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.