[php classes and objects] overloading

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Overloading

Definition: Refers to dynamically "creating" class attributes and methods.

Implementation: Magic method.

Overloaded methods will be called when calling class attributes or methods that are undefined or invisible in the current environment.

All overloaded methods must be declared as public

The parameters of these magic methods cannot be passed by reference.

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Attribute overloading

public void __set ( string $name , mixed $value )
在给不可访问属性赋值时，__set() 会被调用。public mixed __get ( string $name )
读取不可访问属性的值时，__get() 会被调用.public bool __isset ( string $name )
当对不可访问属性调用 isset() 或 empty() 时，__isset() 会被调用。public void __unset ( string $name )
当对不可访问属性调用 unset() 时，__unset() 会被调用。

Parameter $name refers to the name of the variable to be operated. The $value parameter of the __set() method specifies the value of the $name variable.

Attribute overloading can only be done in objects. As of PHP 5.3.0, defining these magic methods as static produces a warning. In static methods, these magic methods will not be called. So none of these methods can be declared static.

【未懂】
在除 isset() 外的其它语言结构中无法使用重载的属性，这意味着当对一个重载的属性使用 empty() 时，重载魔术方法将不会被调用。

为避开此限制，必须将重载属性赋值到本地变量再使用 empty()。

Example #1 使用 __get()，__set()，__isset() 和 __unset() 进行属性重载

<?php
class PropertyTest {     /**  被重载的数据保存在此  */
    private $data = array();     /**  重载不能被用在已经定义的属性  */
    public $declared = 1;     /**  只有从类外部访问这个属性时，重载才会发生 */
    private $hidden = 2;    public function __set($name, $value) 
    {
        echo "Setting &#39;$name&#39; to &#39;$value&#39;\n";
        $this->data[$name] = $value;
    }    public function __get($name) 
    {
        echo "Getting &#39;$name&#39;\n";        if (array_key_exists($name, $this->data)) {            return $this->data[$name];
        }

        $trace = debug_backtrace();
        trigger_error(            &#39;Undefined property via __get(): &#39; . $name .            &#39; in &#39; . $trace[0][&#39;file&#39;] .            &#39; on line &#39; . $trace[0][&#39;line&#39;],
            E_USER_NOTICE);        return null;
    }    /**  PHP 5.1.0之后版本 */
    public function __isset($name) 
    {
        echo "Is &#39;$name&#39; set?\n";        return isset($this->data[$name]);
    }    /**  PHP 5.1.0之后版本 */
    public function __unset($name) 
    {
        echo "Unsetting &#39;$name&#39;\n";
        unset($this->data[$name]);
    }    /**  非魔术方法  */
    public function getHidden() 
    {        return $this->hidden;
    }
}


echo "<pre class="brush:php;toolbar:false">\n";

$obj = new PropertyTest;

$obj->a = 1;   //__set()：  Setting &#39;a&#39; to &#39;1&#39;echo $obj->a . "\n\n";     //__get()：  Getting &#39;a&#39;var_dump(isset($obj->a));  //__isset()：    Is &#39;a&#39; set? bool(true)unset($obj->a);        //__unset()：    Unsetting &#39;a&#39;var_dump(isset($obj->a));  //__isset()：    Is &#39;a&#39; set? bool(false)echo "\n";

echo $obj->declared . "\n\n";  //可访问，__get()不被调用：1echo "Let&#39;s experiment with the private property named &#39;hidden&#39;:\n";
echo "Privates are visible inside the class, so __get() not used...\n";
echo $obj->getHidden() . "\n"; //可访问，__get()不被调用：2echo "Privates not visible outside of class, so __get() is used...\n";
echo $obj->hidden . "\n";  //__get()：  Notice:  Undefined property via __get(): hidden in D:\phpStudy2018\PHPTutorial\WWW\index.php on line 75 in D:\phpStudy2018\PHPTutorial\WWW\index.php on line 31?>

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Method overloading

public mixed __call ( string $name , array $arguments )
在对象中调用一个不可访问方法时，__call() 会被调用。public static mixed __callStatic ( string $name , array $arguments )
在静态上下文中调用一个不可访问方法时，__callStatic() 会被调用。

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Introduction to PHP overloading

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