Class Methods

Usage

UseMethod(name)
NextMethod(name, object, ...)
methods(generic.function, class)

Description

R possesses a simple generic function mechanism which can be used for an object-oriented style of programming. Method despatch takes place based on the class of the first argument to the generic function.

An R ``object'' is a data object which has a class attribute. A class attribute is a vector of character strings giving the names of the classes which the object ``inherits'' from. When a generic function fun is applied to an object with class attribute c("first", "second"), the system searches for a function called fun.first and, if it finds it, applied it to the object. If no such function is found a function called fun.second is tried. If no class name produces a suitable function, the function fun.default is used.

methods can be used to find out about the methods for a particular generic function or class. See the examples below for details.

Now for some obscure details that need to appear somewhere. These comments will be slightly different than those in Appendix A of the White S Book. UseMethod creates a ``new'' function call with arguments matched as they came in to the generic. Any local variables defined before the call to UseMethod are retained (!?). Any statements after the call to UseMethod will not be evaluated as UseMethod does not return.

NextMethod invokes the next method (determined by the class). It does this by creating a special call frame for that method. The arguments will be the same in number, order and name as those to the current method but their values will be promises to evaluate their name in the current method and environment. Any arguments matched to ... are handled specially. They are passed on as the promise that was supplied as an argument to the current environment. (S does this differently!) If they have been evaluated in the current (or a previous environment) they remain evaluated.

Note

The methods function was written by Martin Maechler.

See Also

class

Examples

methods(summary)

methods(print)

methods(class = data.frame)

methods("[")#- does not list the C-internal ones...


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