CODEWITHSUNDEEP
c++castingvoid-pointersdynamic-cast
claudius
claudius

Reputation: 1144

dynamic_cast vs static_cast to void*

In the last two lines of below program, static_cast<void*> and dynamic_cast<void *> behave differently. From what I understand, The result of a dynamic_cast<void*> always resolves to the address of the complete object. So it uses RTTI in some way. Could anyone explain how compilers uses RTTI to differentiate between the two.

#include <iostream>
using namespace std;
class Top {
protected:
int x;
public:
    Top(int n) { x = n; }
    virtual ~Top() {} 
    friend ostream& operator<<(ostream& os, const Top& t) {
        return os << t.x;
    }
};
class Left : virtual public Top {
protected:
    int y;
public:
    Left(int m, int n) : Top(m) { y = n; }
};
class Right : virtual public Top {
protected:
    int z;
public:
    Right(int m, int n) : Top(m) { z = n; }
};
class Bottom : public Left, public Right {
    int w; 
public:
    Bottom(int i, int j, int k, int m): Top(i), Left(0, j), Right(0, k) { w = m; }
    friend ostream& operator<<(ostream& os, const Bottom& b) {
        return os << b.x << ',' << b.y << ',' << b.z<< ',' << b.w;
    }
};
int main() {
    Bottom b(1, 2, 3, 4);
    cout << sizeof b << endl;
    cout << b << endl;
    cout << static_cast<void*>(&b) << endl;
    Top* p = static_cast<Top*>(&b);
    cout << *p << endl;
    cout << p << endl;
    cout << static_cast<void*>(p) << endl;
    cout << dynamic_cast<void*>(p) << endl;
    return 0;
}

Possible output: https://ideone.com/WoX5DI

28
1,2,3,4
0xbfcce604
1
0xbfcce618
0xbfcce618
0xbfcce604

Upvotes: 8

Views: 1220

Answers (1)

manlio
manlio

Reputation: 18902

From 5.2.7 / 7:

If T is "pointer to cv void," then the result is a pointer to the most derived object pointed to by v. Otherwise, a run-time check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.

So using dynamic_cast<void*>(o) you get a pointer to the first byte of the most "derived" object (if o is polymorphic).

The code the compiler generates for dynamic_cast<void *>(...) is something like:

static_cast<void*>(dynamic_cast<most_derived_type *>(...))

This property is often used for serialization.

Upvotes: 9

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