PHP面向对象中的重要知识点(一)

1. __construct: 

     内置构造函数，在对象被创建时自动调用。见如下代码：

<?php
class ConstructTest {
    private $arg1;
    private $arg2;

    public function __construct($arg1, $arg2) {
        $this->arg1 = $arg1;
        $this->arg2 = $arg2;
        print "__construct is called...
";
    }
    public function printAttributes() {
        print '$arg1 = '.$this->arg1.' $arg2 = '.$this->arg2."
";
    }
}
$testObject = new ConstructTest("arg1","arg2"); 
$testObject->printAttributes();

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
__construct is called...
$arg1 = arg1 $arg2 = arg2

2. parent: 

     用于在子类中直接调用父类中的方法，功能等同于Java中的super。 

<?php
class BaseClass {
    protected $arg1;
    protected $arg2;

    function __construct($arg1, $arg2) {
        $this->arg1 = $arg1;
        $this->arg2 = $arg2;
        print "__construct is called...
";
    }
    function getAttributes() {
        return '$arg1 = '.$this->arg1.' $arg2 = '.$this->arg2;
    }
}

class SubClass extends BaseClass {
    protected $arg3;

    function __construct($baseArg1, $baseArg2, $subArg3) {
        parent::__construct($baseArg1, $baseArg2);
        $this->arg3 = $subArg3;
    }
    function getAttributes() {
        return parent::getAttributes().' $arg3 = '.$this->arg3;
    }
}
$testObject = new SubClass("arg1","arg2","arg3"); 
print $testObject->getAttributes()."
";

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
__construct is called...
$arg1 = arg1 $arg2 = arg2 $arg3 = arg3

3. self:

     在类内调用该类静态成员和静态方法的前缀修饰，对于非静态成员变量和函数则使用this。 

<?php
class StaticExample {
    static public $arg1 = "Hello, This is static field.
";
    static public function sayHello() {
        print self::$arg1;
    }
}

print StaticExample::$arg1;
StaticExample::sayHello();

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
Hello, This is static field.
Hello, This is static field.

4. static:

     这里介绍的static关键字主要用于PHP 5.3以上版本新增的延迟静态绑定功能。请看一下代码和关键性注释。

<?php
abstract class Base {
    public static function getInstance() {
        //这里的new static()实例化的是调用该静态方法的当前类。
        return new static();
    }
    abstract public function printSelf(); 
}

class SubA extends Base {
    public function printSelf() {
        print "This is SubA::printSelf.
";
    }
}

class SubB extends Base {
    public function printSelf() {
        print "This is SubB::printSelf.
";
    }
}

SubA::getInstance()->printSelf();
SubB::getInstance()->printSelf();

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
This is SubA::printSelf.
This is SubB::printSelf.

     static关键字不仅仅可以用于实例化。和self和parent一样，static还可以作为静态方法调用的标识符，甚至是从非静态上下文中调用。在该场景下，self仍然表示的是当前方法所在的类。见如下代码： 

<?php
abstract class Base {
    private $ownedGroup;
    public function __construct() {
        //这里的static和上面的例子一样，表示当前调用该方法的实际类。
        //需要另外说明的是，这里的getGroup方法即便不是静态方法，也会得到相同的结果。然而倘若
        //getGroup真的只是普通类方法，那么这里还是建议使用$this。
        $this->ownedGroup = static::getGroup();
    }
    public function printGroup() {
        print "My Group is ".$this->ownedGroup."
";
    }
    public static function getInstance() {
        return new static();
    }
    public static function getGroup() {
        return "default";
    }
}

class SubA extends Base {
}

class SubB extends Base {
    public static function getGroup() {
        return "SubB";
    }
}

SubA::getInstance()->printGroup();
SubB::getInstance()->printGroup(); 

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
My Group is default
My Group is SubB

5. __destruct:

     析构方法的作用和构造方法__construct刚好相反，它只是在对象被垃圾收集器收集之前自动调用，我们可以利用该方法做一些必要的清理工作。

<?php
class TestClass {
    function __destruct() {
        print "TestClass destructor is called.
";
    }
}

$testObj = new TestClass();
unset($testObj);
print "Application will exit.
";

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
TestClass destructor is called.
Application will exit.

6. __clone:

     在PHP 5之后的版本中，对象之间的赋值为引用赋值，即赋值后的两个对象将指向同一地址空间，如果想基于对象赋值，可以使用PHP提供的clone方法。该方法将当前对象浅拷贝之后的副本返回，如果想在clone的过程中完成一些特殊的操作，如深拷贝，则需要在当前类的声明中实现__clone方法，该方法在执行clone的过程中会被隐式调用。另外需要格外注意的是，__clone方法是作用再被拷贝的对象上，即赋值后的对象上执行。

<?php
class InnerClass {
    public $id = 10;
    public function printSelf() {
        print '$id = '.$this->id."
";
    }
}

class OuterClass {
    public $innerClass;
    public function __construct() {
        $this->innerClass = new InnerClass();
    }
    public function __clone() {
        $this->innerClass = clone $this->innerClass;
        print "__clone is called.
";
    }
}

$outerA = new OuterClass();
print "Before calling to clone.
";
$outerB = clone $outerA;
print "After calling to clone.
";
$outerA->innerClass->id = 20;
print "In outerA: ";
$outerA->innerClass->printSelf();
print "In outerB: ";
$outerB->innerClass->printSelf();

     运行结果如下：

Stephens-Air:Desktop$ php Test.php 
Before calling to clone.
__clone is called.
After calling to clone.
In outerA: $id = 20
In outerB: $id = 10

7. const:

    PHP5可以在类中定义常量属性。和全局常量一样，一旦定义就不能改变。常量属性不需要像普通属性那样以$开头，按照惯例，只能用大写字母来命名常量。另外和静态属性一样，只能通过类而不能通过类的实例访问常量属性，引用常量时同样也不需要以$符号作为前导符。另外常量只能被赋值为基础类型，如整型，而不能指向任何对象类型。

<?php
class TestClass {
    const AVAILABLE = 0;
}

print "TestClass::AVAILABLE = ".TestClass::AVAILABLE."
";

    运行结果如下：

0Stephens-Air:Desktop$ php Test.php 
TestClass::AVAILABLE = 0

注：该Blog中记录的知识点，是在我学习PHP的过程中，遇到的一些PHP和其他面向对象语言相比比较特殊的地方，或者是对我本人而言确实需要簿记下来以备后查的知识点。虽然谈不上什么深度，但还是希望能与大家分享。