Ruby Classes

Keep state in an instance variable and read it with attr_reader.

Ruby

class Counter
  attr_reader :value

  def initialize
    @value = 0
  end

  def increment
    @value += 1
  end
end

counter = Counter.new
counter.increment
counter.increment
puts counter.value

Output

Explanation

A Ruby class defines the behavior of its objects. initialize runs when you call Counter.new, and instance variables that start with @ belong to one object.

attr_reader :value generates a method that returns @value, so outside code can read the value but not assign it. Calling counter.increment twice raises @value from 0 to 2.

Explore the dry run

Guided walkthroughReady

1class Counter

2 attr_reader :value

4 def initialize

5 @value = 0

6 end

8 def increment

9 @value += 1

10 end

11end

13counter = Counter.new

14counter.increment

15counter.increment

16puts counter.value

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Generate a read-only value method.	`—`	—
initialize runs and sets @value to 0.	`@value = 0`	—
The first call makes @value 1.	`@value = 1`	—
Read through attr_reader and print.	`@value = 1`	2

Cost and common mistakes

O(1) per method call.

counter.value = 5 raises NoMethodError, because only a reader was defined.
A misspelled instance variable such as @valu silently reads as nil.

Try it yourself

Make Counter writable too.

Reveal answer and reasoning

Replace attr_reader with attr_accessor :value so counter.value = 5 works.

Ruby Classes

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading