Question

I have written a few lines of code which I think should not compile. I am calling a method of a derived class on a static_cast-ed pointer to object of base class as follows:

class B {};    

class D: public B
{
public:
    void bar() { printf("%d\n", m_i); }
private:
    int m_i;
};

int main()
{
    B b;
    D* d = static_cast<D*>(&b);
    d->bar();
    return 0;
}

The value printed is obviously junk but should this even compile? How does gcc manage to do that?

Answer 1

gcc can’t guarantee that it’s incorrect, except in enough of a minority of cases that it really isn’t worth checking. When you use static_cast, then you are promising the compiler that you know wtf you’re doing.

There are two kinds of casts here. static_cast, which is, you are telling the compiler that pointer to a base IS a pointer to derived, and shut up and get on with it. dynamic_cast, which is, you are asking the compiler to check if that pointer to base is indeed a pointer to derived. You used static_cast, so the compiler shut up and did as you said.

Edit: John accurately pointed out that there are no virtual functions in your inheritance hierarchy, for which you should be fired from C++, and dynamic_cast is only valid for virtual functions.