Section 15: Base Class Pointer Derived Class Object

1. Concept: Base Pointer to Derived Object

We can have a Base Class Pointer pointing to a Derived Class Object.

• Rule: When a base class pointer holds a derived class object, we can only call functions present in the base class.
• Restriction: We cannot call functions that are defined exclusively in the derived class.
• Reasoning: The pointer is of type Base. The compiler determines accessible members based on the pointer type, not the object type at runtime (unless virtual functions are used, though that is a separate topic).

Visual Representation

The following diagram illustrates the accessibility of functions based on your notes:

classDiagram
    class Base {
        +void func1()
        +void func2()
        +void func3()
    }
    class Derived {
        +void func4()
        +void func5()
    }

    Base <|-- Derived : Inherits from

    note for Base "Code: Base *p = new Derived();"
    note for Base "p->func1();  ✅ OK (In Base)\np->func2();  ✅ OK (In Base)\np->func4();  ❌ Error (Not in Base)"

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2. Inverse Case: Derived Pointer to Base Object

What if the pointer is of the Derived class and we try to assign an object of the Base class?

• Result: It is NOT possible.
• Reason: A Derived pointer expects to see all the features of the Derived class. A Base object does not have the extra features (like func4 or func5), so assigning it would lead to undefined behavior when accessing those missing members.

Derived *p;
p = new Base(); // ❌ Error: Cannot assign Base object to Derived pointer

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3. Real World Analogy: The Car

Think of a Basic Car and an Advanced Car (where Advanced Car inherits from Basic Car).

• Inheritance: The Advanced Car has all the features of a Basic Car, plus extra features.
• Logic:
• You can call an Advanced Car a "Basic Car" (because it has all basic features).

• You cannot call a Basic Car an "Advanced Car" (because it lacks the advanced features).

• In Code: The methods of the Advanced Car are not present in the Basic Car. Therefore, a pointer to a "Basic Car" cannot access the exclusive methods of the "Advanced Car," even if it is actually looking at one.

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4. Code Example

class Base {
public:
    void func1() { }
    void func2() { }
};

class Derived : public Base {
public:
    void func4() { }
    void func5() { }
};

int main() {
    Derived d;

    // Base pointer pointing to Derived object
    // This is valid because Derived IS A Base
    Base *ptr = &d;      // Stack allocation example
    // Or: Base *ptr = new Derived(); // Heap allocation example

    // Function Calls
    ptr->func1();        // ✅ Valid: func1 is in Base
    ptr->func2();        // ✅ Valid: func2 is in Base

    // ptr->func4();     // ❌ Error: func4 is not in Base class

    // Inverse Case (Invalid)
    Base b;
    // Derived *ptr2 = &b; // ❌ Error: Cannot point Derived pointer to Base object

    return 0;
}