Modern R Programming Cookbook

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Though you can directly use the new() function to define a new S4 class, there is another more generalized way to define a new S4 class using a constructor function as follows:

    > robustSummary <- setClass("robustSummary",      slots=list(median="numeric", mad="numeric",       q1="numeric", q3="numeric"))    > robustSummary    class generator function for class "robustSummary" from package     '.GlobalEnv'    function (...)    new("robustSummary", ...)