More on pointer problems

A base class pointer can point to a base class object without problems.
A derived class pointer can point to a derived class object without problems.
Let’s see what happens if we aim a base class pointer at a derived class object, using a static_cast.

class B {
public:
void f() { }
};

class D: public B {
public:
void m() { }
};

Pointing the base class pointer at a derived object loses the resolution if it’s not polymorphic.

int main() {
B* p = static_cast<B*>( new D ); // casting
p->m();
}

Error: m is not a member of B

We have a function m() in the derived D. The base B doesn’t have the function m(). Use of the static_cast allows p to point to a D object.

The compilation error from the statement
p->m();
... occurs because p is of type B*, and B doesn’t have the required function m()

We are “okay” if we don’t try and use D functionality .

It doesn’t make sense to have a derived pointer pointing to a base object, since the base object “is not” a derived object.

class B {
// Whatever
};

class D : public B {
// More whatever
};

int main()
{
D* p;
p = new B;
}

Error: Cannot use B* to initialize D*.

But using a static_cast we seemly can do this ...

#include <iostream>

using namespace std;

class B {
public:
B() {}
void m() { cout << "Base " << endl; }
};

class D : public B {
int data;
public:
D() : B(), data(1) {}
void m() { cout << "Derived " << data << endl; }
};

int main()
{
D* p = static_cast<D*>( new B );
p->m();
cout << typeid(p).name() << endl;
cout << typeid(*p).name() << endl;

return 0;
}

Only B::B() is invoked - data isn’t initialised.

// calls D::m(), data incorrect

// pointer to D

// D ... incomplete object