class MyClass
  # class definition
end

MyClass.class_eval do
  # code executed in the context of MyClass
  def new_method
    puts "Hello from class_eval!"
  end
end

class Example
  def existing_method
    puts "I exist"
  end
end

Example.class_eval do
  def dynamic_method
    puts "I was added dynamically!"
  end
end

# Now you can use the new method
obj = Example.new
obj.dynamic_method  # => "I was added dynamically!"

class MyClass
end

MyClass.class_eval("def hello; puts 'Hello!'; end")

class Calculator
  OPERATIONS = [:add, :subtract, :multiply, :divide]
end

Calculator.class_eval do
  OPERATIONS.each do |op|
    define_method(op) do |a, b|
      case op
      when :add then a + b
      when :subtract then a - b
      when :multiply then a * b
      when :divide then a / b
      end
    end
  end
end

class User
  attr_accessor :name, :email
end

# Dynamically add validation methods
User.class_eval do
  [:name, :email].each do |field|
    define_method("validate_#{field}") do
      !send(field).nil? && !send(field).empty?
    end
  end
end

class Plugin
  def self.register(plugin_name, &block)
    class_eval(&block)
  end
end

Plugin.register(:logger) do
  def log(message)
    puts "[LOG] #{message}"
  end
end

class Config
  def self.configure(&block)
    class_eval(&block)
  end
end

Config.configure do
  def database_url
    "postgresql://localhost/myapp"
  end

  def api_key
    ENV['API_KEY']
  end
end

class Example
  def self.class_method
    puts "I'm a class method"
  end

  def instance_method
    puts "I'm an instance method"
  end
end

# class_eval - can define class methods
Example.class_eval do
  def self.new_class_method
    puts "New class method"
  end
end

# instance_eval - can define instance methods
Example.instance_eval do
  def new_instance_method
    puts "New instance method"
  end
end

# DANGEROUS - don't do this with user input
user_code = gets.chomp
MyClass.class_eval(user_code)  # Could execute malicious code

class ActiveRecord
  def self.has_many(association_name)
    class_eval do
      define_method(association_name) do
        # Implementation for has_many association
        puts "Fetching #{association_name} for #{self.class}"
      end
    end
  end
end

class User < ActiveRecord
  has_many :posts
end

user = User.new
user.posts  # => "Fetching posts for User"

class MyClass
  def initialize(name)
    @name = name
  end
end

obj = MyClass.new("Alice")

obj.instance_eval do
  # code executed in the context of this specific object
  puts @name  # Can access instance variables
end

class Person
  def initialize(name, age)
    @name = name
    @age = age
  end

  def introduce
    puts "Hi, I'm #{@name}"
  end
end

person = Person.new("Bob", 30)

person.instance_eval do
  puts "Name: #{@name}"      # => "Name: Bob"
  puts "Age: #{@age}"        # => "Age: 30"
  introduce                  # => "Hi, I'm Bob"
end

class BankAccount
  def initialize(balance)
    @balance = balance
  end
end

account = BankAccount.new(1000)

account.instance_eval do
  puts "Current balance: #{@balance}"  # => "Current balance: 1000"
  @balance += 500                      # Modify instance variable
  puts "New balance: #{@balance}"      # => "New balance: 1500"
end

class Example
  def initialize(value)
    @value = value
  end
end

obj = Example.new("test")
obj.instance_eval("puts @value")  # => "test"

class Calculator
  private

  def secret_calculation(x, y)
    x * y + 42
  end
end

calc = Calculator.new

# Access private method in tests
result = calc.instance_eval { secret_calculation(5, 3) }
puts result  # => 57

class Config
  def initialize
    @settings = {}
  end

  def set(key, value)
    @settings[key] = value
  end
end

config = Config.new
config.set(:api_url, "https://api.example.com")
config.set(:timeout, 30)

