Base class pointers in C++

Introduction

Base class pointers let you use a pointer of a parent class to point to objects of child classes. This helps write flexible and reusable code.

When you want to store different child objects in the same pointer type.

When you want to call functions on objects without knowing their exact child type.

When you want to use polymorphism to decide which function runs at runtime.

When you want to write code that works with a family of related classes easily.

Syntax

C++

BaseClass *pointerName;
pointerName = &childObject;

The pointer is declared as the base class type.

It can point to any object of the base class or its derived classes.

Examples

A base class pointer points to a Dog object. It calls the base class version of speak because the function is not virtual.

C++

class Animal {
public:
    void speak() { std::cout << "Animal sound" << std::endl; }
};

class Dog : public Animal {
public:
    void speak() { std::cout << "Bark" << std::endl; }
};

int main() {
    Dog d;
    Animal *ptr = &d;
    ptr->speak();
    return 0;
}

Here, speak is virtual. The base class pointer calls Dog's speak because of polymorphism.

C++

class Animal {
public:
    virtual void speak() { std::cout << "Animal sound" << std::endl; }
};

class Dog : public Animal {
public:
    void speak() override { std::cout << "Bark" << std::endl; }
};

int main() {
    Dog d;
    Animal *ptr = &d;
    ptr->speak();
    return 0;
}

Sample Program

This program shows a base class pointer pointing to different child objects. It calls the correct speak function for each child thanks to virtual functions.

C++

#include <iostream>

class Animal {
public:
    virtual ~Animal() {}
    virtual void speak() {
        std::cout << "Animal sound" << std::endl;
    }
};

class Dog : public Animal {
public:
    void speak() override {
        std::cout << "Bark" << std::endl;
    }
};

class Cat : public Animal {
public:
    void speak() override {
        std::cout << "Meow" << std::endl;
    }
};

int main() {
    Dog dog;
    Cat cat;

    Animal *ptr;

    ptr = &dog;
    ptr->speak();  // Calls Dog's speak

    ptr = &cat;
    ptr->speak();  // Calls Cat's speak

    return 0;
}

Important Notes

Without virtual, the base class function is called even if the pointer points to a child object.

Base class pointers allow writing code that works with many child types easily.

Remember to use virtual destructors in base classes when using base class pointers to avoid memory issues.

Summary

Base class pointers can point to objects of derived classes.

They enable polymorphism when used with virtual functions.

This helps write flexible and reusable code that works with related classes.

Practice

1. What does a base class pointer in C++ allow you to do?

easy

A. Only point to base class objects

B. Point to objects of derived classes

C. Only point to primitive data types

D. Prevent any pointer arithmetic

Solution

1. Step 1: Understand base and derived classes

   A base class is a general class, and derived classes extend it with more features.

2. Step 2: Role of base class pointers

   A base class pointer can point to any object of the base class or its derived classes, enabling polymorphism.

3. Final Answer:

   Point to objects of derived classes -> Option B

4. Quick Check:

   Base pointer can point to derived objects [OK]

Hint: Base pointers can point to derived objects [OK]

Common Mistakes:

• Thinking base pointers only point to base objects
• Confusing pointers with references
• Assuming base pointers can't access derived objects

2. Which of the following is the correct syntax to declare a base class pointer named ptr for a class Base?

easy

A. Base *ptr;

B. ptr Base*;

C. Base ptr*;

D. *Base ptr;

Solution

1. Step 1: Recall pointer declaration syntax

   In C++, a pointer to a type is declared as Type *pointerName;.

2. Step 2: Apply to Base class

   For class Base, the pointer declaration is Base *ptr;.

3. Final Answer:

   Base *ptr; -> Option A

4. Quick Check:

   Pointer syntax = Type *name [OK]

Hint: Pointer syntax is Type *name; [OK]

Common Mistakes:

• Placing * after variable name incorrectly
• Swapping type and variable name
• Using invalid syntax like *Base ptr

3. Consider the 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;
}

What is the output when this code runs?

medium

A. Derived

B. Base

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand virtual function behavior

   The show() function is virtual in Base, so calls use the actual object's method.

2. Step 2: Pointer points to Derived object

   The pointer ptr points to a Derived object, so Derived::show() is called.

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Virtual function calls Derived method [OK]

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

Common Mistakes:

• Ignoring virtual keyword and expecting Base output
• Confusing pointer type with object type
• Assuming compilation or runtime error

4. What is wrong with this code snippet?

class Base {
public:
    void display() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void display() { std::cout << "Derived"; }
};

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

medium

A. delete ptr causes memory leak

B. Pointer ptr cannot point to Derived object

C. Derived class must not redefine display()

D. display() should be declared virtual in Base for polymorphism

Solution

1. Step 1: Check function overriding and polymorphism

   Without virtual, base class pointer calls base version even if pointing to derived object.

2. Step 2: Identify missing virtual keyword

   To enable polymorphism, display() in Base must be declared virtual.

3. Final Answer:

   display() should be declared virtual in Base for polymorphism -> Option D

4. Quick Check:

   Missing virtual causes base method call [OK]

Hint: Declare base methods virtual for polymorphism [OK]

Common Mistakes:

• Assuming pointer can't point to derived
• Thinking delete causes leak here
• Believing derived can't redefine base methods

5. You have a base class Animal with a virtual function sound(). Two derived classes Dog and Cat override sound(). You want to store multiple animals in an array and call sound() on each. Which approach correctly uses base class pointers to achieve this?

hard

A. Create an array of Animal objects and assign Dog and Cat objects directly

B. Create an array of Dog* and cast Cat* to Dog*

C. Create an array of Animal*, store new Dog() and new Cat(), then loop and call sound()

D. Use an array of void* pointers and cast to Animal* before calling sound()

Solution

1. Step 1: Understand polymorphic storage

   To store different derived objects, use an array of base class pointers (Animal*).

2. Step 2: Store derived objects via pointers

   Store new Dog() and new Cat() in the Animal* array, then call sound() polymorphically.

3. Final Answer:

   Create an array of Animal*, store new Dog() and new Cat(), then loop and call sound() -> Option C

4. Quick Check:

   Base pointer array + virtual functions = polymorphism [OK]

Hint: Use base class pointer array for polymorphic objects [OK]

Common Mistakes:

• Using object array causes slicing
• Casting unrelated pointers unsafely
• Using void* loses type safety and polymorphism