Method objects with method() in Ruby - Deep Dive

Overview - Method objects with method()

What is it?

In Ruby, every method belongs to an object. The method() function lets you get a special object that represents a method. This method object can be stored, passed around, or called later. It turns a method into a first-class object you can work with like any other value.

Why it matters

Without method objects, you can only call methods directly on objects, limiting flexibility. Method objects let you treat methods like data, enabling powerful patterns like callbacks, delayed execution, and dynamic method calls. This makes your code more flexible and reusable.

Where it fits

Before learning method objects, you should understand Ruby methods and objects basics. After this, you can explore advanced topics like Procs, lambdas, and blocks, which also deal with passing behavior around.

Mental Model

Core Idea

A method object is a handle to a method that you can store, pass, and call anytime, just like a variable holding a value.

Think of it like...

Imagine a TV remote control that can turn on a specific TV channel. The method object is like that remote: it points to a specific channel (method) and lets you activate it whenever you want, without needing to be at the TV itself.

Object ── has method ──> Method
  │                      ↑
  │                      │
  └──── method() returns ─┘

You get a Method object from an Object using method(), then call it later.

Build-Up - 6 Steps

1

Foundation

Understanding Ruby Methods and Objects

Concept: Methods belong to objects and can be called directly.

In Ruby, you write methods inside classes or modules. When you create an object, you can call its methods like this: class Dog def bark puts 'Woof!' end end dog = Dog.new dog.bark # Calls the bark method directly

Result

Output: Woof!

Knowing that methods are tied to objects is the base for understanding how method objects work.

2

Foundation

What is a Method Object?

Concept: A method object is a special object representing a method, which you can store and call later.

You can get a method object by calling method() on an object with the method name as a symbol: method_obj = dog.method(:bark) This method_obj now holds the bark method and can be called with method_obj.call

Result

Calling method_obj.call prints: Woof!

This shows methods can be treated like data, not just called immediately.

3

Intermediate

Storing and Passing Method Objects

🤔Before reading on: do you think method objects can be passed as arguments to other methods? Commit to your answer.

Concept: Method objects can be stored in variables or passed to other methods as arguments.

You can pass method objects to other methods to call them later: def call_method_later(meth) puts 'Calling method now:' meth.call end call_method_later(method_obj) # Passes the bark method object

Result

Output: Calling method now: Woof!

Understanding this unlocks flexible code patterns like callbacks and event handlers.

4

Intermediate

Converting Method Objects to Procs

🤔Before reading on: do you think method objects and Procs are the same? Commit to your answer.

Concept: Method objects can be converted to Proc objects, which are blocks of code you can pass around and call.

You can convert a method object to a Proc using to_proc: proc_obj = method_obj.to_proc proc_obj.call # Calls bark method This lets you use method objects where Procs are expected.

Result

Output: Woof!

Knowing this helps integrate method objects with Ruby's block and Proc system.

5

Advanced

Using Method Objects for Dynamic Dispatch

🤔Before reading on: do you think method objects can call private methods? Commit to your answer.

Concept: Method objects can call private methods and enable dynamic method calls based on runtime data.

Even private methods can be accessed via method objects: class Cat private def meow puts 'Meow!' end end cat = Cat.new meth = cat.method(:meow) meth.call # Calls private method You can also select methods dynamically: method_name = :meow cat.method(method_name).call

Result

Output: Meow!

This reveals method objects bypass some normal access restrictions, enabling powerful metaprogramming.

6

Expert

Method Objects and Memory: Binding and Garbage Collection

🤔Before reading on: do you think method objects keep their original object alive? Commit to your answer.

Concept: Method objects keep a reference to their original object, affecting memory and garbage collection.

When you create a method object, it holds a reference to the object it came from. This means: obj = Dog.new meth = obj.method(:bark) obj = nil # Even if obj is nil, meth keeps the Dog alive This can cause memory leaks if method objects are stored long-term without care.

Result

The Dog object stays in memory as long as meth exists.

Understanding this prevents subtle bugs and memory leaks in long-running Ruby programs.

Under the Hood

When you call method(:name) on an object, Ruby creates a Method object that internally stores a reference to the original object and the method's internal code pointer. Calling call on this Method object invokes the method on the stored object, preserving context like instance variables and private access. The Method object acts as a closure binding the method code to its receiver.

