Why Instance_variable_get and set in Ruby? - Purpose & Use Cases

The Big Idea

What if you could peek inside any object and change its secrets without extra code?

The Scenario

Imagine you have a Ruby object with many hidden details stored in instance variables. You want to peek inside or change these details without writing a special method for each one.

The Problem

Manually writing getter and setter methods for every instance variable is slow and boring. It clutters your code and makes it hard to change or explore variables dynamically.

The Solution

Using instance_variable_get and instance_variable_set lets you access or change any instance variable by name at runtime. This means you can work with variables flexibly without extra methods.

Before vs After

✗ Before

def get_name
  @name
end

def set_name(value)
  @name = value
end

✓ After

obj.instance_variable_get(:@name)
obj.instance_variable_set(:@name, 'new value')

What It Enables

This lets you explore and modify an object's hidden details on the fly, making your code more dynamic and powerful.

Real Life Example

When debugging or building tools that inspect objects, you can quickly read or change any instance variable without changing the original class.

Key Takeaways

Manually writing access methods is slow and rigid.

instance_variable_get and instance_variable_set access variables by name dynamically.

This makes your Ruby code more flexible and easier to maintain.

Practice

1. What does the Ruby method instance_variable_get do?

easy

A. It reads the value of an instance variable by its name.

B. It deletes an instance variable from an object.

C. It creates a new instance variable with a given name.

D. It lists all instance variables of an object.

Solution

1. Step 1: Understand the method purpose

   instance_variable_get is used to access the value of an instance variable by passing its name as a symbol or string.

2. Step 2: Compare with other options

   It does not delete, create, or list variables; it only reads the value.

3. Final Answer:

   It reads the value of an instance variable by its name. -> Option A

4. Quick Check:

   instance_variable_get reads variable value [OK]

Hint: Remember: get = read value of variable [OK]

Common Mistakes:

• Confusing get with set (changing value)
• Thinking it deletes variables
• Assuming it lists all variables

2. Which of the following is the correct syntax to set an instance variable @name to "Alice" using instance_variable_set?

easy

A. object.instance_variable_set(:nam, "Alice")

B. object.instance_variable_set("nam", "Alice")

C. object.instance_variable_set(@name, "Alice")

D. object.instance_variable_set(:@name, "Alice")

Solution

1. Step 1: Understand the argument format

   The first argument to instance_variable_set must be the instance variable name as a symbol or string, including the '@' prefix.

2. Step 2: Check each option

   A: object.instance_variable_set(:nam, "Alice") misses '@'. B: object.instance_variable_set("nam", "Alice") misses '@'. C: passes a variable instead of symbol/string. D: uses :@name symbol correctly.

3. Final Answer:

   object.instance_variable_set(:@name, "Alice") -> Option D

4. Quick Check:

   Use symbol with '@' prefix for setting [OK]

Hint: Use symbol with '@' prefix to set variable [OK]

Common Mistakes:

• Omitting '@' in variable name
• Passing variable instead of symbol/string
• Using string without '@' prefix

3. What will be the output of this Ruby code?

class Person
  def initialize(name)
    @name = name
  end
end

p = Person.new("Bob")
puts p.instance_variable_get(:@name)

medium

A. Bob

B. nil

C. @name

D. Error

Solution

1. Step 1: Understand instance variable initialization

   The @name variable is set to "Bob" in the constructor.

2. Step 2: Use instance_variable_get to read value

   Calling instance_variable_get(:@name) returns the value "Bob" stored in @name.

3. Final Answer:

   Bob -> Option A

4. Quick Check:

   instance_variable_get returns stored value [OK]

Hint: instance_variable_get returns actual stored value [OK]

Common Mistakes:

• Expecting variable name instead of value
• Confusing with nil if variable not set
• Thinking it raises error if variable exists

4. Identify the issue in this Ruby code snippet:

class Car
  def initialize
    @speed = 0
  end

  def set_speed(value)
    instance_variable_set(:speed, value)
  end
end

car = Car.new
car.set_speed(50)
puts car.instance_variable_get(:@speed)

medium

A. The variable @speed is not initialized.

B. Cannot call instance_variable_set inside a method.

C. Missing '@' in instance_variable_set argument name.

D. instance_variable_get should use string, not symbol.

Solution

1. Step 1: Check instance_variable_set argument

   The argument to instance_variable_set must include '@' prefix to identify the instance variable.

2. Step 2: Identify the error

   Here, :speed is missing '@', so Ruby will not find the variable and will create a new one without '@', causing unexpected behavior.

3. Final Answer:

   Missing '@' in instance_variable_set argument name. -> Option C

4. Quick Check:

   Always include '@' in variable name for instance_variable_set [OK]

Hint: Always prefix variable name with '@' in set/get methods [OK]

Common Mistakes:

• Omitting '@' in variable name
• Thinking instance_variable_set can't be used in methods
• Confusing symbol and string usage

5. Given a class Box with private instance variable @content, how can you change its value to "Books" from outside the class using instance_variable_set?

hard

A. box.instance_variable_set(:conten, "Books")

B. box.instance_variable_set(:@content, "Books")

C. box.@content = "Books"

D. box.set_content("Books")

Solution

1. Step 1: Understand private variable access

   Private instance variables cannot be accessed or changed directly from outside the class using normal assignment.

2. Step 2: Use instance_variable_set with correct name

   Using instance_variable_set with the symbol including '@' allows changing the private variable from outside.

3. Step 3: Check options

   box.instance_variable_set(:@content, "Books") correctly uses :@content. box.instance_variable_set(:conten, "Books") misses '@', box.@content = "Books" is invalid syntax, box.set_content("Books") assumes a setter method which may not exist.

4. Final Answer:

   box.instance_variable_set(:@content, "Books") -> Option B

5. Quick Check:

   Use instance_variable_set with '@' to change private vars [OK]

Hint: Use '@' prefix symbol to set private variables externally [OK]

Common Mistakes:

• Omitting '@' in variable name
• Trying direct assignment to private variable
• Assuming setter method exists