Exception hierarchy in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the root class for all exceptions in Ruby?

The root class for all exceptions in Ruby is <code>Exception</code>. All other exception classes inherit from it.

intermediate

What is the difference between StandardError and Exception in Ruby?

<code>StandardError</code> is a subclass of <code>Exception</code> and is the default for rescue clauses. Most common errors inherit from <code>StandardError</code>, while <code>Exception</code> includes system errors that are usually not rescued.

beginner

Name two common subclasses of StandardError.

Two common subclasses of StandardError are NoMethodError (raised when calling a method that doesn't exist) and ArgumentError (raised when wrong arguments are passed to a method).

intermediate

Why should you avoid rescuing Exception directly in Ruby?

Rescuing Exception catches all exceptions, including system errors like Interrupt or SystemExit, which can prevent your program from exiting properly. It's safer to rescue StandardError or specific exceptions.

intermediate

What class do syntax errors belong to in Ruby's exception hierarchy?

Syntax errors belong to the <code>SyntaxError</code> class, which inherits directly from <code>ScriptError</code>, a subclass of <code>Exception</code>. They are not subclasses of <code>StandardError</code>.

Which class is the parent of most common Ruby exceptions?

A Exception

B StandardError

C RuntimeError

D ScriptError

Most common exceptions inherit from StandardError, which itself inherits from Exception.

What happens if you rescue Exception in Ruby?

A You rescue only syntax errors

B You rescue only runtime errors

C You rescue all exceptions, including system errors

D You rescue only standard errors

Rescuing Exception catches all exceptions, including system-level ones like Interrupt.

Which class is NOT a subclass of StandardError?

A NoMethodError

B ArgumentError

C RuntimeError

D SyntaxError

SyntaxError inherits from ScriptError, not StandardError.

Which exception class is used for errors raised when a method is called with wrong arguments?

A ArgumentError

B NameError

C TypeError

D NoMethodError

ArgumentError is raised when wrong arguments are passed to a method.

Which class is the direct parent of SyntaxError?

A ScriptError

B StandardError

C Exception

D RuntimeError

SyntaxError inherits directly from ScriptError.

Explain the Ruby exception hierarchy starting from Exception down to common error classes.

Think about which exceptions are rescued by default and which are not.

Why is it recommended to rescue StandardError instead of Exception in Ruby?

Consider what happens if you catch all exceptions including system interrupts.

Practice

1.

Which class is the root of the exception hierarchy in Ruby?

easy

A. RuntimeError

B. StandardError

C. Exception

D. IOError

Solution

1. Step 1: Understand Ruby's exception hierarchy

   In Ruby, all exceptions inherit from the Exception class, which is the root of the hierarchy.

2. Step 2: Identify the root class

   Other exception classes like StandardError and RuntimeError are subclasses of Exception.

3. Final Answer:

   Exception -> Option C

4. Quick Check:

   Root exception class = Exception [OK]

Hint: Remember: all errors come from Exception root [OK]

Common Mistakes:

• Confusing StandardError as root
• Thinking RuntimeError is root
• Choosing specific error classes instead of root

2.

Which of the following is the correct way to rescue only StandardError exceptions in Ruby?

begin
  # some code
rescue ???
  puts 'Error caught'
end

easy

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Understand rescue syntax

   The rescue keyword can specify which exception class to catch by naming it after rescue.

2. Step 2: Choose the correct exception class

   To catch only StandardError exceptions, write rescue StandardError.

3. Final Answer:

   StandardError -> Option A

4. Quick Check:

   Rescue specific error = StandardError [OK]

Hint: Use rescue with exact error class name [OK]

Common Mistakes:

• Using Exception catches too many errors
• Using RuntimeError misses other StandardErrors
• Using IOError catches unrelated errors

3.

What will be the output of this Ruby code?

begin
  raise IOError, 'File error'
rescue StandardError => e
  puts e.class
end

medium

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Identify the raised exception

   The code raises an IOError with message 'File error'.

2. Step 2: Check rescue clause

   The rescue catches StandardError and its subclasses. IOError is a subclass of StandardError, so it is caught.

3. Step 3: Output the exception class

   The code prints e.class, which is IOError.

4. Final Answer:

   IOError -> Option D

5. Quick Check:

   Raised error class = IOError [OK]

Hint: Raised error class prints even if rescued [OK]

Common Mistakes:

• Assuming rescue prints StandardError
• Confusing Exception with StandardError
• Thinking RuntimeError is printed

4.

Find the error in this Ruby code snippet:

begin
  raise 'Oops!'
rescue IOError
  puts 'IO error caught'
end

medium

A. The raise statement must specify an exception class

B. The rescue clause will not catch the raised error

C. The rescue clause syntax is incorrect

D. No error; code runs fine

Solution

1. Step 1: Analyze the raised error

   Raising with a string 'Oops!' creates a RuntimeError by default.

2. Step 2: Check rescue clause type

   The rescue only catches IOError, but the raised error is RuntimeError, so it is not caught.

3. Final Answer:

   The rescue clause will not catch the raised error -> Option B

4. Quick Check:

   Rescue type mismatch causes uncaught error [OK]

Hint: Raised error type must match rescue type [OK]

Common Mistakes:

• Thinking rescue catches all errors by default
• Believing string raise is invalid syntax
• Assuming rescue syntax is wrong

5.

You want to catch all exceptions except system-exiting ones like SystemExit and Interrupt. Which class should you rescue to catch most errors but allow system exits?

hard

A. StandardError

B. Exception

C. RuntimeError

D. NoMethodError

Solution

1. Step 1: Understand exception classes for system exit

   Classes like SystemExit and Interrupt inherit directly from Exception, not StandardError.

2. Step 2: Choose rescue class to exclude system exits

   Rescuing StandardError catches most runtime errors but excludes system-exiting exceptions.

3. Step 3: Confirm other options

   Rescuing Exception catches everything including system exits, which is usually not desired.

4. Final Answer:

   StandardError -> Option A

5. Quick Check:

   Rescue StandardError to exclude system exits [OK]

Hint: Rescue StandardError to avoid catching system exits [OK]

Common Mistakes:

• Rescuing Exception catches system exits too
• Choosing RuntimeError misses some errors
• Confusing NoMethodError as base class