Accessors

Prev | Contents | Next

| Ruby user's guide | Accessors |

### What is an accessor?

We briefly discussed instance variables in an earlier chapter, but haven't done much with them yet. An object's instance variables are its attributes, the things that generally distinguish it from other objects of the same class. It is important to be able to write and read these attributes; doing so requires writing methods called attribute accessors. We'll see in a moment that we don't always have to write accessor methods explicitly, but let's go through all the motions for now. The two kinds of accessors are writers and readers.

    ruby> class Fruit
        |   def set_kind(k)  # a writer
        |     @kind = k
        |   end
        |   def get_kind     # a reader
        |     @kind
        |   end
        | end
       nil
    ruby> f1 = Fruit.new
       #<Fruit:0xfd7e7c8c>
    ruby> f1.set_kind("peach")  # use the writer
       "peach"
    ruby> f1.get_kind           # use the reader
       "peach"
    ruby> f1                    # inspect the object
       #<Fruit:0xfd7e7c8c @kind="peach">

Simple enough; we can store and retrieve information about what kind of fruit we're looking at. But our method names are a little wordy. The following is more concise, and more conventional:

    ruby> class Fruit
        |   def kind=(k)
        |     @kind = k
        |   end
        |   def kind
        |     @kind
        |   end
        | end
       nil
    ruby> f2 = Fruit.new
       #<Fruit:0xfd7e7c8c>
    ruby> f2.kind = "banana"
       "banana"
    ruby> f2.kind
       "banana"

### The `inspect` method

A short digression is in order. You've noticed by now that when we try to look at an object directly, we are shown something cryptic like `#<anObject:0x83678>`. This is just a default behavior, and we are free to change it. All we need to do is add a method named `inspect`. It should return a string that describes the object in some sensible way, including the states of some or all of its instance variables.

    ruby> class Fruit
        |   def inspect
        |     "a fruit of the " + @kind + " variety"
        |   end
        | end
       nil
    ruby> f2
       "a fruit of the banana variety"

A related method is `to_s` (convert to string), which is used when printing an object. In general, you can think of `inspect` as a tool for when you are writing and debugging programs, and `to_s` as a way of refining program output. `eval.rb` uses `inspect` whenever it displays results. You can use the `p` method to easily get debugging output from programs.

    # These two lines are equivalent:
    p anObject
    print anObject.inspect, "\n"

### Making accessors the easy way

Since many instance variables need accessor methods, Ruby provides convenient shortcuts for the standard forms.

Shortcut | Effect
`attr_reader :v` | `def v; @v; end`
`attr_writer :v` | `def v=(value); @v=value; end`
`attr_accessor :v` | `attr_reader :v; attr_writer :v`
`attr_accessor :v, :w` | `attr_accessor :v; attr_accessor :w`

Let's take advantage of this and add freshness information. First we ask for an automatically generated reader and writer, and then we incorporate the new information into `inspect`:

    ruby> class Fruit
        |   attr_accessor :condition
        |   def inspect
        |     "a " + @condition + @kind"
        |   end
        | end
       nil
    ruby> f2.condition = "ripe"
       "ripe"
    ruby> f2
       "a ripe banana"

### More fun with fruit

If nobody eats our ripe fruit, perhaps we should let time take its toll.

    ruby> class Fruit
        |   def time_passes
        |     @condition = "rotting"
        |   end
        | end
       nil
    ruby> f2
       "a ripe banana"
    ruby> f2.time_passes
       "rotting"
    ruby> f2
       "a rotting banana"

But while playing around here, we have introduced a small problem. What happens if we try to create a third piece of fruit now? Remember that instance variables don't exist until values are assigned to them.

    ruby> f3 = Fruit.new
    ERR: failed to convert nil into String

It is the `inspect` method that is complaining here, and with good reason. We have asked it to report on the kind and condition of a piece of fruit, but as yet `f3` has not been assigned either attribute. If we wanted to, we could rewrite the `inspect` method so it tests instance variables using the`defined?` method and then only reports on them if they exist, but maybe that's not very useful; since every piece of fruit has a kind and condition, it seems we should make sure those always get defined somehow. That is the topic of the next chapter.

Prev | Contents | Next