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 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.

class Fruit
   def set_kind(k)  # a writer
     @kind = k
   end

   def get_kind     # a reader
     @kind
   end
end

f1 = Fruit.new
f1.set_kind("peach")  # use the writer "peach"
f1.get_kind           # use the reader
f1                    # inspect the object

Abbreviated Output

ruby> f1 = Fruit.new
   #<Fruit:0xb7c80dac>
ruby> f1.set_kind("peach")  # use the writer "peach"
   "peach"
ruby> f1.get_kind           # use the reader
   "peach"
ruby> f1                    # inspect the object
   #<Fruit:0xb7c80dac @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:

class Fruit
   def kind=(k)
     @kind = k
   end

   def kind
     @kind
   end
end

f2 = Fruit.new
f2.kind = "banana"
f2.kind

Abbreviated Output

ruby> f2 = Fruit.new
   #<Fruit:0xb7d27dac>
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.

class Fruit
   def kind=(k)
     @kind = k
   end

   def kind
     @kind
   end

   def inspect
     "a fruit of the #{@kind} variety"
   end
end

f2 = Fruit.new
f2.kind = "banana"
f2

Abbreviated Output

ruby> f2 = Fruit.new
   a fruit of the  variety
ruby> f2.kind = "banana"
   "banana"
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.

class Fruit
   def kind=(k)
     @kind = k
   end

   def kind
     @kind
   end
end

f2 = Fruit.new
f2.kind = "banana"
p f2
puts f2.inspect
puts f2.to_s

Making accessors the easy way

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

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:

class Fruit
   attr_accessor :condition, :kind

   def inspect
     "a #{@condition} #{@kind}"
   end
end

f2 = Fruit.new
f2.condition = "ripe"
f2.kind = "mango"
f2

Abbreviated Output

ruby> f2 = Fruit.new
   a  
ruby> f2.condition = "ripe"
   "ripe"
ruby> f2.kind = "mango"
   "mango"
ruby> f2
   a ripe mango

More fun with fruit

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

class Fruit
   attr_accessor :condition, :kind

   def inspect
     "a #{@condition} #{@kind}"
   end

   def time_passes
     @condition = "rotting"
   end
end

f2 = Fruit.new
f2.condition = "ripe"
f2.kind = "mango"
f2
f2.time_passes
f2

Abbreviated Output

ruby> f2 = Fruit.new
   a  
ruby> f2.condition = "ripe"
   "ripe"
ruby> f2.kind = "mango"

   "mango"
ruby> f2
   a ripe mango
ruby> f2.time_passes
   "rotting"
ruby> f2
   a rotting mango

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.

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.