Why designed this way?

Ruby treats methods as first-class objects to support flexible programming styles like metaprogramming and callbacks. The design keeps method objects tied to their receiver to maintain correct context and access control. Alternatives like just storing method names would lose this binding, making dynamic calls less safe and predictable.

┌─────────────┐
│   Object    │
│ (receiver)  │
└─────┬───────┘
      │ method(:name)
      ▼
┌─────────────┐
│  Method Obj │
│ - receiver  │
│ - method ptr│
└─────┬───────┘
      │ call()
      ▼
┌─────────────┐
│  Executes   │
│ method code │
│ on receiver │
└─────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does calling method() create a copy of the method code? Commit to yes or no.

Common Belief: Calling method() creates a new copy of the method's code.

Reality: Method objects do not copy code; they reference the original method's code and the receiver object.

Why it matters: Thinking method() copies code leads to confusion about memory use and behavior when methods change dynamically.

Quick: Can method objects be called without their original object? Commit to yes or no.

Common Belief: Method objects can be called independently of their original object.

Reality: Method objects always call the method on the original object they were created from.

Why it matters: Misunderstanding this causes bugs when expecting method objects to work on different objects.

Quick: Do method objects ignore method visibility like private or protected? Commit to yes or no.

Common Belief: Method objects respect method visibility and cannot call private methods.

Reality: Method objects can call private and protected methods, bypassing normal visibility rules.

Why it matters: This can lead to unexpected access and security issues if not understood.

Quick: Are method objects the same as Procs? Commit to yes or no.

Common Belief: Method objects and Procs are the same and interchangeable without conversion.

Reality: Method objects and Procs are different types; method objects can be converted to Procs but are not identical.

Why it matters: Confusing them leads to errors when passing method objects where Procs are expected.

Expert Zone

1

Method objects keep a strong reference to their receiver, which can unintentionally prevent garbage collection.

2

Calling method() on a singleton method returns a Method object bound to that singleton, enabling metaprogramming tricks.

3

Method objects can be unbound and rebound to different objects of the same class, allowing flexible method reuse.

When NOT to use

Avoid method objects when you only need simple callbacks; Procs or lambdas are lighter and more flexible. Also, do not use method objects for methods that change frequently at runtime, as they bind to the method version at creation time.

Production Patterns

Method objects are used in event-driven systems as callbacks, in metaprogramming to dynamically invoke methods, and in frameworks like Rails for filters and hooks. They enable clean separation of behavior and data.

Connections

First-class functions

Method objects make Ruby methods behave like first-class functions.

Understanding method objects helps grasp how Ruby treats behavior as data, similar to functional programming languages.

Closures in programming

Method objects act like closures by binding method code to an object context.

Knowing this clarifies how method objects preserve state and environment when called later.

Remote procedure calls (RPC)

Method objects conceptually resemble RPC handles that point to a procedure to call remotely.

Seeing method objects as handles helps understand distributed systems where calls are deferred or sent over networks.

Common Pitfalls

#1 Storing method objects without realizing they keep the original object alive.

Wrong approach:

meth = obj.method(:foo)
obj = nil
# meth still holds obj alive, causing memory leak

Correct approach:

meth = nil
obj = nil
# Release method object to allow garbage collection

Root cause:

Not knowing method objects hold strong references to their receiver objects.

#2 Trying to call a method object on a different object than it was created from.

Wrong approach:

meth = obj1.method(:foo)
meth.call(obj2) # Wrong: method object ignores argument as receiver

Correct approach:

meth = obj2.method(:foo)
meth.call # Correct: method object bound to obj2

Root cause:

Misunderstanding that method objects are bound to their original receiver.

#3 Assuming method objects respect method visibility rules.

Wrong approach:

meth = obj.method(:private_method)
meth.call # Unexpectedly works despite private

Correct approach:

Use send(:private_method) explicitly or understand method objects bypass visibility

Root cause:

Not realizing method objects can call private methods, bypassing normal access control.

Key Takeaways

Method objects let you treat methods as objects you can store, pass, and call anytime.

They keep a reference to the original object, preserving context and access to private methods.

Method objects can be converted to Procs to integrate with Ruby's block system.

Understanding method objects unlocks powerful patterns like callbacks, dynamic dispatch, and metaprogramming.

Be careful with method objects holding references to avoid memory leaks in long-running programs.

