Runtime polymorphism in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is runtime polymorphism in C++?

Runtime polymorphism means that the program decides which function to call during execution, not while compiling. It allows a base class pointer to call derived class functions using virtual functions.

beginner

How do you enable runtime polymorphism in C++?

By declaring a function as virtual in the base class. This tells the compiler to use dynamic dispatch to call the correct function at runtime.

intermediate

What happens if a base class function is not virtual but overridden in a derived class?

The base class pointer will call the base class version of the function, not the derived class version. This is because without <code>virtual</code>, the function call is resolved at compile time.

advanced

Explain the role of the vtable in runtime polymorphism.

The vtable is a hidden table created by the compiler that stores pointers to virtual functions. Each class with virtual functions has a vtable. At runtime, the program uses the vtable to call the correct function version.

intermediate

What is a pure virtual function and how does it relate to runtime polymorphism?

A pure virtual function is declared by assigning 0 in the base class (e.g., <code>virtual void f() = 0;</code>). It makes the class abstract and forces derived classes to implement the function, enabling polymorphism.

Which keyword enables runtime polymorphism in C++?

A virtual

B static

C const

D inline

The virtual keyword tells the compiler to use dynamic dispatch for function calls, enabling runtime polymorphism.

What will happen if a base class pointer calls a non-virtual function overridden in a derived class?

A Compilation error

B Derived class function is called

C Base class function is called

D Program crashes

Without virtual, the function call is resolved at compile time, so the base class function is called.

What is the purpose of a vtable?

A Store data members

B Store pointers to virtual functions

C Store global variables

D Store function parameters

The vtable stores pointers to virtual functions to enable dynamic dispatch at runtime.

What does a pure virtual function do?

A Provides a default implementation

B Disables polymorphism

C Prevents inheritance

D Makes the class abstract

A pure virtual function makes the class abstract and forces derived classes to implement the function.

Which of these is true about runtime polymorphism?

A It requires virtual functions

B Function calls are decided at compile time

C It only works with static functions

D It disables inheritance

Runtime polymorphism requires virtual functions to decide which function to call during program execution.

Describe how runtime polymorphism works in C++ and why virtual functions are important.

Think about how the program decides which function to call when using a base class pointer.

Explain the difference between compile-time and runtime polymorphism with examples.

Consider when the program decides which function to call: before running or while running.

Practice

1. What is the main purpose of virtual functions in C++ runtime polymorphism?

easy

A. To allocate memory for objects dynamically

B. To make functions run faster by avoiding function calls

C. To prevent any function from being overridden in derived classes

D. To allow derived classes to override base class methods and decide at runtime which method to call

Solution

1. Step 1: Understand virtual functions role

   Virtual functions allow a base class to declare a function that derived classes can override.

2. Step 2: Explain runtime decision

   At runtime, the program decides which overridden function to call based on the actual object type.

3. Final Answer:

   To allow derived classes to override base class methods and decide at runtime which method to call -> Option D

4. Quick Check:

   Virtual functions enable runtime method selection [OK]

Hint: Virtual means choose function at runtime, not compile time [OK]

Common Mistakes:

• Thinking virtual speeds up function calls
• Believing virtual prevents overriding
• Confusing virtual with memory allocation

2. Which of the following is the correct syntax to declare a virtual function in a base class?

easy

A. void display() virtual;

B. void virtual display();

C. virtual void display();

D. virtual: void display();

Solution

1. Step 1: Recall virtual function syntax

   The keyword virtual comes before the return type in the function declaration.

2. Step 2: Match correct syntax

   virtual void display(); correctly places virtual before void display();

3. Final Answer:

   virtual void display(); -> Option C

4. Quick Check:

   Virtual keyword precedes return type [OK]

Hint: Virtual always goes before return type in declaration [OK]

Common Mistakes:

• Placing virtual after function name
• Using virtual as a label or modifier incorrectly
• Missing semicolon after declaration

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* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Identify virtual function usage

   The show() function in Base is virtual and overridden in Derived.

2. Step 2: Understand pointer type and object type

   The pointer is of type Base* but points to a Derived object, so the overridden show() in Derived is called at runtime.

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls derived version [OK]

Hint: Virtual functions call derived method via base pointer [OK]

Common Mistakes:

• Expecting base class method output
• Confusing pointer type with object type
• Ignoring virtual keyword effect

4. Identify the error in the following code snippet related to runtime polymorphism:

class Animal {
public:
    void sound() { std::cout << "Animal sound"; }
};
class Dog : public Animal {
public:
    void sound() { std::cout << "Bark"; }
};
int main() {
    Animal* a = new Dog();
    a->sound();
    delete a;
    return 0;
}

medium

A. Incorrect pointer type declaration

B. Missing virtual keyword in base class function

C. Derived class function should be static

D. Memory leak due to missing delete

Solution

1. Step 1: Check base class function declaration

   The base class function sound() is not declared virtual, so no runtime polymorphism occurs.

2. Step 2: Effect on function call

   Because sound() is not virtual, calling a->sound() calls the base class version, not the derived one.

3. Final Answer:

   Missing virtual keyword in base class function -> Option B

4. Quick Check:

   Virtual needed for runtime polymorphism [OK]

Hint: Base function must be virtual for polymorphism [OK]

Common Mistakes:

• Assuming override works without virtual
• Thinking pointer type causes error
• Ignoring virtual keyword importance

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

class Shape {
public:
    virtual void identify() { std::cout << "Shape"; }
};
class Circle : public Shape {
public:
    void identify() override { std::cout << "Circle"; }
};
class ColoredCircle : public Circle {
public:
    void identify() override { std::cout << "Colored Circle"; }
};
int main() {
    Shape* ptr = new ColoredCircle();
    ptr->identify();
    delete ptr;
    return 0;
}

hard

A. Colored Circle

B. Circle

C. Shape

D. Compilation error due to multiple overrides

Solution

1. Step 1: Trace virtual function overrides

   identify() is virtual in Shape, overridden in Circle, and again in ColoredCircle.

2. Step 2: Determine actual object type at runtime

   The pointer ptr is of type Shape* but points to a ColoredCircle object, so the most derived override runs.

3. Final Answer:

   Colored Circle -> Option A

4. Quick Check:

   Most derived override called at runtime [OK]

Hint: Runtime calls most derived override in inheritance chain [OK]

Common Mistakes:

• Expecting base or intermediate class output
• Thinking multiple overrides cause errors
• Ignoring virtual override chain