php - 扩展 PHP 中的单例类

Question

<?php
class LoveBase
{
    protected static $_instance = NULL;
    protected function __construct() {}
    public static function app()
    {
        if(self::$_instance == NULL) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }

    public function get()
    {
        return 'LoveBase';
    }

}

class Love extends LoveBase
{
    public static function app()
    {
        if(self::$_instance == NULL) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }
    public function get()
    {
        return 'Love';
    }
}

// Print "LoveLove" in this case(first case)
echo Love::app()->get(); 
echo LoveBase::app()->get();

// Print "LoveBaseLoveBase" in this case(second case)
// echo LoveBase::app()->get();
// echo Love::app()->get();

1. 为什么两种不同的方法得出相同的结果？

2. 比较这两种情况，当它的类首先实例化时，该方法将起作用。

（对不起，我英语不好，希望你能说得通）

Answer 1

您定义了两个静态函数，它们都使用相同的静态变量 ($_instance) - 基类的静态成员也可以通过子类访问（只要它不是私有的）。请记住，静态的东西（方法和变量）会被继承，但不会被克隆。

解决方案：将成员变量设为私有，并为每个类创建一个。

class LoveBase
{
    private static $_instance = NULL;
    // ...

class Love extends LoveBase
{
    private static $_instance = NULL;
    // ...

Answer 2

// Print "LoveLove" in this case(first case)

//Set self::$_instance to Love object id
echo Love::app()->get(); 

//Static property $_instance is now already set, so LoveBase::app() won't create new self(), it will just return created and saved Love object
echo LoveBase::app()->get();

// Print "LoveBaseLoveBase" in this case(second case)

// Here is the same case, but static property $_instance filled with new self() in LoveBase class
// echo LoveBase::app()->get();
// echo Love::app()->get();