Ruby's Exception Hierarchy

Direct Answer

Ruby exceptions form a class hierarchy rooted at Exception. Most application failures inherit from StandardError, including RuntimeError, ArgumentError, and NoMethodError; a rescue with no class catches this branch. Process-control and serious runtime conditions such as SystemExit, Interrupt, and NoMemoryError sit outside StandardError, so normal application code should not catch the Exception root.

Deep Dive & Architecture

A rescue clause uses the same ancestry idea as is_a?. If an exception object is an instance of the listed class or one of its descendants, the clause can handle it. This allows a library to publish a parent error for broad boundary handling while still offering subclasses for precise cases.

StandardError is a deliberate safety boundary. Most mistakes in normal input, method calls, files, networks, and application rules live below it. Ruby's default rescue stays on this branch so common error handling does not accidentally consume signals or exit requests.

The order of clauses narrows behaviour. A JSON::ParserError handler can produce an invalid-document response, while a later StandardError handler at a top-level job boundary can record an unexpected job failure. Reversing those clauses would make the specialised response impossible because the parent matches first.

Hierarchy alone does not decide what should be caught. The current layer must be able to restore a valid state, translate the failure for its caller, or add context before re-raising. A class being catchable is not evidence that continuing is safe.

Code Example

require 'json'

def parse_preferences(text)
  JSON.parse(text)
rescue JSON::ParserError => error
  { 'error' => "invalid JSON: #{error.message}" }
rescue StandardError => error
  warn "unexpected parser failure: #{error.class}"
  raise
end

parse_preferences('{bad json}')
# => {"error"=>"invalid JSON: ..."}

Common Mistakes & Pitfalls

Rescuing Exception (catches SystemExit/signals), or not knowing bare rescue catches StandardError (not Exception).