Heuristic test for an object from an S4 class

Description

Returns TRUE if object has been generated from a formally defined (“S4”) class. DEPRECATED: use isS4(object) instead.

Usage

seemsS4Object(object)

Arguments

Details

The class of the object is examined for the "package" attribute included when objects are generated from an S4 class. The test in this function has been superseded by an internal bit set when S4 objects are generated.

The seemsS4Object function is deprecated and will be removed.

The test can be fooled in at least two ways:

1. It will give TRUE incorrectly if someone puts a "package" string attribute on the class of an S3 object. Presumably unlikely.
2. It will give FALSE incorrectly for class definitions and certain other objects for packages that have not been INSTALLed since the seemsS4Object was added to R. See the Warning below.

Value

Always TRUE or FALSE for any object.

Warnings

One motivation for this function is to prevent standard S3 vector operations from being applied to S4 objects that are not vectors. Note that seemsS4Object() alone is not that test. One also needs to check that the object does not inherit from class "vector". See the examples.

The existence of a class definition for the object's class is also not equivalent. S4 class definitions are recorded for S3 classes registered via setOldClass, but registering does not change the class of such objects, so they are not judged to be S4 objects (and should not be).

Certain other S4 objects used to be generated without the "package" attribute in earlier builds of R, notably class definitions. Packages using S4 objects must be reinstalled with a version of R recent enough to contain the seemsS4Object function (e.g., R 2.3.0 or later).

Examples

seemsS4Object(1) # FALSE

seemsS4Object(getClass(class(1)))  #TRUE

## how to test for an S4 object that is not a vector

S4NotVector <- function(object) seemsS4Object(object) && !is(object, "vector")

setClass("classNotNumeric", representation(x="numeric", y="numeric"))

setClass("classWithNumeric", representation(y="numeric"), contains = "numeric")

obj1 <- new("classNotNumeric", x=1, y=2)

obj2 <- new("classWithNumeric", 1, y=2)

seemsS4Object(obj1); seemsS4Object(obj2)  # TRUE, TRUE
S4NotVector(obj1); S4NotVector(obj2)  # TRUE, FALSE