C++ object model: calling virtual function by vptr leads to crash

Troskyvs 0
Viewed 56

I've got this code snippet, it tries to call virtual function through an object's vptr (pointing to virtual function table) and uses object pointer to convert to p->vptr, like this:

#include<iostream>
using namespace std;
struct C {
    virtual int f() {
        return 7;
    }
};
typedef int (*pf)();
int main() {
    C c1;
    pf *pvtable = (pf *) &c1;
    cout << (*pvtable[0])() << endl;
    return 0;
}

I used clang++14 to compile/link. On running it, programs returns 139, and no cout line is shown, seems it has crashed.

Why it doesn't work and how to fix it?

object c++
1 Answers

Why it doesn't work

You are casting a pointer-to-C to a pointer-to-int(*)().

This cast has no meaning in the C++ language and using the resulting pointer is explicitly Undefined Behavior.

and how to fix it?

There is no reliable way.

C++ does not promise the existence of a vtable pointer in any program, and if there is one C++ does not offer any method to access it.

Drew Dormann0
Related