class - Types and classes of variables

Question

Two R questions:

1. What is the difference between the type (returned by typeof) and the class (returned by class) of a variable? Is the difference similar to that in, say, C++ language?
2. What are possible types and classes of variables?

Answer 1

In R every "object" has a mode and a class. The former represents how an object is stored in memory (numeric, character, list and function) while the later represents its abstract type. For example:

d <- data.frame(V1=c(1,2))
class(d)
# [1] "data.frame"
mode(d)
# [1] "list"
typeof(d)
# list

As you can see data frames are stored in memory as list but they are wrapped into data.frame objects. The latter allows for usage of member functions as well as overloading functions such as print with a custom behavior.

typeof(storage.mode) will usually give the same information as mode but not always. Case in point:

typeof(c(1,2))
# [1] "double"
mode(c(1,2))
# [1] "numeric"

The reasoning behind this can be found here:

The R specific function typeof returns the type of an R object

Function mode gives information about the mode of an object in the sense of Becker, Chambers & Wilks (1988), and is more compatible with other implementations of the S language

The link that I posted above also contains a list of all native R basic types (vectors, lists etc.) and all compound objects (factors and data.frames) as well as some examples of how mode, typeof and class are related for each type.