Custom exception classes in Ruby

Introduction

Custom exception classes help you create your own error types. This makes your program clearer and easier to fix when something goes wrong.

When you want to signal a specific problem in your program that built-in errors don't cover.

When you want to handle different errors in different ways.

When you want to add extra information to an error.

When you want your error messages to be clearer for users or developers.

Syntax

Ruby

class MyError < StandardError
  def initialize(msg = "My custom error happened")
    super(msg)
  end
end

Custom exceptions usually inherit from StandardError to fit well with Ruby's error handling.

You can add your own methods or data to the custom error class if needed.

Examples

A simple custom error class that inherits from StandardError.

Ruby

class MyError < StandardError
end

Custom error with a default message you can change when raising it.

Ruby

class MyError < StandardError
  def initialize(msg = "Something went wrong")
    super(msg)
  end
end

Custom error that stores an extra code value for more details.

Ruby

class MyError < StandardError
  attr_reader :code
  def initialize(msg = "Error", code = 100)
    @code = code
    super(msg)
  end
end

Sample Program

This program defines a custom error MyError. It raises this error if the input value is not positive. The error is caught and its message is printed.

Ruby

class MyError < StandardError
  def initialize(msg = "Custom error happened")
    super(msg)
  end
end

def test_error(value)
  raise MyError, "Value must be positive" if value <= 0
  "Value is #{value}"
end

begin
  puts test_error(-1)
rescue MyError => e
  puts "Caught a custom error: #{e.message}"
end

Important Notes

Always inherit from StandardError unless you have a special reason.

Use raise to trigger your custom error with a message.

Catch your custom errors with rescue YourErrorClass => e to handle them.

Summary

Custom exception classes let you create clear, specific errors for your program.

They help separate different error types for better handling.

Use inheritance from StandardError and add messages or extra info as needed.

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