s4 - What does the @ symbol mean in R?

Question

In packages like marray and limma, when complex objects are loaded, they contain "members variables" that are accessed using the @ symbol. What does this mean and how does it differ from the $ symbol?

Answer 1

See ?'@':

• Description:

  Extract the contents of a slot in a object with a formal (S4) class structure.

• Usage:

  object@name

  ...

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The S language has two object systems, known informally as S3 and S4.

• S3 objects, classes and methods have been available in R from the beginning, they are informal, yet very interactive. S3 was first described in the White Book (Statistical Models in S).
• S3 is not a real class system, it mostly is a set of naming conventions.
• S4 objects, classes and methods are much more formal and rigorous, hence less interactive. S4 was first described in the Green Book (Programming with Data). In R it is available through the methods package, attached by default since version 1.7.0.

See also this document: S4 Classes and Methods.