Calling a Derived Class method from a Void Pointer cast to a Base Object

Question

I want to call an overridden method in a derived class from a void pointer. This fails at runtime because the virtual Base::foo() method is invoked, rather than Derived::foo(). This is how the relevant code works:

Class Base  
{  
public:  
    Base();  
    virtual void foo() = 0;
}

Class Derived: public Base
{
public:
    Derived();
    void foo();
}

int main()
{
    Derived* dv = new Derived();
    void* ptr = dv;

    static_cast<Base*>(ptr)->foo();
}

Works fine for me (derived method is called). However I believe what you are trying to do is actually undefined behaviour, because the standard does not appear to guarantee that if you convert Derived* to void* and then to Base* that it'll actually point to the right object. — Brian Bi
For example, the compiler is allowed to put the Base subobject at the end of the Derived object instead of at the beginning, in which case static_cast<Base*>(ptr) likely won't point to the Base subobject, since it'll point to the beginning of the Derived object. — Brian Bi

David Schwartz David Schwartz · Accepted Answer · 2014-03-14T06:02:52

There's no way this can work other than by luck. Since this is a static_cast, the conversion must be known at compile time. But because the pointer is a void *, there's no way the compiler can know how the pointer needs to converted to point to the instance of the base inside the derived.

Calling a Derived Class method from a Void Pointer cast to a Base Object

1 Answers