Method_missing for catch-all in Ruby - Step-by-Step Execution

Concept Flow - Method_missing for catch-all

Call an undefined method

↓

Ruby looks for method

↓

Method not found?

↓

Call method_missing

↓

Handle call or raise error

↓

Return result

When you call a method that does not exist, Ruby calls method_missing to handle it or raise an error.

Execution Sample

Ruby

class CatchAll
  def method_missing(name, *args)
    "Called #{name} with #{args}"
  end
end
obj = CatchAll.new
puts obj.any_method(1, 2)

This code catches calls to any undefined method and returns a string describing the call.

Execution Table

Step	Action	Method Called	Arguments	Result
1	Create object	N/A	N/A	obj is a CatchAll instance
2	Call undefined method	any_method	[1, 2]	No method found
3	Invoke method_missing	method_missing	any_method, [1, 2]	"Called any_method with [1, 2]"
4	Print result	N/A	N/A	Called any_method with [1, 2]

💡 method_missing handles the undefined method call and returns a string

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
obj	undefined	CatchAll instance	CatchAll instance	CatchAll instance	CatchAll instance
name	undefined	undefined	undefined	:any_method	:any_method
args	undefined	undefined	undefined	[1, 2]	[1, 2]
result	undefined	undefined	undefined	"Called any_method with [1, 2]"	"Called any_method with [1, 2]"

Key Moments - 3 Insights

Why does Ruby call method_missing instead of raising an error?

Because the method called (any_method) does not exist, Ruby automatically calls method_missing to handle it, as shown in step 3 of the execution_table.

What are the arguments passed to method_missing?

The first argument is the method name as a symbol (:any_method), and the second is an array of arguments ([1, 2]), as shown in the 'Arguments' column in step 3.

What happens if method_missing does not handle the call?

If method_missing does not handle the call or raise an error, Ruby will raise a NoMethodError. Here, method_missing returns a string, so no error occurs.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' at step 3?

A CatchAll instance

B "Called any_method with [1, 2]"

C No method found

D undefined

💡 Hint

Check the 'Result' column at step 3 in the execution_table.

At which step does Ruby realize the method is undefined and calls method_missing?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the step where 'Invoke method_missing' happens in the execution_table.

If method_missing did not return a string, what would happen?

A Ruby would call method_missing again

B Ruby would print nil

C Ruby would raise NoMethodError

D Ruby would ignore the call

💡 Hint

Recall the key_moments explanation about what happens if method_missing does not handle the call.

Concept Snapshot

method_missing(name, *args) is called when an undefined method is invoked.
It receives the method name and arguments.
You can handle the call or raise an error.
Useful for catch-all or dynamic method handling.
If not handled, Ruby raises NoMethodError.

Full Transcript

When you call a method that does not exist on an object, Ruby tries to find it but fails. Then Ruby calls the method_missing method on that object. This method receives the name of the missing method and any arguments passed. You can write code inside method_missing to handle these calls, like returning a message or performing some action. If method_missing does not handle the call, Ruby raises a NoMethodError. In the example, calling any_method with arguments 1 and 2 triggers method_missing, which returns a string describing the call. This way, method_missing acts as a catch-all for undefined methods.