Exception hierarchy in Ruby - Step-by-Step Execution

Concept Flow - Exception hierarchy

Start

↓

Raise Exception

↓

Check Exception Type

↓

Is it StandardError?

Yes→Handle StandardError

↓

Is it Exception?

Yes→Handle Exception

↓

Unhandled Exception

↓

End

When an exception is raised, Ruby checks its type against the hierarchy from StandardError down to Exception, handling it if matched or leaving unhandled otherwise.

Execution Sample

Ruby

begin
  raise ZeroDivisionError, "divided by zero"
rescue StandardError => e
  puts e.class
end

This code raises a ZeroDivisionError and rescues it as a StandardError, printing the exception class.

Execution Table

Step	Action	Exception Raised	Exception Type Checked	Handled?	Output
1	Begin block starts	None	None	No
2	Raise ZeroDivisionError	ZeroDivisionError	Is it StandardError?	Yes
3	Rescue block catches exception	ZeroDivisionError	Handled as StandardError	Yes	ZeroDivisionError
4	End of begin-rescue block	None	None	No

💡 Exception ZeroDivisionError is a subclass of StandardError, so it is handled and output is printed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
e	nil	ZeroDivisionError instance	ZeroDivisionError instance	ZeroDivisionError instance

Key Moments - 2 Insights

Why does rescuing StandardError catch ZeroDivisionError?

Because ZeroDivisionError is a subclass of StandardError, as shown in execution_table step 2 and 3, so the rescue matches it.

What happens if an exception is not a StandardError subclass?

It is not caught by rescue StandardError and will be unhandled, as the flow shows in the concept_flow after checking StandardError.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, which exception type is raised?

A Exception

B StandardError

C ZeroDivisionError

D RuntimeError

💡 Hint

Check the 'Exception Raised' column at step 2 in the execution_table.

At which step is the exception handled in the execution_table?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for 'Handled?' column with 'Yes' and output printed.

If we raise an exception not inheriting from StandardError, what happens?

A It is unhandled and causes program to crash

B It is automatically converted to StandardError

C It is handled by rescue StandardError

D It is ignored silently

💡 Hint

Refer to concept_flow where exceptions not matching StandardError are unhandled.

Concept Snapshot

Exception hierarchy in Ruby:
- Exception is the root class
- StandardError is a common subclass
- rescue without args catches StandardError and subclasses
- Specific exceptions inherit from StandardError
- Exceptions outside StandardError are not caught by default rescue
- Use rescue Exception to catch all exceptions (not recommended)

Full Transcript

This visual execution shows how Ruby handles exceptions using its hierarchy. When an exception like ZeroDivisionError is raised, Ruby checks if it is a subclass of StandardError. Since it is, the rescue block catches it and prints the exception class. If an exception is not a StandardError subclass, it remains unhandled and may crash the program. This hierarchy helps organize error handling in Ruby programs.