构造函数重载

Question

Normally when I want to create a class constructor that accepts different types of parameters, I'll use a kludgy overloading principle of not defining any args in the constructor definition: e.g. for an ECEF coordinate class constructor, I want it to accept either $x, $y and $z arguments, or to accept a single array argument containg x, y and z values, or to accept a single LatLong object I'd create a constructor looking something like:

function __construct()
    {
        //  Identify if any arguments have been passed to the constructor
        if (func_num_args() > 0) {
            $args = func_get_args();
            //  Identify the overload constructor required, based on the datatype of the first argument
            $argType = gettype($args[0]);
            switch($argType) {
                case 'array' :
                     //  Array of Cartesian co-ordinate values
                     $overloadConstructor = 'setCoordinatesFromArray';
                     break;
                case 'object' :
                     //  A LatLong object that needs converting to Cartesian co-ordinate values
                     $overloadConstructor = 'setCoordinatesFromLatLong';
                     break;
                default :
                     //  Individual Cartesian co-ordinate values
                     $overloadConstructor = 'setCoordinatesFromXYZ';
                     break;
            }
            //  Call the appropriate overload constructor
            call_user_func_array(array($this,$overloadConstructor),$args);
        }
    }   //  function __construct()

I'm looking at an alternative: to provide a straight constructor with $x, $y and $z as defined arguments, and to provide static methods of createECEFfromArray() and createECEFfromLatLong() that handle all the necessary extraction of x, y and z; then create a new ECEF object using the standard constructor, and return that

Which option is cleaner from an OO purists perspective?

Answer 1

I've been thinking of the suggestions offered here and by others to clean up the overloading of the object constructor. The method I've decided upon is, I think, OO elegant, simple to implement and intuitive to use.

As a solution, I decided to implement a common interface for the constructor arguments: I began by creating an interface.

interface Geodetic_XyzFormat
{
    public function getX();
    public function getY();
    public function getZ();
}

I added an abstract to implement the getters defined in the interface, together with setters and a few other methods that would be common across several child classes.

abstract class Geodetic_ECEF_Coordinates implements Geodetic_XyzFormat
{
    protected $_xCoordinate;
    protected $_yCoordinate;
    protected $_zCoordinate;

    protected function setX($xCoordinate)
    {
        $this->_xCoordinate = $xCoordinate;
    }

    public function getX()
    {
        return $this->_xCoordinate;
    }

    protected function setY($yCoordinate)
    {
        $this->_yCoordinate = $yCoordinate;
    }

    public function getY()
    {
        return $this->_yCoordinate;
    }

    protected function setZ($zCoordinate)
    {
        $this->_zCoordinate = $zCoordinate;
    }

    public function getZ()
    {
        return $this->_zCoordinate;
    }

    protected function setCoordinates($xDistance,
                                      $yDistance,
                                      $zDistance,
                                      $uom)
    {
        $this->setX(
            ($xDistance instanceof Geodetic_Distance) ? $xDistance : new Geodetic_Distance($xDistance, $uom)
        );

        $this->setY(
            ($yDistance instanceof Geodetic_Distance) ? $yDistance : new Geodetic_Distance($yDistance, $uom)
        );

        $this->setZ(
            ($zDistance instanceof Geodetic_Distance) ? $zDistance : new Geodetic_Distance($zDistance, $uom)
        );
    }

}

For my main class constructor, I type hinted for it to accept classes that extended the interface definition:

class Geodetic_ECEF_TestClass
{
    protected $_xCoordinate;
    protected $_yCoordinate;
    protected $_zCoordinate;

    public function __construct(Geodetic_XyzFormat $xyzCoordinates = NULL)
    {
        if (!is_null($xyzCoordinates)) {
            $this->_xCoordinate = $xyzCoordinates->getX();
            $this->_yCoordinate = $xyzCoordinates->getY();
            $this->_zCoordinate = $xyzCoordinates->getZ();
            return;
        }

        //    Defaults
          $this->_xCoordinate = new Geodetic_Distance();
           $this->_yCoordinate = new Geodetic_Distance();
           $this->_zCoordinate = new Geodetic_Distance();
    }
}

