static_cast on objects c++

Question

I am trying to understand why static_cast works different when it is performed on object than when it is performed on a pointer to object.

class A
{
    int myA;
public:
    A() { myA = 11; };
    virtual void Do() {  printf("\n A class is executed "); }
};
class B : public A
{
    int myB;
public:
    B(){ myB = 22; }

    void Do() {   printf("\n B class is executed "); }
};

When the following cast is performed, A::Do() is executed. Why? Why B's virtual table is neglected?

A t_a;
B t_b;
(static_cast<A>(t_b)).Do();  //output: A class is executed

But when it is done via pointer, B::Do() is executed.

A* pA = new B;
pA->Do();
(static_cast<A*>(pA))->Do();   //output: B class is executed

Can you give me a full explanation (including memory layout) to understand the difference.

You don't need the last `static_cast(pA)` because `pA` is already a pointer to `A`. The first static cast tells the compiler to treat `t_b` object as an instance of class `A`; in fact, every object of type `B` is also of type `A` due to inheritance. What will happen, that is somewhat hidden in the example, is object slicing: you lose access to members and methods of `B`. The second example makes use of runtime polymorphism and virtual dispatching to access a method of a derived class through a pointer to a base class. — rturrado, Nov 12 '22 at 20:29

static_cast on objects c++

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