# Access settings dynamically
config.instance_eval do
  puts @settings[:api_url]   # => "https://api.example.com"
  puts @settings[:timeout]   # => 30
end

class ComplexObject
  def initialize
    @data = { a: 1, b: 2, c: 3 }
    @metadata = { created: Time.now }
  end
end

obj = ComplexObject.new

obj.instance_eval do
  puts "All instance variables:"
  instance_variables.each do |var|
    puts "#{var}: #{instance_variable_get(var)}"
  end
end

class Builder
  def initialize
    @result = []
  end

  def build(&block)
    instance_eval(&block)
    @result
  end
end

builder = Builder.new
result = builder.build do
  @result << "item1"
  @result << "item2"
  @result << "item3"
end

puts result  # => ["item1", "item2", "item3"]

class Example
  def initialize
    @value = "instance value"
  end

  @@class_var = "class variable"
end

obj = Example.new

# instance_eval - context is the object
obj.instance_eval do
  puts @value        # => "instance value"
  puts self.class    # => Example
end

# class_eval - context is the class
Example.class_eval do
  puts @@class_var   # => "class variable"
  puts self          # => Example
end

class QueryBuilder
  def initialize
    @conditions = []
  end

  def where(field, value)
    @conditions << "#{field} = '#{value}'"
    self
  end

  def build
    @conditions.join(" AND ")
  end
end

query = QueryBuilder.new.instance_eval do
  where("name", "John")
  where("age", 25)
  build
end

puts query  # => "name = 'John' AND age = '25'"

class AppConfig
  def initialize
    @config = {}
  end

  def configure(&block)
    instance_eval(&block)
  end

  def database(url)
    @config[:database] = url
  end

  def api_key(key)
    @config[:api_key] = key
  end

  def get_config
    @config
  end
end

config = AppConfig.new
config.configure do
  database "postgresql://localhost/myapp"
  api_key ENV['API_KEY']
end

puts config.get_config

class Serializable
  def to_hash
    result = {}
    instance_eval do
      instance_variables.each do |var|
        key = var.to_s.delete('@').to_sym
        result[key] = instance_variable_get(var)
      end
    end
    result
  end
end

class User < Serializable
  def initialize(name, email)
    @name = name
    @email = email
  end
end

user = User.new("Alice", "alice@example.com")
puts user.to_hash  # => {:name=>"Alice", :email=>"alice@example.com"}

# DANGEROUS - don't do this with user input
user_code = gets.chomp
obj.instance_eval(user_code)  # Could execute malicious code

# RSpec example
describe User do
  it "has a name" do
    user = User.new("John")
    expect(user.instance_eval { @name }).to eq("John")
  end
end

# Basic eval usage
eval("puts 'Hello from eval!'")

# With variables
x = 10
y = 20
result = eval("x + y")
puts result  # => 30

def calculate(a, b, operation)
  eval("#{a} #{operation} #{b}")
end

puts calculate(10, 5, "+")   # => 15
puts calculate(10, 5, "*")   # => 50
puts calculate(10, 5, "-")   # => 5

name = "Alice"
age = 25
city = "New York"

eval("puts \"#{name} is #{age} years old and lives in #{city}\"")
# => "Alice is 25 years old and lives in New York"

class Calculator
  def add(x, y)
    x + y
  end

  def multiply(x, y)
    x * y
  end
end

calc = Calculator.new
method_name = "add"
args = [5, 3]

result = eval("calc.#{method_name}(#{args.join(', ')})")
puts result  # => 8

# config.rb
APP_NAME = "MyApp"
DEBUG_MODE = true
DATABASE_URL = "postgresql://localhost/myapp"

# Loading configuration
config_content = File.read('config.rb')
eval(config_content)

puts APP_NAME      # => "MyApp"
puts DEBUG_MODE    # => true

class MathEvaluator
  def self.evaluate(expression)
    eval(expression)
  rescue => e
    "Error: #{e.message}"
  end
end

puts MathEvaluator.evaluate("2 + 3 * 4")        # => 14
puts MathEvaluator.evaluate("Math.sqrt(16)")    # => 4.0
puts MathEvaluator.evaluate("10 / 0")           # => "Error: divided by 0"

class Template
  def initialize(template)
    @template = template
  end

  def render(binding)
    eval("\"#{@template}\"", binding)
  end
end

template = Template.new("Hello <%= name %>, you are <%= age %> years old!")
name = "Bob"
age = 30

result = template.render(binding)
puts result  # => "Hello Bob, you are 30 years old!"

