Exception hierarchy in Ruby

Introduction

Exceptions help your program handle errors without crashing. The exception hierarchy organizes these errors so you can catch and fix them easily.

When you want to handle different types of errors in your program.

When you want to catch all errors that come from a common group.

When you want to create your own error types that fit into Ruby's error system.

Syntax

Ruby

begin
  # code that might cause an error
rescue ExceptionType => e
  # code to handle the error
end

begin starts the block where errors might happen.

rescue catches errors of a specific type or its subtypes.

Examples

This catches the error when dividing by zero.

Ruby

begin
  1 / 0
rescue ZeroDivisionError => e
  puts "Cannot divide by zero!"
end

This catches an error when accessing an invalid array index.

Ruby

begin
  [].fetch(1)
rescue IndexError => e
  puts "Index is out of range!"
end

This raises and catches a general standard error.

Ruby

begin
  raise StandardError, "Oops!"
rescue StandardError => e
  puts e.message
end

Sample Program

This program tries to open a file that does not exist. It catches different errors based on Ruby's exception hierarchy.

Ruby

begin
  puts "Trying to open a file..."
  File.open("nonexistent.txt")
rescue IOError => e
  puts "IO error happened: #{e.message}"
rescue SystemCallError => e
  puts "System call error happened: #{e.message}"
rescue Exception => e
  puts "Some other error: #{e.message}"
end

Important Notes

All error classes in Ruby inherit from Exception.

StandardError is the default for rescue if no exception type is given.

Use specific exceptions first, then more general ones to avoid catching errors too broadly.

Summary

Exceptions are organized in a hierarchy starting from Exception.

You can catch errors by their type or any of their parent types.

Handling exceptions helps keep your program running smoothly even when errors happen.

Practice

1.

Which class is the root of the exception hierarchy in Ruby?

easy

A. RuntimeError

B. StandardError

C. Exception

D. IOError

Solution

1. Step 1: Understand Ruby's exception hierarchy

   In Ruby, all exceptions inherit from the Exception class, which is the root of the hierarchy.

2. Step 2: Identify the root class

   Other exception classes like StandardError and RuntimeError are subclasses of Exception.

3. Final Answer:

   Exception -> Option C

4. Quick Check:

   Root exception class = Exception [OK]

Hint: Remember: all errors come from Exception root [OK]

Common Mistakes:

• Confusing StandardError as root
• Thinking RuntimeError is root
• Choosing specific error classes instead of root

2.

Which of the following is the correct way to rescue only StandardError exceptions in Ruby?

begin
  # some code
rescue ???
  puts 'Error caught'
end

easy

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Understand rescue syntax

   The rescue keyword can specify which exception class to catch by naming it after rescue.

2. Step 2: Choose the correct exception class

   To catch only StandardError exceptions, write rescue StandardError.

3. Final Answer:

   StandardError -> Option A

4. Quick Check:

   Rescue specific error = StandardError [OK]

Hint: Use rescue with exact error class name [OK]

Common Mistakes:

• Using Exception catches too many errors
• Using RuntimeError misses other StandardErrors
• Using IOError catches unrelated errors

3.

What will be the output of this Ruby code?

begin
  raise IOError, 'File error'
rescue StandardError => e
  puts e.class
end

medium

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Identify the raised exception

   The code raises an IOError with message 'File error'.

2. Step 2: Check rescue clause

   The rescue catches StandardError and its subclasses. IOError is a subclass of StandardError, so it is caught.

3. Step 3: Output the exception class

   The code prints e.class, which is IOError.

4. Final Answer:

   IOError -> Option D

5. Quick Check:

   Raised error class = IOError [OK]

Hint: Raised error class prints even if rescued [OK]

Common Mistakes:

• Assuming rescue prints StandardError
• Confusing Exception with StandardError
• Thinking RuntimeError is printed

4.

Find the error in this Ruby code snippet:

begin
  raise 'Oops!'
rescue IOError
  puts 'IO error caught'
end

medium

A. The raise statement must specify an exception class

B. The rescue clause will not catch the raised error

C. The rescue clause syntax is incorrect

D. No error; code runs fine

Solution

1. Step 1: Analyze the raised error

   Raising with a string 'Oops!' creates a RuntimeError by default.

2. Step 2: Check rescue clause type

   The rescue only catches IOError, but the raised error is RuntimeError, so it is not caught.

3. Final Answer:

   The rescue clause will not catch the raised error -> Option B

4. Quick Check:

   Rescue type mismatch causes uncaught error [OK]

Hint: Raised error type must match rescue type [OK]

Common Mistakes:

• Thinking rescue catches all errors by default
• Believing string raise is invalid syntax
• Assuming rescue syntax is wrong

5.

You want to catch all exceptions except system-exiting ones like SystemExit and Interrupt. Which class should you rescue to catch most errors but allow system exits?

hard

A. StandardError

B. Exception

C. RuntimeError

D. NoMethodError

Solution

1. Step 1: Understand exception classes for system exit

   Classes like SystemExit and Interrupt inherit directly from Exception, not StandardError.

2. Step 2: Choose rescue class to exclude system exits

   Rescuing StandardError catches most runtime errors but excludes system-exiting exceptions.

3. Step 3: Confirm other options

   Rescuing Exception catches everything including system exits, which is usually not desired.

4. Final Answer:

   StandardError -> Option A

5. Quick Check:

   Rescue StandardError to exclude system exits [OK]

Hint: Rescue StandardError to avoid catching system exits [OK]

Common Mistakes:

• Rescuing Exception catches system exits too
• Choosing RuntimeError misses some errors
• Confusing NoMethodError as base class