Custom exceptions in Ruby allow developers to create specific error types tailored to their application's needs. They enhance error handling and provide more meaningful information about exceptional situations.

Defining Custom Exceptions

To create a custom exception in Ruby, simply define a new class that inherits from the StandardError class or any of its subclasses.


class MyCustomError < StandardError
  def initialize(msg="A custom error occurred")
    super
  end
end

This example defines a basic custom exception called MyCustomError. It inherits from StandardError and provides a default error message.

Raising Custom Exceptions

Once defined, you can raise your custom exception using the raise keyword, just like built-in exceptions.


def risky_operation(value)
  raise MyCustomError.new("Invalid value: #{value}") if value < 0
  # Perform operation
end

begin
  risky_operation(-5)
rescue MyCustomError => e
  puts "Caught an error: #{e.message}"
end

In this example, we raise our custom exception when an invalid value is provided. The begin-rescue block catches and handles the exception.

Adding Custom Functionality

Custom exceptions can include additional methods or attributes to provide more context about the error.


class DatabaseConnectionError < StandardError
  attr_reader :error_code

  def initialize(msg="Database connection failed", error_code=500)
    @error_code = error_code
    super(msg)
  end
end

begin
  raise DatabaseConnectionError.new("Unable to connect to the database", 503)
rescue DatabaseConnectionError => e
  puts "Error: #{e.message}"
  puts "Error Code: #{e.error_code}"
end

This more advanced example includes an error_code attribute, allowing for more detailed error reporting.

Best Practices

Name your custom exceptions clearly and descriptively.
Inherit from the most appropriate exception class for your use case.
Provide meaningful error messages to aid in debugging.
Use custom exceptions to differentiate between various error conditions in your application.
Document your custom exceptions thoroughly, especially if they're part of a public API.

Conclusion

Custom exceptions in Ruby offer a powerful way to handle application-specific errors. By creating and using custom exceptions, you can improve error handling, make debugging easier, and provide more meaningful feedback when things go wrong.

Remember to use custom exceptions judiciously and in conjunction with Ruby's built-in exception handling mechanisms for robust error management in your applications.