class CodeGenerator
  def self.generate_method(method_name, body)
    eval("
      def #{method_name}
        #{body}
      end
    ")
  end
end

class MyClass
  CodeGenerator.generate_method(:greet, "puts 'Hello, World!'")
  CodeGenerator.generate_method(:calculate, "2 + 2")
end

obj = MyClass.new
obj.greet        # => "Hello, World!"
puts obj.calculate  # => 4

class DynamicClassCreator
  def self.create_class(class_name, methods = {})
    eval("
      class #{class_name}
        #{methods.map { |name, body| "def #{name}; #{body}; end" }.join("\n")}
      end
    ")
  end
end

DynamicClassCreator.create_class("Person", {
  greet: "puts 'Hello!'",
  age: "25"
})

person = Person.new
person.greet  # => "Hello!"
puts person.age  # => 25

class ExpressionParser
  def self.parse(expression, variables = {})
    # Create a safe binding with variables
    binding_obj = binding
    variables.each do |key, value|
      binding_obj.local_variable_set(key, value)
    end

    eval(expression, binding_obj)
  end
end

result = ExpressionParser.parse("x * y + z", { x: 5, y: 3, z: 10 })
puts result  # => 25

class AppConfig
  def self.load_from_string(config_string)
    eval(config_string)
  end
end

config_string = "
  APP_NAME = 'MyApplication'
  DEBUG = true
  DATABASE = {
    host: 'localhost',
    port: 5432,
    name: 'myapp'
  }
"

AppConfig.load_from_string(config_string)
puts APP_NAME  # => "MyApplication"
puts DATABASE[:host]  # => "localhost"

# NEVER do this with user input!
user_input = gets.chomp
eval(user_input)  # User could input: system('rm -rf /')

# Dangerous with file input
file_content = File.read('user_provided_file.rb')
eval(file_content)  # Could contain malicious code

# Instead of eval, use safer alternatives
class SafeCalculator
  def self.calculate(expression)
    # Use a math library or parser instead
    case expression
    when /^\d+\s*\+\s*\d+$/
      # Safe addition
      eval(expression)
    else
      raise "Unsafe expression"
    end
  end
end

def create_binding(variables = {})
  binding_obj = binding
  variables.each do |key, value|
    binding_obj.local_variable_set(key, value)
  end
  binding_obj
end

my_binding = create_binding(x: 10, y: 20)
result = eval("x + y", my_binding)
puts result  # => 30

class ContextExample
  def initialize(value)
    @value = value
  end

  def evaluate_in_context(code)
    eval(code)
  end
end

obj = ContextExample.new("test")
result = obj.evaluate_in_context("@value")
puts result  # => "test"

# Bad - using eval
method_name = "calculate"
eval("result = #{method_name}(5, 3)")

# Good - using send
result = send(method_name, 5, 3)

class SafeEvaluator
  ALLOWED_OPERATIONS = %w[+ - * /]

  def self.evaluate(expression)
    operation = expression.match(/(\d+)\s*([+\-*/])\s*(\d+)/)
    return "Invalid expression" unless operation

    op = operation[2]
    return "Operation not allowed" unless ALLOWED_OPERATIONS.include?(op)

    eval(expression)
  end
end

class SandboxedEval
  def self.safe_eval(code, timeout: 1)
    Timeout::timeout(timeout) do
      eval(code)
    end
  rescue Timeout::Error
    "Execution timed out"
  rescue => e
    "Error: #{e.message}"
  end
end

# ERB template example
require 'erb'

template = ERB.new("Hello <%= name %>!")
name = "World"
result = template.result(binding)
puts result  # => "Hello World!"

