R: Class Methods

methods {base} | R Documentation

## Class Methods

### 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 or on the object supplied as an argument to `UseMethod` or `NextMethod`.

### Usage

    UseMethod(generic, object)
    NextMethod(generic = NULL, object = NULL, ...)
    methods(generic.function, class)

### Arguments

`generic` | a character string naming a function.
`object` | an object whose class will determine the method to be dispatched. Defaults to the first argument of the enclosing function.
`...` | further arguments to be passed to the method.
`generic.function` | a generic function, or a character string naming a generic function.
`class` | a symbol or character string naming a class: only used if `generic.function` is not supplied.

### Details

An R ``object'' is a data object which has a `class` attribute. A class attribute is a character vector 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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