PHP类成员的访问方式和权限
PHP5的访问方式允许限制对类成员的访问. 这是在PHP5中新增的功能,但在许多面向对象语言中都早已存在. 有了访问方式,才能开发一个可靠的面向对象应用程序,并且构建可重用的面向对象类库. 像C++和Java一样,PHP有三种访问方式:public,private和protected. 对于一个类成员的访问方式,可以是其中之一. 如果你没有指明访问方式,默认地访问方式为public. 你也可以为静态成员指明一种访问方式,将访问方式放在static关键字之前(如public static). Public成员可以被毫无限制地访问.类外部的任何代码都可以读写public属性. 你可以从脚本的任何地方调用一个public方法. 在PHP的前几个版本中,所有方法和属性都是public, 这让人觉得对象就像是结构精巧的数组. Private(私有)成员只在类的内部可见. 你不能在一个private属性所在的类方法之外改变或读取它的值. 同样地,只有在同一个类中的方法可以调用一个private方法. 继承的子类也不能访问父类中的private 成员. 要注意,类中的任何成员和类的实例都可以访问private成员. 看例子6.8,equals方法将两个widget进行比较.==运算符比较同一个类的两个对象,但这个例子中每个对象实例都有唯一的ID.equals 方法只比较name和price. 注意equals方法如何访问另一个Widget实例的private属性. Java和C都允许这样的操作. <?php public function __construct($name, $price) //TRUE //FALSE //FALSE, == includes id in comparison 如果你对面向对象编程不熟悉,你可能想知道用private成员的目的是什么. 你可以回忆一下封装和耦合的想法,这在本章开头我们有讨论过. Private成员有助于封装数据. 他们可以隐藏在一个类内部而不被类外部的代码接触到. 同时他们还有助于实现松散的耦合. 如果数据结构外的代码不能直接访问内部属性,那么就不会产生一个隐性的关联性. 当然,大部分private属性仍然可以被外部代码共享. 解决方法是用一对public方法,一个是get(获取属性的值),另一个是set(设置属性的值). 构造函数也接受属性的初始值. 这使得成员间的交流通过一个狭窄的,经过良好限定的接口来进行. 这也提供改变传递给方法的值的机会. 注意在例子6.8中,构造函数如何强制使price成为一个float数(floadval()). Protected(受保护的) 成员能被同个类中的所有方法和继承出的类的中所有方法访问到. Public属性有违封装的精神,因为它们允许子类依赖于一个特定的属性来书写.protected方法则不会带来这方面的担忧.一个使用 protected方法的子类需要很清楚它的父类的结构才行. 注意Widget现在有一个叫作getName的protected方法. 如果Widget的实例试图调用protected方法将会出错: $w1->getName()产生了一个错误. 但子类Thing中的getName方法可以调用这个protected方法.当然对于证明Widget::getName方法是protected,这个例子显得过于简单. 在实际情况下,使用protected方法要依赖于对对象的内部结构的理解. public function __construct($name, $price) //checks if two widgets are the same protected function getName() class Thing extends Widget public function setColor($color) public function getColor() public function getName() $w1 = new Widget(Cog, 5.00); //TRUE (still!) 结果仍然为真 //print Cog 输出 Cog 一个子类可能改变通过覆写父类方法来改变方法的访问方式,尽管如此,仍然有一些限制. 如果你覆写了一个public类成员,他子类中必须保持public. 如果你覆写了一个protected成员,它可保持protected或变成public.Private成员仍然只在当前类中可见. 声明一个与父类的private成员同名的成员将简单地在当前类中建立一个与原来不同的成员. 因此,在技术上你不能覆写一个private成员. Final关键字是限制访问成员方法的另一个方法. 子类不能覆写父类中标识为final的方法. Final关键字不能用于属性.
class Widget
{
private $name;
private $price;
private $id;
{
$this->name = $name;
$this->price = floatval($price);
$this->id = uniqid();
}
//checks if two widgets are the same 检查两个widget是否相同
public function equals($widget)
{
return(($this->name == $widget->name)AND
($this->price == $widget->price));
}
}
$w1 = new Widget(Cog, 5.00);
$w2 = new Widget(Cog, 5.00);
$w3 = new Widget(Gear, 7.00);
if($w1->equals($w2))
{
print("w1 and w2 are the same
");
}
if($w1->equals($w3))
{
print("w1 and w3 are the same
");
}
if($w1 == $w2) //不等,因为ID不同
{
print("w1 and w2 are the same
");
}
?>
<?php
class Widget
{
private $name;
private $price;
private $id;
{
$this->name = $name;
$this->price = floatval($price);
$this->id = uniqid();
}
public function equals($widget)
{
return(($this->name == $widget->name)AND
($this->price == $widget->price));
}
{
return($this->name);
}
}
{
private $color;
{
$this->color = $color;
}
{
return($this->color);
}
{
return(parent::getName());
}
}
$w2 = new Thing(Cog, 5.00);
$w2->setColor(Yellow);
if($w1->equals($w2))
{
print("w1 and w2 are the same
");
}
print($w2->getName());
?>

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