class MyClass
  define_method :dynamic_method do |*args|
    puts "Hello from dynamic method with args: #{args}"
  end
end

obj = MyClass.new
obj.dynamic_method("test", 123)  # => "Hello from dynamic method with args: [\"test\", 123]"

class Calculator
  # Define methods dynamically based on operations
  [:add, :subtract, :multiply, :divide].each do |operation|
    define_method operation do |a, b|
      case operation
      when :add then a + b
      when :subtract then a - b
      when :multiply then a * b
      when :divide then a / b
      end
    end
  end
end

calc = Calculator.new
puts calc.add(5, 3)      # => 8
puts calc.multiply(4, 2)  # => 8
puts calc.divide(10, 2)   # => 5

class Person
  def initialize(name, age)
    @name = name
    @age = age
  end

  # Create getter methods dynamically
  [:name, :age].each do |attribute|
    define_method attribute do
      instance_variable_get("@#{attribute}")
    end
  end
end

person = Person.new("Alice", 25)
puts person.name  # => "Alice"
puts person.age   # => 25

class DynamicAPI
  define_method :api_call do |endpoint, params = {}|
    puts "Calling #{endpoint} with params: #{params}"
    # Simulate API call
    "Response from #{endpoint}"
  end
end

api = DynamicAPI.new
api.api_call("/users", { id: 1 })  # => "Calling /users with params: {:id=>1}"

class Model
  def self.attr_accessor(*attributes)
    attributes.each do |attribute|
      # Define getter
      define_method attribute do
        instance_variable_get("@#{attribute}")
      end

      # Define setter
      define_method "#{attribute}=" do |value|
        instance_variable_set("@#{attribute}", value)
      end
    end
  end

  attr_accessor :name, :email, :age
end

user = Model.new
user.name = "John"
user.email = "john@example.com"
puts user.name   # => "John"
puts user.email  # => "john@example.com"

class User
  def initialize(attributes = {})
    attributes.each do |key, value|
      instance_variable_set("@#{key}", value)
    end
  end

  # Create validation methods dynamically
  [:name, :email, :age].each do |field|
    define_method "validate_#{field}" do
      value = instance_variable_get("@#{field}")
      case field
      when :name
        !value.nil? && !value.empty?
      when :email
        value =~ /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z\d\-]+)*\.[a-z]+\z/i
      when :age
        value.is_a?(Integer) && value > 0 && value < 150
      end
    end
  end
end

user = User.new(name: "Alice", email: "alice@example.com", age: 25)
puts user.validate_name   # => true
puts user.validate_email  # => true
puts user.validate_age    # => true

class APIClient
  def initialize(base_url)
    @base_url = base_url
  end

  # Generate API methods for different resources
  [:users, :posts, :comments].each do |resource|
    define_method "get_#{resource}" do |id = nil|
      endpoint = id ? "#{resource}/#{id}" : resource
      puts "GET #{@base_url}/#{endpoint}"
      # Actual HTTP request would go here
    end

    define_method "create_#{resource.singularize}" do |data|
      puts "POST #{@base_url}/#{resource}"
      puts "Data: #{data}"
      # Actual HTTP request would go here
    end
  end
end

api = APIClient.new("https://api.example.com")
api.get_users           # => "GET https://api.example.com/users"
api.get_users(123)      # => "GET https://api.example.com/users/123"
api.create_user(name: "John")  # => "POST https://api.example.com/users"

class ActiveRecord
  def self.has_many(association_name)
    define_method association_name do
      puts "Fetching #{association_name} for #{self.class}"
      # Actual database query would go here
      []
    end
  end

  def self.belongs_to(association_name)
    define_method association_name do
      puts "Fetching #{association_name} for #{self.class}"
      # Actual database query would go here
      nil
    end
  end
end

class User < ActiveRecord
  has_many :posts
  belongs_to :company
end

user = User.new
user.posts    # => "Fetching posts for User"
user.company  # => "Fetching company for User"

class DynamicCalculator
  def self.create_operation(operation_name, &block)
    define_method operation_name do |*args|
      instance_eval(&block)
    end
  end

