What does it mean to use the name of a class for string interpolation?

Question

The following code is an extract from rubykoans about_classes.rb:

class Dog7
  def initialize(initial_name)
    @name = initial_name
  end
  def to_s
    @name
  end
end

I created an instance of Dog7:

fido = Dog7.new("Fido")

I understand the following:

"My dog is " + fido.to_s # => "My dog is Fido"
"My dog is #{fido.to_s}" # => "My dog is Fido"

I do not understand why the following interpolation makes sense and gives the same result:

"My dog is #{fido}" # => "My dog is Fido"

fido is not a string.

Answer 1

The statement #{fido} implicitly calls fido.to_s. That's why you get the "Fido", which is the value of @name.

Actually, "My dog is #{fido.to_s}" is redundant, because the #{} bit will call to_s.

Here is another way of formatting strings:

"My dog is %s" % fido

This is pretty much another version of the #{} syntax. Above, the %s indicates to the formatter that it needs to call to_s on fido. It would be redundant to do "My dog is %s" % fido.to_s, however, it would still work.