魔术吸气剂/定型器未被调用

Question

When I was reading the OOP chapter in the Zend PHP Certification study guide, 5.5, I found a question that gave me a shock from its answer. This question is:

class Magic
{
    public $a = "A";
    protected $b = array( "a" => "A" , "b" => "B" , "c" => "C" );
    protected $c = array( 1 , 2 , 3 );

    public function __get( $v )
    {
        echo "$v";
        return $this->b[$v];
    }

    public function __set( $var , $val )
    {
        echo "$var: $val,";
        $this->$var = $val;
    }
}

$m = new Magic();
echo $m->a . ", " . $m->b . ", " . $m->c . ", ";
$m->c = "CC";
echo $m->a . ", " . $m->b . ", " . $m->c . ", ";

The output for this code is:

b, c, A, B, C, c: CC, b, c, A, B, C

Why does this code not print a, and how does it work?

Answer 1

__get is only invoked for non-existent or invisible properties. In other words, when you write

$obj->prop

if prop is defined and is visible in the current context, it will be returned "as is", without calling __get.

Example:

class X {

    public   $pub = 1;
    private  $pri = 2;

    function __get($v) {
        echo "[get $v]
";
        return 42;
    }

    function test() {
        echo $this->foo, "
";  // __get invoked
        echo $this->pri, "
";  // no __get
        echo $this->pub, "
";  // no __get

    }
}

$x = new X;
$x->test();

echo $x->foo, "
"; // __get invoked
echo $x->pri, "
"; // __get invoked (property not visible)
echo $x->pub, "
"; // no __get

This explains why magic->a doesn't invoke the getter. Now, since you also have the setter defined, magic->c = CC actually changes the protected member of the class, therefore, when you echo magic->c later on, this still invokes the getter (due to c's invisibility), and the getter returns this->b[c], not the actual value of this->c.

Here's your code, slightly rewritten for clarity:

class Magic
{
    public $a = "publicA";
    protected $values = array( "a" => "valA" , "b" => "valB" , "c" => "valC" );
    protected $c = "oldC";

    public function __get( $v )
    {
        echo "[get $v]
";
        return $this->values[$v];
    }

    public function __set( $var , $val )
    {
        echo "[set $var=$val]
";
        $this->$var = $val;
    }
}

$m = new Magic();
echo $m->a . ", " . $m->b . ", " . $m->c . "
";
$m->c = "newC";
echo $m->a . ", " . $m->b . ", " . $m->c . "
";

Results:

[get b]      
[get c]
publicA, valB, valC  # no getter for `a` 
[set c=newC]
[get b]
[get c]              # getter still invoked for `c`
publicA, valB, valC  # no getter for `a`

An exercise to the reader:

Why is the output different if you replace dots . in the echo statements with commas:

$m = new Magic();
echo $m->a , ", " , $m->b , ", " , $m->c , "
";
$m->c = "newC";
echo $m->a . ", " , $m->b , ", " , $m->c , "
";