Object#respond_to?

obj.respond_to?(method_name, include_all=false) -> true or false

The respond_to? method is a core Ruby reflection method that checks if an object responds to a particular method. It takes a method name (as a Symbol or String) and returns true if the object has a method with that name defined, or false otherwise. This method is fundamental to Ruby’s duck typing philosophy, allowing you to write flexible code that works with any object that “quacks” like a duck.

Syntax

obj.respond_to?(method_name)
obj.respond_to?(method_name, include_all)

Parameters

Parameter	Type	Default	Description
method_name	Symbol or String	—	The name of the method to check for
include_all	Boolean	false	When true, also checks for private methods

Examples

Basic usage

str = "hello"
str.respond_to?(:upcase)
# => true

str.respond_to?(:foobar)
# => false

# Works with strings too
str.respond_to?("downcase")
# => true

Checking for private methods

By default, respond_to? returns false for private methods. Pass true as the second argument to include them:

class User
  def public_method
    "I'm public!"
  end

  private
  def private_method
    "I'm secret!"
  end
end

user = User.new
user.respond_to?(:private_method)
# => false

user.respond_to?(:private_method, true)
# => true

Dynamic method dispatch

Use respond_to? before calling methods dynamically to avoid NoMethodError:

def process_object(obj)
  if obj.respond_to?(:to_json)
    puts obj.to_json
  elsif obj.respond_to?(:to_s)
    puts obj.to_s
  else
    puts obj.inspect
  end
end

Common Patterns

Plugin systems

Many gems use respond_to? to check for optional capabilities:

class PluginManager
  def execute_plugins(object)
    plugins.each do |plugin|
      next unless object.respond_to?(plugin.required_method)
      plugin.call(object)
    end
  end
end

Conditional method calls

Build flexible APIs that work with different object types:

def format_output(item)
  formatter = item.respond_to?(:format) ? item : item.to_s
  formatter.format
end

Testing for method availability

Check for Ruby version-specific methods:

# Only call if available (Ruby 2.5+)
result = str.respond_to?(:delete_suffix) ? str.delete_suffix("!") : str.chomp

Errors

respond_to? never raises an error. It always returns true or false, even for non-existent method names.

See Also

• Object#clone
• Object#dup
• Object#frozen?