Custom Exceptions

Custom Exceptions

You can define your own exception class by inheriting from StandardError (or a more specific existing exception), letting calling code catch a precise, meaningful error type for your domain. Custom exceptions usually forward a message to the parent constructor via super. This is standard practice for libraries reporting domain-specific failures, rather than raising a generic error.

Note: Inherit custom exceptions from StandardError, not the broader Exception class, so they're caught by a plain rescue clause by default.

Example: Custom Exceptions

class InsufficientFundsError < StandardError; end

class Account
  attr_reader :balance

  def initialize(balance)
    @balance = balance
  end

  def withdraw(amount)
    raise InsufficientFundsError, "Not enough funds" if amount > @balance
    @balance -= amount
  end
end

account = Account.new(100)

begin
  account.withdraw(500)
rescue InsufficientFundsError => e
  puts e.message
end