Question

The questions are:
a) Is the following code legal or not ? (considering it crashes at run-time)
b) If there is any compiler flag for gcc or MVC to show at compile time, a potential problem in the following code ?

#include <iostream>
using namespace std;
class A
{
public:
    void write(){   cout<<"A"; }    
};
class B
{
public:
    virtual void write(){ cout<<"B"; }  
};
int main()
{

    A *pa=(A*) new B();
    pa->write();
    B *pb=(B*) new A() ;
    pb->write();

    delete pa;
    delete pb;
    return 0;
}

Thanks!

Answer 1

No, it’s not legal. B doesn’t derive from A, nor vice versa. Forcing a cast like this results in undefined behaviour. You’re telling the compiler “shut up, I know what I’m doing”, which leads to all sorts of trouble. This is one reason to avoid old C-style casts, and use C++-style casts instead (static_cast, etc.).