/**
* Constructs an instance of this class.
*
* @param Unit $parent
* @param string $symbol
* @param string $quantity
*/
public function __construct(Unit $parent, $symbol, $quantity)
{
if (!$parent->isStandardUnit()) {
throw new \InvalidArgumentException(sprintf('Parent unit "%s" is not a standard unit.', $parent->getSymbol()));
}
parent::__construct($quantity);
$this->parent = $parent;
$this->symbol = $symbol;
}
