Access control in MATLAB - Step-by-Step Execution

Concept Flow - Access control

Define Class with Properties

↓

Set Property Access Levels

↓

Create Object Instance

↓

Try to Access Properties

↓

Allowed

↓

Access Succeeds

↓

End

This flow shows how MATLAB controls access to class properties based on their access levels when an object tries to use them.

Execution Sample

MATLAB

classdef Person
  properties (Access = private)
    age
  end
  methods
    function obj = Person(a)
      obj.age = a;
    end
    function a = getAge(obj)
      a = obj.age;
    end
  end
end

p = Person(30);
disp(p.getAge());
disp(p.age);

This code defines a class with a private property and shows accessing it via a method and directly.

Execution Table

Step	Action	Code Line	Result	Notes
1	Define class Person with private property age	classdef Person ... properties (Access = private) age end	Class Person created	age is private, not accessible outside class methods
2	Create object p with age 30	p = Person(30);	Object p created with age=30	Constructor sets private age property
3	Call method getAge() on p	disp(p.getAge());	30	Access allowed via public method
4	Try to access p.age directly	disp(p.age);	Error: Cannot access private property	Direct access denied due to private access
5	End of execution			Execution stops after error

💡 Execution stops because direct access to private property 'age' is denied.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
p.age	undefined	30	30	Error (access denied)	Error

Key Moments - 2 Insights

Why does disp(p.getAge()) work but disp(p.age) causes an error?

Because 'age' is private, it can only be accessed inside class methods like getAge(). Direct access outside the class (step 4) is blocked, as shown in the execution_table.

What does 'Access = private' mean for a property?

It means only methods inside the class can read or write that property. External code cannot access it directly, which is why step 4 fails.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value returned by p.getAge() at step 3?

A Error

B 30

C Undefined

D 0

💡 Hint

Check the 'Result' column in row for step 3 in execution_table.

At which step does the program stop due to access control?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Notes' and 'Result' columns for step 4 in execution_table.

If the property 'age' was public instead of private, what would happen at step 4?

A Direct access to p.age would succeed

B getAge() would fail

C Error still occurs

D Object creation would fail

💡 Hint

Refer to the meaning of 'Access = private' in key_moments and how it affects direct access.

Concept Snapshot

Access control in MATLAB classes:
- Use 'properties (Access = private)' to hide data
- Private properties accessible only inside class methods
- Public methods can access private data safely
- Direct access to private properties from outside causes error
- Use getter/setter methods to control property access

Full Transcript

This example shows how MATLAB controls access to class properties. We define a class Person with a private property 'age'. When we create an object p with age 30, the private property is set inside the constructor. Calling the public method getAge() returns the age successfully because methods inside the class can access private properties. However, trying to access p.age directly from outside the class causes an error because 'age' is private. This protects the data from unwanted external changes. The execution table traces each step, showing where access is allowed or denied. The variable tracker shows the value of p.age at each step, ending with an error on direct access. Key moments clarify why private access works only inside methods and not outside. The quiz tests understanding of these access rules. This teaches how access control helps keep data safe and controlled in MATLAB classes.

Practice

1. What does the private access modifier do in a MATLAB class?

easy

A. It makes properties or methods accessible only in the command window.

B. It allows access to properties or methods from anywhere in the program.

C. It allows access only to child classes and the class itself.

D. It restricts access to class properties or methods only within the class itself.

Solution

1. Step 1: Understand the role of private in MATLAB classes

   The private keyword restricts access to properties or methods so that only the class itself can use them.

2. Step 2: Compare with other access modifiers

   public allows access from anywhere, protected allows access from child classes, but private is the most restrictive.

