Function overriding in C++ - Step-by-Step Execution

Concept Flow - Function overriding

Base class defines function

↓

Derived class defines function with same name

↓

Create derived class object

↓

Call function on derived object

↓

Derived class function runs, overrides base

↓

End

Function overriding happens when a derived class provides its own version of a function defined in its base class. When called on a derived object, the derived version runs.

Execution Sample

C++

#include <iostream>
class Base {
public:
  virtual void show() { std::cout << "Base show"; }
};
class Derived : public Base {
public:
  void show() override { std::cout << "Derived show"; }
};
int main() {
  Derived d;
  d.show();
}

This code shows a base class and a derived class both having a show() function. Calling show() on derived object runs derived version.

Execution Table

Step	Action	Function Called	Output
1	Create Derived object d	None
2	Call d.show()	Derived::show()	Derived show
3	Program ends	None

💡 No more code to run, program ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
d (Derived object)	None	Created	Exists	Exists

Key Moments - 2 Insights

Why does calling show() on derived object run Derived::show() and not Base::show()?

Because Derived class defines its own show() with the same name, it overrides Base's show(). See execution_table step 2 where Derived::show() runs.

What if Derived did not have show()?

Then calling d.show() would run Base::show() because Derived inherits it. Overriding only happens if Derived defines the function.

Visual Quiz - 3 Questions

Look at the execution_table, what function is called at step 2?

A Base::show()

B Derived::show()

C No function called

D Both Base::show() and Derived::show()

💡 Hint

Check the 'Function Called' column at step 2 in execution_table

At which step is the Derived object created?

A Step 2

B Step 3

C Step 1

D No object created

💡 Hint

Look at the 'Action' column in execution_table for object creation

If Derived did not override show(), what would be the output at step 2?

A Base show

B Derived show

C No output

D Compilation error

💡 Hint

Refer to key_moments explanation about overriding and inheritance

Concept Snapshot

Function overriding means a derived class has a function with the same name as in base class.
When called on derived object, derived function runs, hiding base version.
Overriding requires same function signature.
If derived does not override, base function runs.
Used for customizing behavior in derived classes.

Full Transcript

Function overriding in C++ happens when a derived class defines a function with the same name as one in its base class. When you create an object of the derived class and call that function, the derived class's version runs instead of the base class's. In the example, the base class has a show() function that prints "Base show". The derived class also has a show() function that prints "Derived show". When we create a derived object and call show(), it prints "Derived show". This is because the derived class's function overrides the base class's. If the derived class did not have its own show(), calling show() on the derived object would run the base class's version. This lets derived classes customize or replace behavior from base classes.

Practice

1. What is function overriding in C++?

easy

A. A child class provides its own version of a parent class's virtual function.

B. A function with the same name but different parameters in the same class.

C. A function that is called automatically when an object is created.

D. A function that cannot be changed once defined.

Solution

1. Step 1: Understand function overriding concept

   Function overriding means a child class changes the behavior of a function defined in its parent class.

2. Step 2: Identify correct description

   A child class provides its own version of a parent class's virtual function. correctly describes overriding as providing a new version of a parent's virtual function in the child class.

3. Final Answer:

   A child class provides its own version of a parent class's virtual function. -> Option A

4. Quick Check:

   Function overriding = child changes parent's virtual function [OK]

Hint: Overriding means child changes parent's virtual function [OK]

Common Mistakes:

• Confusing overriding with overloading
• Thinking overriding happens without virtual keyword
• Believing overriding applies to non-virtual functions

2. Which of the following is the correct syntax to override a function in C++11 and later?

easy

A. void func() virtual override;

B. override void func();

C. void func() override;

D. virtual void func() override;

Solution

1. Step 1: Recall override syntax in child class

   In C++11+, the child class function uses the same signature and adds the 'override' keyword after the function declaration.

2. Step 2: Identify correct syntax

   virtual void func() override; shows the correct syntax: 'virtual void func() override;'. The 'virtual' keyword can be used in the child class but is optional; however, it is valid and common to include it.

3. Final Answer:

   virtual void func() override; -> Option D

4. Quick Check:

   Child function uses virtual and override keywords correctly [OK]

Hint: Use 'override' after function signature in child class; 'virtual' is optional but allowed [OK]

Common Mistakes:

• Adding 'virtual' keyword after function name
• Placing 'override' before function name
• Omitting 'override' keyword entirely

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

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

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand virtual function behavior

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

2. Step 2: Trace the function call

   Calling b->show() invokes Derived::show(), printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls child's version [OK]

Hint: Virtual functions call child's version via base pointer [OK]

Common Mistakes:

• Expecting base class output
• Forgetting to make base function virtual
• Confusing pointer type with object type

4. Identify the error in the following code snippet:

class Parent {
public:
    void display() { std::cout << "Parent"; }
};
class Child : public Parent {
public:
    void display() override { std::cout << "Child"; }
};

medium

A. Child's display() should not use override keyword.

B. Parent's display() must be declared virtual to override.

C. display() must return int, not void.

D. No error, code is correct.

Solution

1. Step 1: Check virtual keyword in parent

   For overriding to work, the base class function must be declared virtual.

2. Step 2: Analyze override keyword usage

   Child uses 'override' but Parent's display() is not virtual, causing a compilation error.

3. Final Answer:

   Parent's display() must be declared virtual to override. -> Option B

4. Quick Check:

   Base function must be virtual for override [OK]

Hint: Base function must be virtual to allow override [OK]

Common Mistakes:

• Ignoring virtual keyword in base class
• Thinking override works without virtual
• Assuming override keyword is optional

5. Given the classes below, what will be the output of obj->print();?

class A {
public:
    virtual void print() { std::cout << "A"; }
};
class B : public A {
public:
    void print() override { std::cout << "B"; }
};
class C : public B {
public:
    void print() override { std::cout << "C"; }
};

int main() {
    A* obj = new C();
    obj->print();
    delete obj;
    return 0;
}

hard

A. A

B. B

C. C

D. Compilation error

Solution

1. Step 1: Understand pointer and object types

   Pointer 'obj' is of type A*, but it points to an object of class C.

2. Step 2: Follow virtual function overriding chain

   Since print() is virtual and overridden in B and C, the most derived override (C::print) is called.

3. Final Answer:

   C -> Option C

4. Quick Check:

   Virtual call resolves to most derived override [OK]

Hint: Virtual calls use most derived override at runtime [OK]

Common Mistakes:

• Expecting base class output
• Confusing pointer type with object type
• Ignoring virtual function behavior