Question

In a class definition, what is the difference between these two methods?

def func(var)
...
end

def func=(var)
...
end

Is there any, or is one of them not valid?

Answer 1

To explain some things about reader/writer AKA getter/setter methods in Ruby:

Ruby doesn’t force us to use = in the method definition for a setter. We get to choose whether the method has one.

Consider this:

class Foo

  # automagically creates:
  #   .v
  #   .v=
  attr_accessor :v

  def initialize(v)
    puts "inside initialize(#{ v })"
    @v = v
  end

  def setter(v)
    puts "inside setter(#{ v })" 
    @v = v 
  end

  def setter=(v) 
    puts "inside setter=(#{ v })"
    @v = v 
  end

end

f = Foo.new(1)
puts f.v

f.setter(2)
puts f.v

f.setter = 3
puts f.v

f.setter=(4)
puts f.v

f.v = 5
puts f.v

f.v=(6)
puts f.v

Running the code outputs:

inside initialize(1)
1
inside setter(2)
2
inside setter=(3)
3
inside setter=(4)
4
5
6

The = is simply another letter in the method name because Ruby is smart enough to know if it sees f.setter = 3 it should use the setter=(v) method.

Ruby doesn’t force using = to set a variable, you can decide if it makes more sense to you when you define the method. It is idiomatic that we use = because it helps make a setter look like an assignment, removing the urge to name all the setters something like set_v(v).