C++ types of casting, what they're doing, and which variables are valid?

Question

Ok I'm having some trouble understanding the different types of casting and whats going on with them. I created two classes: A and B. B is derived from A.

I made a bunch of pointers and dumb them to the screen. I need help understanding what's going on and which variables are actually valid.

Here's the code:

#include <iostream>

using namespace std;


class A {
public:
    A(int iValue){
            m_iValue = iValue;
    }

    A(){
            m_iValue = 10;
    }

    virtual void doSomething(){
            cout << "The value is " << getValue() << endl;
    }

    void doSomethingElse(){
            cout << "This is something else" << endl;
    }

    virtual int getValue(){
            return m_iValue;
    }

private:
    int m_iValue;
};

class B : public A{

public:
 B() : A(5){

    }

    virtual void doSomething(){

            cout << "This time, the value is " << getValue() << endl;
    }

    void doSomethingElse(){

            cout << "This is something else entirely" << endl;
    }

    virtual void doAnotherThing(){

            cout << "And this is a third thing" << endl;
    }
};


int main(int argc, char * argv[]){
    A * pA1 = new B;
    A * pA2 = new A;

    B * pB1;
    B * pB2;

    pB1 = (B*)pA1;
    pB2 = (B*)pA2;

    cout << "Step 1 - c style cast:" << endl;
    cout << "  pA1 cast to pointer pB1: " << pB1 << endl;
    cout << "  pA2 cast to pointer pB2: " << pB2 << endl << endl;

    B * pB3;
    B * pB4;

    pB3 = dynamic_cast<B*>(pA1);
    pB4 = dynamic_cast<B*>(pA2);

    cout << "Step 2 - dynamic_cast:" << endl;
    cout << "  pA1 cast to pointer pB3: " << pB3 << endl;
    cout << "  pA2 cast to pointer pB4: " << pB4 << endl << endl;


    B * pB5;
    B * pB6;

    pB5 = static_cast<B*>(pA1);
    pB6 = static_cast<B*>(pA2);

    cout << "Step 3 - static_cast:" << endl;
    cout << "  pA1 cast to pointer pB5: " << pB5 << endl;
    cout << "  pA2 cast to pointer pB6: " << pB6 << endl << endl;

    B * pB7;
    B * pB8;

    pB7 = reinterpret_cast<B*>(pA1);
    pB8 = reinterpret_cast<B*>(pA2);

    cout << "Step 4 - reinterpret_cast:" << endl;
    cout << "  pA1 cast to pointer pB7: " << pB7 << endl;
    cout << "  pA2 cast to pointer pB8: " << pB8 << endl << endl;

    return 0;
}

Here's what it outputs:

Step 1 - c style cast:
  pA1 cast to pointer pB1: 0x1853010
  pA2 cast to pointer pB2: 0x1853030

Step 2 - dynamic_cast:
  pA1 cast to pointer pB3: 0x1853010
  pA2 cast to pointer pB4: 0

Step 3 - static_cast:
  pA1 cast to pointer pB5: 0x1853010
  pA2 cast to pointer pB6: 0x1853030

Step 4 - reinterpret_cast:
  pA1 cast to pointer pB7: 0x1853010
  pA2 cast to pointer pB8: 0x1853030

People, why don't you search and/or read your books properly before asking... — Griwes, Apr 05 '13 at 17:48
Another related question: http://stackoverflow.com/q/332030/10077 — Fred Larson, Apr 05 '13 at 17:50
Note for the C-style cast: you have a typo, printing `pB1` twice; if you print `pB2`, you'd get the same result as in other non-dynamic cast scenarios. — Sergey Kalinichenko, Apr 05 '13 at 17:50
@dasblinkenlight - nice call. And Fred, I'll check it out. Thanks — Habit, Apr 05 '13 at 19:46
[The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) is a good place to start. Seriously, you will learn more, faster, and avoid common pitfalls if you study the `C++` basics with a good book. — Blastfurnace, Apr 05 '13 at 22:22

C++ types of casting, what they're doing, and which variables are valid?

0 Answers0