Method overriding in C++ - Time & Space Complexity

Time Complexity: Method overriding

O(n)

Understanding Time Complexity

When we use method overriding, we replace a method in a child class with a new version.

We want to see how this affects the time it takes to run the program.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Base {
public:
    virtual void display() {
        // simple print
        std::cout << "Base display" << std::endl;
    }
};

class Derived : public Base {
public:
    void display() override {
        // overridden print
        std::cout << "Derived display" << std::endl;
    }
};

void callDisplay(Base* obj) {
    obj->display();
}
    

This code shows a base class with a method and a derived class that overrides it. We call the method through a base pointer.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the display() method once per object.
• How many times: Each callDisplay call triggers exactly one method call.

How Execution Grows With Input

Each call to display runs a simple print statement, which takes about the same time regardless of input size.

Input Size (n)	Approx. Operations
10	10 method calls, each simple print
100	100 method calls, each simple print
1000	1000 method calls, each simple print

Pattern observation: The total work grows directly with the number of calls.

Final Time Complexity

Time Complexity: O(n)

This means if you call the method n times, the total time grows in a straight line with n.

Common Mistake

[X] Wrong: "Overriding a method makes the program slower because it adds extra work."

[OK] Correct: Overriding just changes which code runs, but each call still takes about the same time as before.

Interview Connect

Understanding how method overriding affects time helps you explain object-oriented design choices clearly and confidently.

Self-Check

"What if the overridden method called another method with a loop inside? How would the time complexity change?"

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