Object#instance_of?

obj.instance_of?(klass) -> true or false

instance_of? is a Kernel method that checks whether an object is an instance of exactly the given class — not its superclasses. Unlike is_a? and kind_of?, it does not consider the inheritance chain.

Syntax

obj.instance_of?(klass)

Parameters

Parameter	Type	Description
klass	Class	The exact class to check against

Examples

Exact class matching

"hello".instance_of?(String)
# => true

"hello".instance_of?(Object)
# => false (it's a String, not Object directly)

[1, 2].instance_of?(Array)
# => true

[1, 2].instance_of?(Object)
# => false

The key difference from is_a?

This is where instance_of? differs from is_a?:

num = 42

num.is_a?(Integer)       # => true
num.is_a?(Numeric)       # => true (Numeric is superclass)
num.is_a?(Object)        # => true (Object is root)

num.instance_of?(Integer)   # => true
num.instance_of?(Numeric)   # => false (it's an Integer, not Numeric)
num.instance_of?(Object)    # => false (it's an Integer, not Object)

Practical use case

Use instance_of? when you need strict type checking that ignores inheritance:

class Config
  def initialize(data)
    # Must be exactly a Hash, not a subclass
    unless data.instance_of?(Hash)
      raise ArgumentError, "expected Hash, got #{data.class}"
    end
    @data = data
  end
end

When to use instance_of?

• Strict type checking: When you need to reject subclasses
• Security-sensitive code: When object type matters exactly
• API contracts: When a method requires a specific class, not a compatible one

Most of the time, is_a? is more flexible and appropriate since subclasses typically behave the same as their parents.

Errors

Passing a non-Class raises TypeError:

"test".instance_of?("string")
# => TypeError: class or module required

See Also

• Object#is_a? — Checks class and ancestors
• Object#kind_of? — Alias for is_a?