open_closed.md

Open closed principle

ENTITIES SHOULD BE OPEN FOR EXTENSION, BUT CLOSED FOR MODIFICATION

Software entities should be open for EXTENSION, but closed for MODIFICATION. Allow behaviour to be extended without modifying the source-code

Real world example

Code

Without OCP

class Square
{
    public $width;
    public $height;

    public function __construct($height, $width)
    {
        $this->width = $width;
        $this->height = $height;
    }
}

class Circle
{
    public $radius;

    public function __construct($radius)
    {
        $this->radius = $radius;
    }
}

class AreaCalculator
{
    public function calculate($shapes)
    {
        foreach ($shapes as $shape) {
            // ringer that we are breaking the open-closed principle
            // when we do something like this, with conditionals
            if (is_a($shape, 'Square')) {
                $area[] = $shape->width * $shape->height;
            } else {
                $area[] = $shape->radius * $shape->radius * pi();
            }
        }

        return array_sum($area);
    }
}

With OCP

interface Shape
{
    public function area();
}

class Square implements Shape {
    public $width;
    public $height;

    public function __construct($height, $width)
    {
        $this->width = $width;
        $this->height = $height;
    }

    public function area()
    {
        return $this->width * $this->height;
    }
}

class Circle implements Shape
{
    public $radius;

    public function __construct($radius)
    {
        $this->radius = $radius;
    }

    public function area()
    {
        return $this->radius * $this->radius * pi();
    }
}

class AreaCalculator
{
    public function calculate($shapes)
    {
        foreach ($shapes as $shape) {
            $area[] = $shape->area();
        }

        return array_sum($area);
    }
}