Access control in MATLAB

Introduction

Access control helps you decide who can use or change parts of your program. It keeps your data safe and organized.

When you want to hide details inside a function or class so others can't change them by mistake.

When you want to allow only certain parts of your program to change important data.

When you want to protect your program from bugs caused by unexpected changes.

When you want to make your code easier to understand by clearly showing what can be used outside.

When you want to control how users interact with your program's data.

Syntax

MATLAB

classdef ClassName
    properties (Access = public) % or private or protected
        % variables here
    end
    methods (Access = public) % or private or protected
        % functions here
    end
end

public means anyone can use it.

private means only inside the class can use it.

protected means the class and its subclasses can use it.

Examples

This class hides the variable secretNumber so it can't be changed from outside. You can only get it using getSecret.

MATLAB

classdef MyClass
    properties (Access = private)
        secretNumber
    end
    methods
        function obj = MyClass(num)
            obj.secretNumber = num;
        end
        function n = getSecret(obj)
            n = obj.secretNumber;
        end
    end
end

This class has a public property visibleNumber that anyone can read or change.

MATLAB

classdef MyClass
    properties (Access = public)
        visibleNumber
    end
end

This property protectedData can be used inside ParentClass and any class that inherits from it.

MATLAB

classdef ParentClass
    properties (Access = protected)
        protectedData
    end
end

Sample Program

This program creates a BankAccount class with a private balance. You can only change the balance by using deposit and withdraw methods. Trying to change balance directly from outside causes an error.

MATLAB

classdef BankAccount
    properties (Access = private)
        balance
    end
    methods
        function obj = BankAccount(initialAmount)
            obj.balance = initialAmount;
        end
        function obj = deposit(obj, amount)
            obj.balance = obj.balance + amount;
        end
        function obj = withdraw(obj, amount)
            if amount <= obj.balance
                obj.balance = obj.balance - amount;
            else
                disp('Not enough balance');
            end
        end
        function b = getBalance(obj)
            b = obj.balance;
        end
    end
end

% Using the class
account = BankAccount(100);
account = account.deposit(50);
account = account.withdraw(30);
disp(['Balance: ', num2str(account.getBalance())]);
account.balance = 1000; % This will cause an error because balance is private

Important Notes

Private properties and methods cannot be accessed from outside the class.

Use public methods to safely interact with private data.

Protected access is useful when you plan to create child classes.

Summary

Access control keeps parts of your program safe and organized.

Use private to hide data inside a class.

Use public to allow access from anywhere.

Use protected to share with child classes only.

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