3. Final Answer:

   It restricts access to class properties or methods only within the class itself. -> Option D

4. Quick Check:

   private means class-only access [OK]

Hint: Private means only the class itself can access [OK]

Common Mistakes:

• Confusing private with public access
• Thinking private allows child class access
• Assuming private is accessible from outside

2. Which of the following is the correct syntax to declare a protected property named data in a MATLAB class?

easy

A. properties protected
data
end

B. properties (protected)
data
end

C. properties (public)
data
end

D. properties (private)
data
end

Solution

1. Step 1: Recall MATLAB property access syntax

   Access modifiers are specified in parentheses after the properties keyword, e.g., properties (protected).

2. Step 2: Identify correct syntax for protected

   The correct syntax uses parentheses: properties (protected). properties protected data end misses parentheses, so it's invalid.

3. Final Answer:

   properties (protected) data end -> Option B

4. Quick Check:

   Use parentheses for access modifiers [OK]

Hint: Use parentheses after properties for access [OK]

Common Mistakes:

• Omitting parentheses around access modifier
• Using wrong access modifier for the question
• Placing access modifier outside properties block

3. Consider this MATLAB class snippet:

classdef MyClass
    properties (Access = private)
        secret = 42;
    end
    methods
        function val = getSecret(obj)
            val = obj.secret;
        end
    end
end

obj = MyClass();
disp(obj.secret);

What happens when you run this code?

medium

A. It causes an error because secret is private.

B. It displays 42.

C. It displays an empty value.

D. It displays the class name.

Solution

1. Step 1: Identify access level of secret

   The property secret is declared private, so it cannot be accessed directly outside the class.

2. Step 2: Analyze the code trying to access secret

   The line disp(obj.secret); tries to access the private property directly, which is not allowed and causes an error.

3. Final Answer:

   It causes an error because secret is private. -> Option A

4. Quick Check:

   Private properties block outside access [OK]

Hint: Private properties can't be accessed outside class [OK]

Common Mistakes:

• Assuming private properties can be accessed directly
• Confusing method access with property access
• Ignoring access modifiers in property declaration

4. This MATLAB class has an error. Find and fix it:

classdef Parent
    properties (Access = protected)
        value
    end
end

classdef Child < Parent
    methods
        function setValue(obj, val)
            obj.value = val;
        end
    end
end

c = Child();
c.setValue(10);
disp(c.value);

medium

A. Add a getter method in Child to access value.

B. Make setValue a static method.

C. Change value access to public.

D. Remove inheritance from Child.

Solution

1. Step 1: Understand protected access

   The property value is protected, so Child can set it, but outside code cannot access it directly.

2. Step 2: Identify why disp(c.value) fails

   Outside code tries to access value directly, which is not allowed for protected properties.

3. Step 3: Fix by adding a getter method

   Adding a method in Child to return value allows controlled access without changing access level.

4. Final Answer:

   Add a getter method in Child to access value. -> Option A

5. Quick Check:

   Use getter for protected property access outside class [OK]

Hint: Use getter methods to access protected properties outside class [OK]

Common Mistakes:

• Trying to access protected property directly outside class
• Changing access to public unnecessarily
• Not using inheritance properly

5. You want to create a MATLAB class BankAccount where the balance is private, but child classes can update it. You also want to allow anyone to check the balance. Which access control setup is correct?

hard

A. Declare balance as public; anyone can read and update balance.

B. Declare balance as private and provide a public getter method; child classes cannot update balance directly.

C. Declare balance as protected and provide a public getter method; child classes can update balance, others can only read it.

D. Declare balance as private with no getter; only the class can access balance.

Solution

1. Step 1: Identify access needs for balance

   Balance should be hidden from outside for updates but readable by anyone, and updatable by child classes.

2. Step 2: Choose access modifier and methods

   Protected allows child classes to update balance. A public getter method allows anyone to read balance safely.

3. Final Answer:

   Declare balance as protected and provide a public getter method; child classes can update balance, others can only read it. -> Option C

4. Quick Check:

   Protected + public getter fits requirements [OK]

Hint: Protected for update by children, public getter for read [OK]

Common Mistakes:

• Making balance public allowing unwanted updates
• Using private without getter blocking read access
• Not providing a getter method for balance