User Guide #26 - Ruby - Accessors

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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 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 .

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>

inspect

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.

# Эти две строки эквивалентны:
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

f3

inspect

defined?