  create_operation :custom_add do
    args = method(__method__).parameters.map { |_, name| local_variable_get(name) }
    args.inject(:+)
  end

  create_operation :power do
    base, exponent = method(__method__).parameters.map { |_, name| local_variable_get(name) }
    base ** exponent
  end
end

calc = DynamicCalculator.new
puts calc.custom_add(1, 2, 3, 4)  # => 10
puts calc.power(2, 3)             # => 8

class FeatureToggle
  def self.define_feature_method(feature_name, enabled = true)
    if enabled
      define_method feature_name do
        puts "Feature #{feature_name} is enabled"
        # Feature implementation
      end
    else
      define_method feature_name do
        puts "Feature #{feature_name} is disabled"
        # Fallback or no-op
      end
    end
  end

  define_feature_method :new_ui, true
  define_feature_method :beta_feature, false
end

app = FeatureToggle.new
app.new_ui        # => "Feature new_ui is enabled"
app.beta_feature  # => "Feature beta_feature is disabled"

class FlexibleAPI
  def self.define_flexible_method(method_name)
    define_method method_name do |*args, **kwargs, &block|
      puts "Method: #{method_name}"
      puts "Arguments: #{args}"
      puts "Keyword arguments: #{kwargs}"
      puts "Block given: #{block_given?}"

      # Process based on arguments
      if block_given?
        block.call(args.first)
      elsif !kwargs.empty?
        kwargs.values.first
      else
        args.first
      end
    end
  end

  define_flexible_method :process
end

api = FlexibleAPI.new
api.process("hello")                    # => "hello"
api.process(data: "world")              # => "world"
api.process("test") { |x| x.upcase }    # => "TEST"

class MethodAliaser
  def self.create_aliases(base_method, *aliases)
    aliases.each do |alias_name|
      define_method alias_name do |*args, &block|
        send(base_method, *args, &block)
      end
    end
  end

  def original_method
    puts "Original method called"
  end

  create_aliases :original_method, :alias1, :alias2, :alternative_name
end

obj = MethodAliaser.new
obj.alias1              # => "Original method called"
obj.alternative_name    # => "Original method called"

class PerformanceExample
  # Methods defined at class definition time (faster)
  define_method :static_method do
    puts "Static method"
  end

  def self.create_dynamic_methods
    # Methods defined at runtime (slower)
    1000.times do |i|
      define_method "dynamic_method_#{i}" do
        puts "Dynamic method #{i}"
      end
    end
  end
end

class MemoryEfficient
  # More memory efficient - methods defined once
  METHODS = [:method1, :method2, :method3]

  METHODS.each do |method_name|
    define_method method_name do
      puts "Called #{method_name}"
    end
  end
end

# Good
define_method :calculate_total do
  # implementation
end

# Bad
define_method :m1 do
  # implementation
end

class SafeMethodDefiner
  def self.define_safe_method(method_name, &block)
    define_method method_name do |*args|
      begin
        instance_eval(&block)
      rescue => e
        puts "Error in #{method_name}: #{e.message}"
        nil
      end
    end
  end
end

class DocumentedClass
  # Dynamically creates getter methods for attributes
  # @param attributes [Array<Symbol>] list of attribute names
  def self.create_getters(*attributes)
    attributes.each do |attr|
      define_method attr do
        instance_variable_get("@#{attr}")
      end
    end
  end

  create_getters :name, :email
end

# Rails-like example
class ActiveRecord
  def self.validates_presence_of(*attributes)
    attributes.each do |attribute|
      define_method "validate_#{attribute}_presence" do
        value = send(attribute)
        value.nil? || value.to_s.empty?
      end
    end
  end
end

class User < ActiveRecord
  validates_presence_of :name, :email
end

class MyClass
  def method_missing(method_name, *args, &block)
    puts "Method '#{method_name}' called with args: #{args}"
    # Handle the missing method call
  end
end

obj = MyClass.new
obj.undefined_method("hello", 123)  # => "Method 'undefined_method' called with args: [\"hello\", 123]"

class DynamicHandler
  def method_missing(method_name, *args, &block)
    puts "Trying to call: #{method_name}"
    puts "Arguments: #{args}"
    puts "Block given: #{block_given?}"

