# A virtual class has no objects of its own
setClass("Car", representation("VIRTUAL"))
# Methods every car must have
setGeneric("start_engine", function(car) {
standardGeneric("start_engine")
})
# [1] "start_engine"
setGeneric("stop_engine", function(car) {
standardGeneric("stop_engine")
})
# [1] "stop_engine"
setClass("SportCar", contains = "Car")
setMethod("start_engine", "SportCar",
function(car) {
cat("start engine\n")
TRUE
})
setMethod("stop_engine", "SportCar",
function(car) {
cat("stop engine\n")
})
car <- new("SportCar")
started <- start_engine(car)
# prints "start engine"
stop_engine(car)
# prints "stop engine"
cat("started =", started, "\n")
# A method is missing: the object is created,
# and the call fails
setClass("NoStopCar", contains = "Car")
no_stop <- new("NoStopCar")
call_error <- tryCatch(stop_engine(no_stop),
error = conditionMessage)
cat("call_error =", call_error, "\n")
# call_error = unable to find an inherited
# method for function ‘stop_engine’ for
# signature ‘car = "NoStopCar"’