Finally, I created a couple of classes extending my abstract that would each handle the different options for the constructor arguments; in this case, an array of values, and individual values... I'll write the LatLong variant later, but it will use the same basic principles and extend Geodetic_ECEF_Coordinates in the same way.

class Geodetic_ECEF_CoordinateArray extends Geodetic_ECEF_Coordinates
{
    public function __construct(array $coordinates = NULL, $uom = Geodetic_Distance::METRES)
    {
        if (is_null($coordinates))
            throw new Geodetic_Exception('An array of vector coordinates must be passed');
        if (count($coordinates) == 3) {
            list ($xDistance, $yDistance, $zDistance) = array_values($coordinates);
        } else {
            throw new Geodetic_Exception('Invalid number of vectors in array');
        }

        $this->setCoordinates($xDistance, $yDistance, $zDistance, $uom);
    }

}


class Geodetic_ECEF_CoordinateValues extends Geodetic_ECEF_Coordinates
{
    public function __construct($xDistance = NULL,
                                $yDistance = NULL,
                                $zDistance = NULL,
                                $uom = Geodetic_Distance::METRES)
    {
        $this->setCoordinates($xDistance, $yDistance, $zDistance, $uom);
    }

}

So now, when I instantiate an ECEF object, I pass it the appropriate Geodetic_XyzFormat object:

//    Nothing passed to constructor
$dummyECEF1 = new Geodetic_ECEF_TestClass();
var_dump($dummyECEF1);

//    Array of values passed to constructor
$dummyECEF2 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateArray(
        array(1.2, 3.4, 5.6)
    )
);
var_dump($dummyECEF2);

//    Individual values passed to constructor
$dummyECEF3 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateValues(7.8, 9.1, 2.3)
);
var_dump($dummyECEF3);

//    Individual values passed to constructor (including a NULL, which should be treated as a 0)
$dummyECEF4 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateValues(4.5, NULL, 6.7)
);
var_dump($dummyECEF4);

$xDistance = new Geodetic_Distance(11.11, Geodetic_Distance::MILES);
$yDistance = new Geodetic_Distance(22.22, Geodetic_Distance::MILES);
$zDistance = new Geodetic_Distance(33.33, Geodetic_Distance::MILES);
//    Array of distances passed to constructor
$dummyECEF5 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateArray(
        array($xDistance, $yDistance, $zDistance)
    )
);
var_dump($dummyECEF5);

$xDistance = new Geodetic_Distance(44.44, Geodetic_Distance::MILES);
$yDistance = new Geodetic_Distance(55.55, Geodetic_Distance::MILES);
$zDistance = new Geodetic_Distance(66.66, Geodetic_Distance::MILES);
//    Individual distances passed to constructor
$dummyECEF6 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateValues($xDistance, $yDistance, $zDistance)
);
var_dump($dummyECEF6);

$xDistance = new Geodetic_Distance(11.11, Geodetic_Distance::MILES);
$yDistance = new Geodetic_Distance(22.22, Geodetic_Distance::KILOMETRES);
$zDistance = new Geodetic_Distance(33.33, Geodetic_Distance::MILES);
//    Array of mixed values and distances passed to constructor
$dummyECEF7 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateArray(
        array(11, $yDistance, 33), 
        Geodetic_Distance::MILES
    )
);
var_dump($dummyECEF7);

$xDistance = new Geodetic_Distance(44.44, Geodetic_Distance::MILES);
$yDistance = new Geodetic_Distance(55.55, Geodetic_Distance::KILOMETRES);
$zDistance = new Geodetic_Distance(66.66, Geodetic_Distance::INCHES);
//    Individual mixture of distances and values passed to constructor
$dummyECEF8 = new Geodetic_ECEF_TestClass(
    new Geodetic_ECEF_CoordinateValues($xDistance, 55, $zDistance, Geodetic_Distance::NAUTICAL_MILES)
);
var_dump($dummyECEF8);

It doesn't require all the kludgy tests for different argument types (whether in a factory, or in my main class), nor the use of statics (so it should be pretty easy writing unit tests for it)

Thanks for everyone who offered suggestions and gave me food for thought