    # Return a default value or handle the call
    "Handled by method_missing"
  end
end

obj = DynamicHandler.new
result = obj.some_random_method("arg1", "arg2") { puts "block" }
puts result
# Output:
# Trying to call: some_random_method
# Arguments: ["arg1", "arg2"]
# Block given: true
# Handled by method_missing

class FlexibleAPI
  def method_missing(method_name, *args, **kwargs, &block)
    puts "Method: #{method_name}"
    puts "Arguments: #{args}"
    puts "Keyword arguments: #{kwargs}"

    # Handle different method patterns
    case method_name.to_s
    when /^get_(.+)$/
      "Getting #{$1}"
    when /^set_(.+)$/
      "Setting #{$1} to #{args.first}"
    else
      "Unknown method: #{method_name}"
    end
  end
end

api = FlexibleAPI.new
puts api.get_user_info     # => "Getting user_info"
puts api.set_name("Alice") # => "Setting name to Alice"
puts api.random_method     # => "Unknown method: random_method"

class DynamicProperties
  def initialize
    @data = {}
  end

  def method_missing(method_name, *args)
    method_str = method_name.to_s

    if method_str.end_with?('=')
      # Setter method
      property_name = method_str.chomp('=')
      @data[property_name] = args.first
    else
      # Getter method
      @data[method_str]
    end
  end
end

obj = DynamicProperties.new
obj.name = "Alice"
obj.age = 25
obj.city = "New York"

puts obj.name  # => "Alice"
puts obj.age   # => 25
puts obj.city  # => "New York"

class APIClient
  def initialize(base_url)
    @base_url = base_url
  end

  def method_missing(method_name, *args)
    method_str = method_name.to_s

    case method_str
    when /^get_(.+)$/
      resource = $1
      puts "GET #{@base_url}/#{resource}"
      # Actual HTTP GET request
    when /^post_(.+)$/
      resource = $1
      data = args.first || {}
      puts "POST #{@base_url}/#{resource}"
      puts "Data: #{data}"
      # Actual HTTP POST request
    when /^put_(.+)$/
      resource = $1
      data = args.first || {}
      puts "PUT #{@base_url}/#{resource}"
      puts "Data: #{data}"
      # Actual HTTP PUT request
    when /^delete_(.+)$/
      resource = $1
      puts "DELETE #{@base_url}/#{resource}"
      # Actual HTTP DELETE request
    else
      super
    end
  end
end

api = APIClient.new("https://api.example.com")
api.get_users           # => "GET https://api.example.com/users"
api.post_user(name: "John")  # => "POST https://api.example.com/user"
api.put_user(1, name: "Jane")  # => "PUT https://api.example.com/user"
api.delete_user(1)      # => "DELETE https://api.example.com/user"

class Configuration
  def initialize
    @config = {}
  end

  def method_missing(method_name, *args)
    method_str = method_name.to_s

    if method_str.end_with?('=')
      # Setter
      key = method_str.chomp('=')
      @config[key] = args.first
    else
      # Getter
      @config[method_str]
    end
  end

  def to_hash
    @config
  end
end

config = Configuration.new
config.database_url = "postgresql://localhost/myapp"
config.api_key = ENV['API_KEY']
config.debug_mode = true

puts config.database_url  # => "postgresql://localhost/myapp"
puts config.to_hash       # => {"database_url"=>"postgresql://localhost/myapp", ...}

class HTMLBuilder
  def initialize
    @html = []
  end

  def method_missing(tag_name, *args, &block)
    content = args.first || ""
    attributes = args.last.is_a?(Hash) ? args.last : {}

    if block_given?
      @html << "<#{tag_name}#{format_attributes(attributes)}>"
      @html << yield
      @html << "</#{tag_name}>"
    else
      @html << "<#{tag_name}#{format_attributes(attributes)}>#{content}</#{tag_name}>"
    end

