Although we do not use it here, the handler must have an int parameter to receive the id of the control element. There two states for a check box, namely, checked or unchecked. Associated with the states are the int values 1 and 0 that is obtained through the following call to the checkBox object's method

     checkBox->get_int_val()

We convert the int value to the appropriate Boolean value that is passed to the cgWorld's method for receiving this value. In particular, the expression

    (checkBox->get_int_val() == 1)

evaluates to true if the box is checked by the user, which means the lighting should be enabled. Otherwise, it evaluates to false.