Calling Derived class function from a Base class pointer after typecasting it to Derived class pointer

Question

I am fairly new to C++ (& OOP). I am struggling to understand the following piece of code:

#include <iostream>

class Base {
public:
    Base() {
        std::cout << "In Base Constr: " << __FUNCSIG__ << std::endl;
    }

    virtual ~Base() {
        std::cout << "In Base Destr: " << __FUNCSIG__ << std::endl;
    }

    void A() {
        std::cout << "In Base func A " << __FUNCSIG__ << std::endl;
    }
};

class Derived : public Base {
    public:
    Derived() {
        std::cout << "In Derived Constr: " << __FUNCSIG__ << std::endl;
    }

    ~Derived() {
        std::cout << "In Derived Destr: " << __FUNCSIG__ << std::endl;
    }

    void B() {
        std::cout << "In Derived func B " << __FUNCSIG__ << std::endl;
    }
};

void test(Base* b) {
    Derived* d = static_cast<Derived*>(b);
    d->A();
    d->B();              // How is this valid??
}

int main() {
    Base *b = new Derived();
    std::cout << "In main" << std::endl;
    b->A();
    std::cout << __LINE__ << std::endl;

    Base *bb = new Base();
    std::cout << __LINE__ << std::endl;
    test(bb);

    delete b;
    delete bb;
}

I am not sure, why & how the line d->B() works? even though the pointer was typecasted to Derived class, but the Base class object itself should not have that function in memory.

Answer 1

I am not sure, why & how the line b->B() works? [..] the Base class object itself should not have that function in memory

You're right! It doesn't work!

(Well, functions aren't stored "in memory", but…)

The call is invalid. The static_cast says "I promise that this Base* points to a Derived". That promise was broken.

The program has undefined behaviour. That can in practice mean that things "appear" to work, especially if no member variables were touched by the non-existent function...