    self
  end

  private

  def format_attributes(attributes)
    return "" if attributes.empty?
    " " + attributes.map { |k, v| "#{k}=\"#{v}\"" }.join(" ")
  end

  def to_s
    @html.join("\n")
  end
end

builder = HTMLBuilder.new
html = builder.html do
  builder.head do
    builder.title("My Page")
  end
  builder.body(class: "main") do
    builder.h1("Hello World")
    builder.p("This is a paragraph", class: "intro")
  end
end

puts html
# Output:
# <html>
# <head>
# <title>My Page</title>
# </head>
# <body class="main">
# <h1>Hello World</h1>
# <p class="intro">This is a paragraph</p>
# </body>
# </html>

class CachedMethodHandler
  def initialize
    @cache = {}
  end

  def method_missing(method_name, *args)
    method_str = method_name.to_s

    # Check if we should handle this method
    if method_str.start_with?('cached_')
      cache_key = "#{method_str}_#{args.hash}"

      if @cache.key?(cache_key)
        puts "Returning cached result for #{method_name}"
        @cache[cache_key]
      else
        puts "Computing result for #{method_name}"
        result = compute_result(method_str, args)
        @cache[cache_key] = result
        result
      end
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?('cached_') || super
  end

  private

  def compute_result(method_name, args)
    # Simulate expensive computation
    sleep(1)
    "Result for #{method_name} with #{args}"
  end
end

handler = CachedMethodHandler.new
puts handler.cached_expensive_calculation(1, 2, 3)  # Slow
puts handler.cached_expensive_calculation(1, 2, 3)  # Fast (cached)
puts handler.respond_to?(:cached_expensive_calculation)  # => true

class Delegator
  def initialize(target)
    @target = target
  end

  def method_missing(method_name, *args, &block)
    if @target.respond_to?(method_name)
      @target.send(method_name, *args, &block)
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    @target.respond_to?(method_name, include_private) || super
  end
end

class User
  def initialize(name, email)
    @name = name
    @email = email
  end

  def display_info
    "Name: #{@name}, Email: #{@email}"
  end
end

user = User.new("Alice", "alice@example.com")
delegator = Delegator.new(user)
puts delegator.display_info  # => "Name: Alice, Email: alice@example.com"

class QueryBuilder
  def initialize
    @conditions = []
    @order_by = nil
    @limit = nil
  end

  def method_missing(method_name, *args)
    method_str = method_name.to_s

    case method_str
    when /^where_(.+)$/
      field = $1
      value = args.first
      @conditions << "#{field} = '#{value}'"
      self
    when /^order_by_(.+)$/
      field = $1
      direction = args.first || 'ASC'
      @order_by = "#{field} #{direction}"
      self
    when /^limit$/
      @limit = args.first
      self
    when /^execute$/
      build_query
    else
      super
    end
  end

  private

  def build_query
    query = "SELECT * FROM table"
    query += " WHERE #{@conditions.join(' AND ')}" unless @conditions.empty?
    query += " ORDER BY #{@order_by}" if @order_by
    query += " LIMIT #{@limit}" if @limit
    query
  end
end

query = QueryBuilder.new
result = query.where_name("John")
              .where_age(25)
              .order_by_created_at("DESC")
              .limit(10)
              .execute

puts result
# => "SELECT * FROM table WHERE name = 'John' AND age = '25' ORDER BY created_at DESC LIMIT 10"

class EventHandler
  def initialize
    @handlers = {}
  end

  def method_missing(event_name, *args)
    method_str = event_name.to_s

    if method_str.end_with?('=')
      # Register event handler
      handler_name = method_str.chomp('=')
      @handlers[handler_name] = args.first
    else
      # Trigger event
      handler = @handlers[method_str]
      if handler
        handler.call(*args)
      else
        puts "No handler registered for event: #{method_str}"
      end
    end
  end
end

handler = EventHandler.new

# Register event handlers
handler.on_click = ->(x, y) { puts "Clicked at (#{x}, #{y})" }
handler.on_hover = ->(element) { puts "Hovered over #{element}" }

