core-bundle/src/Resources/contao/forms/FormCheckBox.php
public function getAttributes($arrStrip=array())
	{
		// The "required" attribute only makes sense for single checkboxes
		if (\count($this->arrOptions) == 1 && $this->mandatory)
		{
			$this->arrAttributes['required'] = 'required';
		}

		return parent::getAttributes($arrStrip);
	}
public function getAttributes($arrStrip=array())
	{
		// The "required" attribute only makes sense for single checkboxes
		if (\count($this->arrOptions) == 1 && $this->mandatory)
		{
			$this->arrAttributes['required'] = 'required';
		}

		return parent::getAttributes($arrStrip);
	}
Return all attributes as string

@param array $arrStrip An optional array with attributes to strip

@return string The attributes string