Practice

1. What does the Ruby expression obj.method(:foo) return?

easy

A. A Method object representing the method foo of obj

B. The result of calling foo on obj

C. A symbol :foo

D. An error because method is not a valid method

Solution

1. Step 1: Understand method usage

   The method method on an object returns a Method object representing the named method.

2. Step 2: Identify the return type

   Calling obj.method(:foo) returns a Method object, not the method's result or a symbol.

3. Final Answer:

   A Method object representing the method foo of obj -> Option A

4. Quick Check:

   obj.method(:foo) returns Method object [OK]

Hint: Remember: method returns a Method object, not the call result [OK]

Common Mistakes:

• Thinking it calls the method immediately
• Confusing with symbol :foo
• Assuming it returns a string or error

2. Which of the following is the correct syntax to get a Method object for the method greet on object person?

easy

A. person.send(:greet)

B. person.method[b greet]

C. person.method(greet)

D. person.method(:greet)

Solution

1. Step 1: Check argument type for method

   The method method expects a symbol representing the method name.

2. Step 2: Validate options

   person.method(:greet) uses a symbol :greet, which is correct. person.send(:greet) calls the method directly, returning its result instead of a Method object. person.method(greet) passes an undefined variable greet, causing error. person.method[b greet] uses brackets, which is invalid syntax.

3. Final Answer:

   person.method(:greet) -> Option D

4. Quick Check:

   Use symbol argument with method [OK]

Hint: Use a symbol like :greet inside method() [OK]

Common Mistakes:

• Confusing method() with send()
• Using brackets instead of parentheses
• Passing undefined variable instead of symbol

3. What is the output of the following Ruby code?

class Calculator
  def add(x, y)
    x + y
  end
end
calc = Calculator.new
adder = calc.method(:add)
puts adder.call(5, 7)

medium

A. 5 7

B. 12

C. add

D. NoMethodError

Solution

1. Step 1: Understand method object creation

   The method(:add) returns a Method object for add on calc.

2. Step 2: Calling the Method object

   Calling adder.call(5, 7) executes add(5, 7), which returns 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   Method object call returns sum 12 [OK]

Hint: Use call on Method object to run method with arguments [OK]

Common Mistakes:

• Expecting method name or arguments printed
• Confusing output with error
• Not using call to invoke method

4. Identify the error in this Ruby code snippet:

class Person
  def name
    "Alice"
  end
end
p = Person.new
m = p.method(name)
m.call

medium

A. Missing quotes around method name in method(name)

B. Cannot call method on an object

C. Method name does not exist

D. Using call on Method object is invalid

Solution

1. Step 1: Check argument to method

   The argument name is used as a variable, but it should be a symbol or string representing the method name.

2. Step 2: Correct usage

   It should be p.method(:name) or p.method('name') to get the Method object.

3. Final Answer:

   Missing quotes around method name in method(name) -> Option A

4. Quick Check:

   Method name must be symbol or string [OK]

Hint: Always pass method name as symbol or string to method() [OK]

Common Mistakes:

• Passing bare method name without quotes or colon
• Thinking method is invalid on objects
• Confusing call usage

5. Given the following code, what will be the output?

class Greeter
  def initialize(name)
    @name = name
  end
  def greet
    "Hello, #{@name}!"
  end
end

g1 = Greeter.new("Bob")
g2 = Greeter.new("Sue")
m1 = g1.method(:greet)
m2 = g2.method(:greet)

puts m1.call
puts m2.call

hard

A. Hello, Sue!\nHello, Sue!

B. Hello, Bob!\nHello, Bob!

C. Hello, Bob!\nHello, Sue!

D. Error: undefined method greet

Solution

1. Step 1: Understand instance variables and method objects

   Each Greeter instance stores its own @name. The method(:greet) returns a Method object bound to that instance.

2. Step 2: Calling Method objects

   Calling m1.call runs greet on g1, printing "Hello, Bob!". Similarly, m2.call prints "Hello, Sue!".

3. Final Answer:

   Hello, Bob! Hello, Sue! -> Option C

4. Quick Check:

   Method objects keep instance context [OK]

Hint: Method objects remember their object, so output matches instance data [OK]

Common Mistakes:

• Assuming method objects lose instance context
• Expecting same output for both calls
• Confusing method object with class method