PHP member variable default value

Question

I have the following PHP code to describe a color. In short, I used PHP 4 way back in the day and am trying to get my head around 5.5 now so this is the first time I'm really using objects in PHP.

Anyway I have a logic error, that I think has to do with the default value set in the Color class. Could someone please explain why my constructors aren't working, or what's going on?

class Color {
    private $red =      1;
    private $green =    1;
    private $blue =     1;
    private $alpha =    1;

    public function __toString() { return "rgb(" . $this->red . ", "
        . $this->green . ", " . $this->blue . ", " . $this->alpha . ")"; }
}

class RGBColor extends Color {
    public function __construct($red, $green, $blue) {
        $this->red = $red;      $this->green = $green;
        $this->blue = $blue;    $this->alpha = 1;
    }
}

class RGBAColor extends Color {
    public function __construct($red, $green, $blue, $alpha) {
        $this->red = $red;      $this->green = $green;
        $this->blue = $blue;    $this->alpha = $alpha;
    }

    public function __toString() { return "rgba(" . $this->red 
        . ", " . $this->green . ", " . $this->blue . ", " . $this->alpha . ")"; }
}

$c = new Color();
echo "Color: " . $c . "<br>";

$c1 = new RGBColor(0.6, 0.4, 1.0);
echo "RGB Color: " . $c1 . "<br>";

$c2 = new RGBAColor(0.6, 0.4, 1.0, 0.5);
echo "RGBA Color: " . $c2 . "<br>";

I get the following output...

Color: rgb(1, 1, 1, 1)
RGB Color: rgb(1, 1, 1, 1)
RGBA Color: rgba(0.6, 0.4, 1, 0.5)

When I should be getting...

Color: rgb(1, 1, 1, 1)
RGB Color: rgb(0.6, 0.4, 1.0)
RGBA Color: rgba(0.6, 0.4, 1, 0.5)

Thanks! -Cody

Putting the $red &c. member variables in the base class looks a bit odd. What if you later want to support HSL and HSLA color values? Will you convert from/to RGB(A) when constructing/outputting the objects? Fair enough if you only want to use RGBA internally, but make certain that you're making the decision rather than falling into it. — outis
– outis, Commented Oct 26, 2013 at 7:01

outis · Accepted Answer · 2013-10-26 06:46:06Z

2

It's not a matter of initialization order but of visibility. A private variable is accessible only by methods defined in the same class. Color::__toString() accesses the variables defined on Color, but the constructors in the child classes access different variables on the child classes. As a simple example:

<?php
class A {
    private $p = __CLASS__;
}

class B extends A {
    function __construct() {
        $this->p = __CLASS__;
    }
}

$b = new B;
var_dump($b);

outputs:

class B#1 (2) {
  private $p =>
  string(1) "A"
  public $p =>
  string(1) "B"
}

If you want a member variable to be accessible in descendants, make it protected rather than private.

answered Oct 26, 2013 at 6:46

outis

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1 Comment

Cody Smith Over a year ago

Oh crap I knew that. Thanks!

Daniele Vrut · Accepted Answer · 2013-10-26 06:47:05Z

2

Use protected not private for variables you want use on children classes. Another approach is to write setters and getters.

answered Oct 26, 2013 at 6:47

Daniele Vrut

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Comments

PPPPPPPPP · Accepted Answer · 2013-10-26 06:53:23Z

0

I am guessing maybe int and float problem , seems PHP round it to integer.

answered Oct 26, 2013 at 6:53

PPPPPPPPP

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