$ins === null){
			static::$ins = new static();
		}
		return static::$ins;
	}
}

class Child extends Singleton
{
	// protected static $ins = null;
}

/*
输出结果为：
bool(true) object(Singleton)#1 (0) { }
问题：对象 $c1, $c2 竟然都是 Singleton 的实例 ???
解决方法：将 getIns() 方法中关键字 self 替换为 static, 利用后期静态绑定的特性
*/
$c1 = Child::getIns();
$c2 = Child::getIns();
var_dump($c1 === $c2); //true
var_dump($c1);

// ------------------------------------------------------------------------
// 另一个问题

/*
输出结果为：
bool(true) object(Child)#1 (0) { }
问题：对象 $c3 竟然是 Child 的实例, 实际上应该是 Singleton 的实例 ???
原因：因为 $ins 属性是从父类 Singleton 继承过来的, 当第一次调用 Child::getIns() 时, $ins = new Child()
	  当再次调用 Singleton::getIns() 时, $ins 已经被实例过了, 而且指向 Child 的实例,
	  所以此时 $c3 变成了 Child 的实例
解决方法：在 Child 类中, 声明自己独有的 $ins 属性
*/
$c3 = Singleton::getIns();
var_dump($c1 === $c3);
var_dump($c3);

The getIns() method of late static binding There is another problem:

If the $ins property of Singleton is set to private, the subclass Child must set its own $ins property,

Because static::$ins first looks for the subclass's own $ins attribute, but since the subclass is not declared and the parent class cannot inherit it, Child::getIns() is called at this time

The method will report an error:

Fatal error: Cannot access property Child::$ins in D:\wamp\www\mycode\DesignPattern\Singleton.php on line 27

Solution:

#Set the $ins attribute of the parent class Singleton to protected or set the subclass Child's own $ins Attributes

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