Custom exception classes in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom exception class in Ruby?

A custom exception class in Ruby is a user-defined class that inherits from the built-in Exception class or one of its subclasses. It allows you to create specific error types for your program to handle distinct error situations clearly.

beginner

How do you define a custom exception class in Ruby?

You define a custom exception class by creating a new class that inherits from StandardError or Exception. For example:<br><pre>class MyError &lt; StandardError; end</pre>

intermediate

Why should custom exceptions inherit from StandardError instead of Exception?

In Ruby, rescuing StandardError catches most common errors. Inheriting from StandardError ensures your custom exceptions behave like typical errors and are caught by default rescue blocks. Inheriting directly from Exception is discouraged because it can catch system errors that should not be rescued.

beginner

How do you raise a custom exception in Ruby?

You raise a custom exception using the raise keyword followed by the exception class. For example:

raise MyError, "Something went wrong!"

beginner

What is the benefit of using custom exception classes?

Custom exception classes help you identify and handle specific error cases clearly. They make your code easier to debug and maintain by distinguishing different error types instead of using generic exceptions.

Which class should a custom exception usually inherit from in Ruby?

A StandardError

B Object

C Exception

D String

Custom exceptions should inherit from StandardError to be caught by typical rescue blocks and avoid catching system errors.

How do you raise a custom exception named MyError with a message?

A throw MyError.new("Error message")

B raise MyError, "Error message"

C error MyError("Error message")

D raise "MyError: Error message"

The correct syntax to raise a custom exception is using raise followed by the exception class and message.

What happens if you inherit a custom exception directly from Exception instead of StandardError?

A It may catch system errors that should not be rescued

B It behaves exactly the same as inheriting from StandardError

C It cannot be raised

D It automatically logs errors

Inheriting from Exception can catch system errors like Interrupt, which should usually not be rescued.

Why use custom exception classes instead of generic exceptions?

A To avoid writing rescue blocks

B To make the program run faster

C To clearly identify and handle specific error types

D To prevent any errors from occurring

Custom exceptions help you handle specific errors clearly and improve code readability.

Which keyword is used to define a custom exception class in Ruby?

A module

B def

C exception

D class

You define a custom exception class using the class keyword followed by inheritance.

Explain how to create and use a custom exception class in Ruby.

Think about class inheritance, raising errors, and rescuing them.

Why is it important to inherit custom exceptions from StandardError instead of Exception?

Consider what kinds of errors you want to catch or avoid.

Practice

1. What is the main purpose of creating a custom exception class in Ruby?

easy

A. To define specific error types for clearer error handling

B. To speed up the program execution

C. To replace all built-in exceptions

D. To automatically fix errors in the code

Solution

1. Step 1: Understand the role of exceptions

   Exceptions represent errors or unexpected events in a program.

2. Step 2: Purpose of custom exceptions

   Custom exceptions let you create specific error types to handle different problems clearly.

3. Final Answer:

   To define specific error types for clearer error handling -> Option A

4. Quick Check:

   Custom exceptions = clearer error handling [OK]

Hint: Custom exceptions clarify error types for better handling [OK]

Common Mistakes:

• Thinking custom exceptions speed up code
• Believing they replace built-in exceptions
• Assuming they fix errors automatically

2. Which of the following is the correct way to define a custom exception class in Ruby?

easy

A. class MyError < StandardError; end

B. class MyError < Exception; end

C. def MyError; end

D. exception MyError inherits StandardError

Solution

1. Step 1: Check Ruby syntax for class inheritance

   In Ruby, custom exceptions inherit from StandardError for best practice.

2. Step 2: Identify correct class definition

   class MyError < StandardError; end correctly defines a class inheriting from StandardError using Ruby syntax.

3. Final Answer:

   class MyError < StandardError; end -> Option A

4. Quick Check:

   Custom exception syntax = class MyError < StandardError [OK]

Hint: Custom exceptions inherit StandardError with class syntax [OK]

Common Mistakes:

• Inheriting directly from Exception (not recommended)
• Using def instead of class
• Wrong inheritance syntax

3. What will be the output of this Ruby code?

class MyError < StandardError; end

begin
  raise MyError, "Something went wrong"
rescue MyError => e
  puts e.message
end

medium

A. MyError

B. StandardError

C. No output

D. Something went wrong

Solution

1. Step 1: Understand raise and rescue

   The code raises a MyError with message "Something went wrong" and rescues it as variable e.

2. Step 2: Output from exception message

   Inside rescue, puts e.message prints the error message string.

3. Final Answer:

   Something went wrong -> Option D

4. Quick Check:

   e.message prints error message string [OK]

Hint: e.message prints the error message string [OK]

Common Mistakes:

• Printing class name instead of message
• Expecting no output
• Confusing exception class with message

4. Identify the error in this Ruby code defining a custom exception:

class MyError
  def initialize(msg)
    super(msg)
  end
end

medium

A. Incorrect method name 'initialize'

B. Missing inheritance from StandardError

C. super should not be called in initialize

D. Class name should be lowercase

Solution

1. Step 1: Check class inheritance

   The class does not inherit from StandardError or any exception class, so it won't behave as an exception.

2. Step 2: Confirm other parts are correct

   Method name and super call are correct for passing message to parent class.

3. Final Answer:

   Missing inheritance from StandardError -> Option B

4. Quick Check:

   Custom exceptions must inherit StandardError [OK]

Hint: Always inherit StandardError for custom exceptions [OK]

Common Mistakes:

• Forgetting to inherit StandardError
• Thinking initialize method name is wrong
• Believing super call is unnecessary

5. You want to create a custom exception that stores an error code along with the message. Which code correctly implements this?

hard

A. class CodeError < StandardError
def initialize(msg)
@code = msg
end
end

B. class CodeError
def initialize(msg, code)
@code = code
super(msg)
end
end

C. class CodeError < StandardError
attr_reader :code
def initialize(msg, code)
super(msg)
@code = code
end
end

D. class CodeError < Exception
def initialize(code)
@code = code
end
end

Solution

1. Step 1: Check inheritance and attribute

   class CodeError < StandardError attr_reader :code def initialize(msg, code) super(msg) @code = code end end inherits from StandardError and defines attr_reader for code to access it.

2. Step 2: Verify initialize method

   It calls super(msg) to set the message and stores code in @code correctly.

3. Final Answer:

   class CodeError < StandardError with attr_reader and initialize(msg, code) -> Option C

4. Quick Check:

   Custom exception with extra data = class CodeError < StandardError attr_reader :code def initialize(msg, code) super(msg) @code = code end end [OK]

Hint: Use attr_reader and call super(msg) in initialize [OK]

Common Mistakes:

• Not inheriting from StandardError
• Calling super after instance variables
• Missing attr_reader for code