# Trigger events
handler.on_click(100, 200)  # => "Clicked at (100, 200)"
handler.on_hover("button")  # => "Hovered over button"
handler.on_keypress         # => "No handler registered for event: on_keypress"

class ProperHandler
  def method_missing(method_name, *args)
    if method_name.to_s.start_with?('dynamic_')
      "Handled: #{method_name}"
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?('dynamic_') || super
  end
end

obj = ProperHandler.new
puts obj.respond_to?(:dynamic_method)  # => true
puts obj.respond_to?(:real_method)     # => false

class PerformanceExample
  def method_missing(method_name, *args)
    # This gets called for EVERY undefined method
    # Can be slow if called frequently
    puts "Handling #{method_name}"
  end
end

# Better approach: Define methods when first called
class BetterExample
  def method_missing(method_name, *args)
    if method_name.to_s.start_with?('dynamic_')
      # Define the method for future calls
      self.class.define_method(method_name) do |*method_args|
        "Handled: #{method_name} with #{method_args}"
      end

      # Call the newly defined method
      send(method_name, *args)
    else
      super
    end
  end
end

class DebuggableHandler
  def method_missing(method_name, *args, &block)
    puts "DEBUG: method_missing called for #{method_name}"
    puts "DEBUG: arguments: #{args}"
    puts "DEBUG: block given: #{block_given?}"

    # Your handling logic here
    super
  end
end

# Rails-like example
class ActiveRecord
  def method_missing(method_name, *args)
    method_str = method_name.to_s

    if method_str.start_with?('find_by_')
      field = method_str.sub('find_by_', '')
      value = args.first
      puts "Finding record where #{field} = #{value}"
      # Actual database query
    else
      super
    end
  end
end

class User < ActiveRecord
end

User.find_by_name("Alice")  # => "Finding record where name = Alice"
User.find_by_email("alice@example.com")  # => "Finding record where email = alice@example.com"

str = "hello"
def str.shout; upcase + '!'; end
# vs.
str.define_singleton_method(:whisper) { downcase + '...' }

module Mixin; end
Mixin.module_eval do
  def helper; "I'm mixed in"; end
end

obj.send(:private_method)
obj.public_send(:public_method)

class MyClass
  def public_method
    "I'm public"
  end

  private
  def secret_method
    "I'm private"
  end
end

obj = MyClass.new

puts obj.send(:public_method)      # ✅ Works: "I'm public"
puts obj.public_send(:public_method) # ✅ Works: "I'm public"

puts obj.send(:secret_method)      # ✅ Works (can access private method)
puts obj.public_send(:secret_method) # ❌ Raises NoMethodError

Object.const_set('DynamicClass', Class.new)
klass = Object.const_get('DynamicClass')

def foo; :bar; end
alias_method :baz, :foo
undef_method :foo

obj.instance_variable_set(:@data, 123)
puts obj.instance_variable_get(:@data) # => 123

class << obj
  def special; 'only me'; end
end

autoload :Parser, 'parser'

def respond_to_missing?(m, include_private=false)
  @target.respond_to?(m) || super
end

module StringExtensions
  refine String do
    def shout; upcase; end
  end
end

using StringExtensions
puts 'hi'.shout # => HI

# routes.rb
Rails.application.routes.draw do
  resources :users do
    resources :posts
  end
end

class Serializer
  %i[name age email].each do |attr|
    define_method("serialize_#{attr}") do |user|
      user.public_send(attr).to_s
    end
  end
end

Rails.application.routes.draw do
  resources :users
end

User.where(active: true)
    .order(created_at: :desc)
    .limit(10)

class CreateProducts < ActiveRecord::Migration[8.0]
  def change
    create_table :products do |t|
      t.string :name
      t.decimal :price
    end
  end
end

class User < ApplicationRecord
  validates :email, presence: true, uniqueness: true
end

class Order < ApplicationRecord
  before_save :calculate_total
  after_commit :send_confirmation_email
end