Method overriding in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is method overriding in C++?

Method overriding occurs when a derived class provides its own version of a function that is already defined in its base class. The function in the derived class has the same name and signature as in the base class.

beginner

Why do we use method overriding?

We use method overriding to change or extend the behavior of a base class method in a derived class. It allows polymorphism, so the program can decide at runtime which method version to call.

beginner

What keyword is used in C++ to indicate a method can be overridden?

The keyword <code>virtual</code> is used in the base class method declaration to allow overriding in derived classes.

intermediate

What happens if a base class method is not declared <code>virtual</code> but a derived class defines a method with the same name?

If the base class method is not virtual, the derived class method hides the base method but does not override it. Calls through base class pointers will use the base class method, not the derived one.

intermediate

Explain the role of the override specifier in C++.

The <code>override</code> specifier is used in the derived class method declaration to indicate that the method is intended to override a base class virtual method. It helps the compiler catch errors if the method does not actually override anything.

Which keyword must be used in the base class to allow method overriding?

A virtual

B override

C static

D final

The virtual keyword in the base class method declaration allows derived classes to override that method.

What does the override keyword do in a derived class method?

A Prevents the method from being overridden

B Declares a new unrelated method

C Makes the method static

D Indicates the method overrides a base class virtual method

The override keyword tells the compiler that the method is meant to override a base class virtual method.

If a base class method is not virtual, what happens when a derived class defines a method with the same name?

A The derived method hides the base method

B The derived method overrides the base method

C Compilation error

D The base method is deleted

Without virtual, the derived method hides the base method but does not override it.

Which of the following is true about method overriding?

A Method signatures must be different

B Method signatures must be the same

C Only static methods can be overridden

D Overriding is only possible with private methods

To override a method, the derived class method must have the same name and signature as the base class method.

What is the benefit of method overriding?

A Prevents inheritance

B Increases program size

C Allows runtime polymorphism

D Makes methods static

Method overriding enables runtime polymorphism, allowing the program to choose the correct method version during execution.

Explain method overriding in C++ and why the virtual keyword is important.

Think about how derived classes change base class behavior.

Describe the difference between method overriding and method hiding in C++.

Consider what happens when base class methods are called through pointers.

Practice

1. What is the main purpose of method overriding in C++?

easy

A. To create a new method with a different name in the child class

B. To allow a child class to provide a specific implementation of a method already defined in its parent class

C. To prevent a method from being used in the child class

D. To copy all methods from the parent class to the child class without changes

Solution

1. Step 1: Understand method overriding concept

   Method overriding allows a child class to change how a method works that it inherits from its parent.

2. Step 2: Identify the correct purpose

   The child class provides its own version of the method, customizing behavior while keeping the same method name.

3. Final Answer:

   To allow a child class to provide a specific implementation of a method already defined in its parent class -> Option B

4. Quick Check:

   Method overriding = child changes parent method [OK]

Hint: Overriding means child changes parent's method behavior [OK]

Common Mistakes:

• Confusing overriding with method overloading
• Thinking overriding creates new method names
• Believing overriding copies methods without changes

2. Which of the following is the correct way to declare a method for overriding in C++?

easy

A. override void display();

B. void display() override;

C. void virtual display();

D. virtual void display();

Solution

1. Step 1: Recall syntax for virtual methods

   In C++, the parent class method must be marked with virtual to allow overriding.

2. Step 2: Check each option

   virtual void display(); correctly uses virtual void display();. void display() override; uses override incorrectly in declaration, it is used in child method definition. Options C and D have wrong syntax.

3. Final Answer:

   virtual void display(); -> Option D

4. Quick Check:

   Parent method uses virtual keyword [OK]

Hint: Parent method must be virtual for overriding [OK]

Common Mistakes:

• Placing override keyword in parent method declaration
• Using wrong order of keywords
• Omitting virtual keyword in parent

3. What will be the output of the following C++ code?

class Base {
public:
    virtual void show() { std::cout << "Base" << std::endl; }
};
class Derived : public Base {
public:
    void show() override { std::cout << "Derived" << std::endl; }
};
int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

medium

A. Derived

B. Runtime error

C. Compilation error

D. Base

Solution

1. Step 1: Understand virtual function behavior

   The pointer is of type Base* but points to Derived object. Because show() is virtual, the Derived version is called.

2. Step 2: Trace the call ptr->show()

   At runtime, the program calls Derived::show(), printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual method calls child's version [OK]

Hint: Virtual calls use actual object method, not pointer type [OK]

Common Mistakes:

• Expecting Base output due to pointer type
• Confusing override keyword effect
• Ignoring virtual keyword

4. Identify the error in the following code snippet related to method overriding:

class Parent {
public:
    void greet() { std::cout << "Hello from Parent" << std::endl; }
};
class Child : public Parent {
public:
    void greet() override { std::cout << "Hello from Child" << std::endl; }
};

medium

A. Parent class method greet() should be marked virtual

B. Child class method greet() should be marked virtual

C. override keyword is not allowed in C++

D. No error, code is correct

Solution

1. Step 1: Check parent method declaration

   For overriding to work properly, the parent method must be declared virtual. Here, it is not.

2. Step 2: Analyze override keyword usage

   The child method uses override, which requires the parent method to be virtual. Since parent method is not virtual, this causes a compilation error.

3. Final Answer:

   Parent class method greet() should be marked virtual -> Option A

4. Quick Check:

   Parent method must be virtual for override [OK]

Hint: Parent method must be virtual if child uses override [OK]

Common Mistakes:

• Thinking child method must be virtual
• Ignoring need for virtual in parent
• Believing override keyword is optional

5. Given the classes below, what will be the output when ptr->display(); is called?

class Animal {
public:
    virtual void display() { std::cout << "Animal" << std::endl; }
};
class Dog : public Animal {
public:
    void display() override { std::cout << "Dog" << std::endl; }
};
class Puppy : public Dog {
public:
    void display() override { std::cout << "Puppy" << std::endl; }
};

int main() {
    Animal* ptr = new Puppy();
    ptr->display();
    delete ptr;
    return 0;
}

hard

A. Animal

B. Dog

C. Puppy

D. Compilation error due to multiple overrides

Solution

1. Step 1: Understand multi-level overriding

   Each class overrides display() with its own version. The pointer is of type Animal* but points to a Puppy object.

2. Step 2: Determine which method is called

   Because display() is virtual, the most derived override (Puppy::display) is called at runtime.

3. Final Answer:

   Puppy -> Option C

4. Quick Check:

   Virtual calls deepest override in inheritance chain [OK]

Hint: Virtual calls deepest override in inheritance chain [OK]

Common Mistakes:

• Expecting Dog or Animal output due to pointer type
• Thinking multiple overrides cause errors
• Ignoring virtual keyword effect