What Is an Instance Method in Ruby?

In Ruby, an instance method is a method declared in a class, that's:

• Created by adding method name after def in a class;
• Callable only on class instance;
• Allowed to declare new instance variables inside;
• Able to access/modify class instance variables and class variables;
• Able to call class methods.

Created by Adding Method Name After def in a Class

An instance method can be created by simply adding the name of the method after the def keyword, for example, like so:

class Foo
    def bar
      "bar"
    end
end

Callable Only on Class Instance

A class instance method can only be called on an instantiated object:

class Foo
    def bar
      "bar"
    end
end

foo = Foo.new
puts foo.bar #=> "bar"

If you try to call it on the class itself, it will raise an error:

# undefined method `bar` for Foo:Class (NoMethodError)
puts Foo.bar

Allowed to Declare New Instance Variables Inside

Ruby allows declaration of new instance variables inside an instance method:

class Foo
    def baz
        @name = "foo"
    end

    def bar
      "#{@name}bar"
    end
end

foo = Foo.new
foo.baz
puts foo.bar #=> "foobar"

Although, this is allowed, you should generally avoid it and initialize instance variables in the constructor method instead.

Able to Access/Modify Class Instance Variables and Class Variables

The following code shows how you can access instance variables and class variables in an instance method:

class Foo
    @@separator = ","

    def initialize()
        @name = "foo"
    end

    def bar
        "#{@name}#{@@separator}bar"
    end
end

foo = Foo.new
puts foo.bar #=> "foo,bar"

Similarly, an instance method may modify values of instance variables and class variables:

class Foo
    @@separator = ","

    def initialize()
        @name = "foo"
    end

    def separator=(val)
        @@separator = val
    end

    def self.separator
      @@separator
    end

    def bar
        "#{@name}#{@@separator}bar"
    end
end

foo = Foo.new
foo.separator = "|"
puts foo.bar #=> "foo|bar"

puts Foo.separator #=> "|"

Able to Call Class Methods

You may call a class method from an instance method, for example, like so:

class Foo
    def self.capitalize(arr)
        arr.map { |item| item.capitalize }
    end

    def bar
        Foo.capitalize(['foo', 'bar'])
    end
end

foo = Foo.new
puts foo.bar #=> "Foo" "Bar"

You may also call the class method as a symbol:

class Foo
    def baz
        "baz"
    end

    def bar
        :baz
    end
end

foo = Foo.new
puts foo.bar #=> "baz"