WP_Widget::_register()

	public function _register() {
		$settings = $this->get_settings();
		$empty = true;

		// When $settings is an array-like object, get an intrinsic array for use with array_keys().
		if ( $settings instanceof ArrayObject || $settings instanceof ArrayIterator ) {
			$settings = $settings->getArrayCopy();
		}

		if ( is_array( $settings ) ) {
			foreach ( array_keys( $settings ) as $number ) {
				if ( is_numeric( $number ) ) {
					$this->_set( $number );
					$this->_register_one( $number );
					$empty = false;
				}
			}
		}

		if ( $empty ) {
			// If there are none, we register the widget's existence with a generic template.
			$this->_set( 1 );
			$this->_register_one();
		}
	}