PHP静态属性行为

Question

I'm trying to understand static property behavior in an abstract class.this is an example code retrieved from php object patterns and practice book (chapter 11-decorative pattern) :

abstract class Expression {

   private static $keycount=0;

   private $key;

   function getKey() {
    if ( ! isset( $this->key ) ) {
        self::$keycount++;
        $this->key=self::$keycount;
      }

    return $this->key;
   }
}

a number of sub classes are extended from this abstract class and then getKey() method will be called at instantiating time.each one check its own $key property and if its null ,then increase $keycount by one and assign it to $key property .
as i understand $keycount save its last value regardless of which subclass its running on . i mean it is in the context of abstract class NOT the context of sub classes. if it was dependent to its subclass then it would be reset to 0 in each subclass.can anyone provide me more insight into this ?

Answer 1

It looks like you answer your own question.

as i understand $keycount save its last value regardless of which subclass its running on

Yes, it works this way. The $keycount is shared between all subclasses. Here is a simple test to check this behavior:

<?php
abstract class Expression {

   private static $keycount=0;

   private $key;

   function getKey() {
    if ( ! isset( $this->key ) ) {
        self::$keycount++;
        $this->key=self::$keycount;
      }

    return $this->key;
   }
}

class ChildOne extends Expression {
}

class ChildTwo extends Expression {
}

$one = new ChildOne();
print 'One get key: ' . $one->getKey().PHP_EOL;
print 'One get key: ' . $one->getKey().PHP_EOL;
$oneMore = new ChildOne();
print 'One more get key: ' . $oneMore->getKey().PHP_EOL;
print 'One more get key: ' . $oneMore->getKey().PHP_EOL;
$two = new ChildTwo();
print 'Two get key: ' . $two->getKey().PHP_EOL;
print 'Two get key: ' . $two->getKey().PHP_EOL;

Output:

One get key: 1
One get key: 1
One more get key: 2
One more get key: 2
Two get key: 3
Two get key: 3