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.
| 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:
| 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.
| 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.
p anObject
puts anObject.inspect
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:
| 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.
| 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.
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.